US20070094753A1

US20070094753A1 - Polycomb genes from maize- Mez1 and Mez2

Info

Publication number: US20070094753A1
Application number: US11/633,204
Authority: US
Inventors: Shawn Kaeppler; Nathan Springer; Timothy Helentjaris; Ronald Phillips
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-07-17
Filing date: 2006-12-04
Publication date: 2007-04-26
Also published as: WO2002006321A3; WO2002006468A2; WO2002006322A2; WO2002006321A2; US20080155717A1; AU2001280611A1; WO2002006322A3; US20020099193A1; US7626078B2; US20060026707A1; US20020170085A1; US20020120125A1; AU2002222935A1; AU2001273477A1; WO2002006468A3

Abstract

The present invention relates to polycomb genes and polypeptides isolated from Zea mays.

Description

RELATED APPLICATION INFORMATION

This application claims priority from U.S. Ser. No. 60/218,745 filed on Jul. 17, 2000.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to plant genetic engineering. More specifically, the present invention relates to polycomb nucleic acids cloned from Zea mays L.

BACKGROUND OF THE INVENTION

In eukaryotes, gene expression patterns are regulated in response to developmental and environmental cues. These changes in gene expression patterns are often the result of specific transcriptional regulators. In many cases, this change in gene expression must be stably maintained through many mitotic cell divisions even though the transcriptional regulator that effected the change in expression is only present transiently. The stable maintenance of a transcription state is performed by a set of nonspecific factors. These factors are important in regulating chromatin states and establishing a chromatin “memory” to effectively maintain the proper gene expression patterns. In Drosophila, the Polycomb group, PcG, genes are involved in nonspecific, long-term stabilization of transcriptional repression. Recently, homologs of some of the polycomb group genes have been shown to affect developmental gene regulation in other species.
There are at least thirteen PcG proteins in Drosophila. Mutations in any of the thirteen identified PcG genes can lead to lethality during early development (See, Simon, J., Current Opinion in Cell Biology, 7(3):376-85 (1995); Pirrotta, V., Curr. Opin. Gen. Dev., 7(2):249-58 (1997); Pirrotta, V., Cell, 93(3):333-6 (1998)). The cause of this lethality is the failure to maintain transcriptional repression of homeotic genes of the Antennopedia/bithorax complex. The expression pattern of these homeotic genes is controlled in the embryo by activators and repressors that define body segments. During gastrulation, these specific factors are no longer present and PcG protein complexes stabilize a silenced state at genes repressed by the specific factors. Importantly, PcG complexes silence different targets in different cell lineages. This indicates that PcG complexes are able to silence based on factors such as transcription state and not just on sequence. An antagonistic set of factors which maintain the active transcriptional state, the trithorax group, also exist in Drosophila.
In addition to playing a role in developmentally regulated repression of gene expression, the PcG proteins are also involved in maintaining a silenced state at other loci. When high copy numbers (>3) of a white-Adh transgene are introduced into the Drosophila genome the level of white-Adh expression becomes reduced via cosuppression (Pal-Bhadra et al., Cell, 90:479-490 (1997)). In addition to reductions in the expression of the transgenes, the expression of the endogenous Adh gene is reduced as well. This cosuppression is relieved by mutations in polycomb (Pc) or polycomblike (pcl). The cosuppression is based on a homology sensing mechanism that leads to repression via PcG proteins (Pal-Bhadra et al., Cell, 99:35-46 (1999)). The PcG protein, enhancer of zeste, E(z), is required for trans-silencing of P-elements (Roche et al., Genetics, 149(4): 1839-55 (1998)). Increased expression of E(z) or the human homolog (EZH2) results in enhancing position effect variegation (PEV) of a heterochromatin associated white locus (Laible et al., EMBO J., 16(11) 3219-32 (1997)). The EZH2 gene was also able to restore telomere mediated gene repression in S. cerevisiae (Laible et al., EMBO J, 16(11) 3219-32 (1997)). These studies suggest that the PcG proteins can play a role in epigenetic inactivation of gene expression distinct from the role of developmental regulation.
Many of the domains present in the PcG proteins that have been cloned are implicated in protein-protein interactions. The esc and E(z) proteins have been shown to interact with each other in a yeast two hybrid system and through in vitro binding assays (Jones et al., Cell Biol., 18(5):2825-34 (1998)). Homotypic and heterotypic interactions based on the SPM domain have been documented for Sex combs on midleg (Scm) and ph (Bornemann et al., Development, 122(5):1621-30 (1996); Peterson et al., Mol. Cell Biol., 17(11):6683-92 (1997)). The Xenopus Pc homolog, Xpc, forms complexes with itself and Bmi-1 (a psc homolog) (Reijnen et al., Mech. Dev., 53(1):35-46 (1995)). In other yeast two-hybrid screens, ph interacts with itself and with Psc, and Psc interacts with Pc (Pirotta, V., Curr. Opin. Gen. Dev., 7(2):249-58 (1997)). These results indicate the presence of a large complex formed by PcG proteins that is formed based on multiple protein-protein interactions among various PcG members.
Recent evidence suggests that PcG proteins actually form two distinct complexes. One complex contains E(z) and esc which have been found to directly interact (van Lohuizen et al., Mol. Cell Biol., 18(6):3572-9 (1998); Jones et al., Mol. Cell Biol., 18(5):2825-34 (1998), Sewalt et al., Mol. Cell Biol., 18(6):3586-95 (1998) Ng et al., Mol. Cell Biol., 20(9):3069-78 (2000)). The second complex is the PRC1 complex (which includes Pc/Ph/Scm/Psc).
Homologs from PcG proteins have been characterized in a number of species. Vertebrates appear to contain the most homologs of PcG proteins (Simon, Current Opinion in Cell Biology, 7(3):376-85 (1995)). Homologs of psc, Pc, ph, E(z) and esc have been cloned in mammals. The role of PcG proteins in mammals is believed to be very similar to the role in Drosophila.
While many of the domains present in PcG proteins are found in yeast proteins, no PcG homologs are present in the S. cerevisiae genome. In C. elegans and Arabidopsis, homologs of two PcG proteins, E(z) and esc are found. A SET domain and a cys-rich region are found in E(z) (Carrington et al., Development, 122(12):4073-83 (1996); Jones et al., Genetics, 126(1):185-99 (1990); Jones, R S, et al., Mol. Cell. Biol., 13(10):6357-66 (1993)). The esc proteins contain a series of seven WD-40 repeats (Gutjahr et al., EMBO J., 14(17):4296-306 (1995); Simon et al., Mech. Devt., 53(2):197-208 (1995)).
The E(z) and esc homologs (maternal effect sterile-2 (mes-2) and maternal effect sterile-6 (mes-6)) from C. elegans were identified in a screen for maternal-effect mutations that result in sterile offspring (Holdeman et al., Development, 125(13):2457-67 (1998), Korf et al., Development, 125(13):2469-78 (1998)). The mes-2 and mes-6 genes are implicated as maternal genes required for germline immortality. Both mes-2 and mes-6 are localized to the nucleus of all embryonic cells and the nuclei of germline cells in larvae and adults. This localization is dependent upon each other and another protein, mes-3 (Holdeman et al., Development, 125(13):2457-67 (1998), Korf et al., Development, 125(13):2469-78 (1998)). Transgene arrays in the C. elegans genome are frequently silenced in germline cells (Kelly et al., Development, 125(13):2451-6 (1998)). Mutations in mes-2 and mes-6 completely alleviate silencing of transgenes in the germline cells (Kelly et al., Development, 125(13):2451-6 (1998). These results suggest that the PcG proteins of C. elegans, mes-2 and mes-6 are involved in transcriptional repression specifically in the germline cells. It is likely that mes-2 and mes-6 repress transcription of genes that would lead to a differentiated state.
Arabidopsis also contains homologs of E(z) and esc (Goodrich et al., Nature, 386(6620):44-51 (1997)), Grossniklaus et al., Science, 280(5362):446-50 (1998); Ohad et al., Plant Cell, 11(3):407-16 (1999)). Arabidopsis contains three E(z)-like genes, curly leaf (clf), Medea (Mea) and E(z)-likeA1 (EZA1) and one esc homolog, fertilization-independent endosperm (FIE1).
Clf mutants display curled leaves, altered maturation times and partial homeotic transformations of floral tissues (Goodrich et al., Nature, 386(6620):44-51 (1997)). Ectopic expression is also observed for the hometoic genes Agamous (AG) and Apetela3 (AP3). These genes are specifically expressed in floral tissues where clf mRNA is also present. This indicates that, similar to the Drosophila PcG proteins, the presence of CLF protein is not sufficient to repress AG and AP3 transcription but requires targeting factors (Goodrich et al., Nature, 386(6620):44-51 (1997)). The homeotic genes AG and AP3 are also ectopicly expressed in Arabidopsis plants with reduced methylation levels (Finnegan et al., Proc. Natl. Acad. Sci. USA, 93(16):8449-8454 (1996)).
Medea was identified in a screen for Arabidopsis gametophyte lethal mutations (Grossniklaus et al., Science, 280(5362):446-50 (1998); Chaudhury et al., Proc. Natl. Acad. Sci., USA, 94(8):4223-8 (1997); Luo et al., Proc. Natl. Acad. Sci. USA, 96(1):296-301 (1999)). A plant heterozygous for mea mutations will produce 50% aborted seeds that collapse and do not germinate. Subsequently it has been found that MEA exhibits an imprinted pattern of gene expression (Kinoshita et al., Plant Cell, 11(10): 1945-52 (1999)); Vielle-Calzada et al., Genes Dev., 13 (22): 2971-82 (1999)). The maternal copy of Medea is expressed while the paternal copy is not. Medea mutants will allow endosperm development to occur in the absence of fertilization (Kiyosue et al., Proc. Natl. Acad. Sci. USA, 96(7):4186-91 (1999)). These results indicate that maternal expression of Medea is required to repress endosperm development. Due to the early lethality of Medea mutants, roles for Medea later in plant development have not been determined. A third E(z)-like gene, EZA1 is present in the Arabidopsis genome (Preuss, D., Plant Cell., 11(5):765-8 (1999)). Presently, the function of EZA1 is unknown.
Mutations in the Arabidopsis esc-like gene, FIE, have phenotypes similar to Medea. A female gametophyte with a FIE mutant allele will undergo replication of the central cell nucleus and endosperm development without a fertilization event (Ohad et al., Plant Cell, 11(3):407-16 (1999)). This indicates that FIE is critical in the repression of endosperm development. As with Medea, due to the early lethality of FIE mutants, the role of FIE in later developmental events has not been determined.
The similar phenotypes of FIE and mea mutants suggests that these two genes may interact functionally like E(z) and esc homologs in other organisms.
SUMMARY OF THE INVENTION
In one embodiment, the present invention relates to an isolated and purified nucleic acid comprising a polynucleotide selected from the group consisting of SEQ ID NO:1, SEQ ID NO:3 and conservatively modified and polymorphic variants thereof. In addition, the present invention relates to an isolated and purified nucleic acid comprising a polynucleotide having at least 60%, 70%, 80%, 90%, or 95% identity to a polynucleotide selected from the group consisting of SEQ ID NO:1 and SEQ ID NO:3.
In yet another embodiment, the present invention relates to an isolated and purified polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4 and conservatively modified variants thereof. In addition, the present invention relates to an isolated and purified polypeptide comprising an amino acid sequence having at least 60%, 70%, 80% or 95% identity to an amino acid sequence selected from the group consisting of: SEQ ID 20 NO:2 and SEQ ID NO:4.
In yet a further embodiment, the present invention relates to an expression cassette containing a promoter sequence operably linked to an isolated and purified nucleic acid comprising a polynucleotide selected from the group consisting of SEQ ID NO:1, SEQ ID NO:3 and conservatively modified and polymorphic variants thereof. Preferably, the expression cassette also contains a polyadenylation signal which is operably linked to the previously described nucleic acid. Examples of promoters which can be used in the expression cassette include constitutive and tissue specific promoters.
In yet another embodiment, the present invention relates to a bacterial cell containing the hereinbefore described expression cassette. The bacterial cell can be an Agrobacterium tumefaciens cell or an Agrobacterium rhizogenes cell.
In still yet another embodiment, the present invention relates to a plant cell transformed with the hereinbefore described expression cassette, a transformed plant containing such a plant cell, and to seed obtained from such a transformed plant. The plant cell, transformed plant and seed can be from Zea mays L.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the Mez1 polynucleotide and amino acid sequences. FIG. 1A shows that the polynucleotide sequence of the Mez1 cDNA is 3180 base pairs (bp). A solid underline indicates that the putative start codon and the first in-frame stop codon is indicated with a wavy underline. FIG. 1B shows the 931 amino acid Mez1 protein.
FIG. 2 shows the Mez2 polynucleotide and amino acid sequences. FIG. 1A shows that the polynucleotide sequence of the Mez2 cDNA is 3030 bp. The putative start codon is indicated by a solid underline while the stop codon is indicated by a wavy underline. The location of several introns is indicated by open arrowheads above the sequence. These introns were identified by sequencing of PCR products amplified from genomic DNA corresponding to bp2032 to bp2587 of the cDNA. The location of the four Mu insertions are indicated by black arrowheads below the sequence. The Mez2-Mu1 allele contains a Mu element inserted into intron 1. The location of the Mez2-Mu2, Mez2-Mu3 and Mez2-Mu4 Mu insertions are all located in exons. The nucleotides that flank the sequence that is removed by alternative splicing are indicated by a double underline. FIG. 1B shows the 893 amino acid Mez2 protein.
FIG. 3 shows the alignment of Mez1 and Mez2. The Mez1 and Mez2 protein sequences were aligned using ClustalW (http://dot.imgen.bcm.tmc.edu:9331/multi-align/Options/clustalw.html). These alignments were then processed using Boxshade to highlight identical residues in black and similar residues in gray. The two proteins are 42% identical and 56% similar over their entire length.
FIG. 4 shows the alignment of E(z) sequences. The sequences of Drosophila E(z) (AAC46462), human EZH1 (AAC50778), human EZH2 (AAC51520), C. elegans MES-2 (AAC27124), Arabidopsis CLF (AAC23781), Arabidopsis MEA (AAC39446), Arabidopsis EZA1 (AAD09108), Mez1 and Mez2 were aligned using ClustalW (http://dot.imgen.bcm.tmc.edu:9331/multi-align/Options/clustalw.html). The alignments were colored using Boxshade to highlight identical residues in black and conserved residues in gray. The location of a putative bipartite nuclear localization signal in the plant sequences is indicated by *'s above the alignments. # symbols are located above the cysteine-rich region. The N-terminal SET domain is indicated by + symbols above the alignment. A putative SANT DNA binding domain is shown with ˆ symbols. $ symbols are placed above all acidic amino acid residues in an acidic region near the C-terminus. A region of high conservation in the plant sequences only containing a CRRC sequence is shown with x's above the alignment. The region between the CRRC domain and the nuclear localization signal is very divergent.
FIG. 5 shows schematic diagrams of E(z)-like proteins. E(z)-like proteins from plants and the Drosophila E(z) are represented by rectangles with the N-terminus located on the left for each protein. The location of the EZD1, EZD2, SANT, Cys-rich, and SET domains are indicated by shading.
FIG. 6 shows the alignment of the SET domains from Drosophila E(z) (AAC46462), human EZH1 (AAC50778), human EZH2 (AAC51520), C. elegans mes-2 (AAC27124), Arabidopsis clf(AAC23781), Arabidopsis Mea (AAC39446), Arabidopsis EZA1 (AAD09108), Mez1 and Mez2 using ClustalW (region indicated by [ ] in FIG. 4). The Arabidopsis sequences are underlined. The maize sequences are in bold text. Bootstrap values are indicated by the numbers at nodes in the tree. Only nodes with bootstrap values greater than 50% are shown.
FIG. 7 shows that the Mez2 transcript is alternatively spliced in different tissues. Three predominant transcripts are found, the full length transcript and two smaller transcripts. The two smaller transcripts were isolated and sequenced to reveal the difference between the transcripts. The MEZ2^a.s.1transcript is lacking base pairs 1016 to 1676 and translation of this sequence results in a truncated protein of 341 amino acids lacking the conserved C-terminal domains. The MEZ2^a.s.2transcript is lacking base pairs 1016 to 1827 and translation of this sequence results in a 624 amino acid protein that lacks the large variable region from the middle of the MEZ2 protein. The MEZ2^a.s.2transcript has been found as the predominant transcript in embryo and endosperm tissues.
FIG. 8 shows the results of a RT-PCR analysis of Mez1 and Mez2 expression pattern. In FIG. 8A, the primer pair Mez1F1-Mez1R1 was used to amplify 2 ng of cDNA from various maize tissues. The PCR products were then separated on a 1% agarose gel stained with ethidium bromide. The arrow indicates the expected size of the PCR product. In FIG. 8B, the primer pair Mez2F4-Mez2R8 was used to amplify 2 ng of cDNA from various maize tissues. The arrows indicate the expected size of Mez2, Mez2^as1and Mez2^as2isoforms. In FIG. 8C, ubiquitin primers were used to amplify 0.2 ng of cDNA from the same maize tissues as a control. The pollen cDNA did not allow the amplification of significant amounts of product indicating that the results using this cDNA are questionable.

DEFINITIONS

Units, prefixes, and symbols can be denoted in the SI accepted form. Numeric ranges are inclusive of the numbers defining the range. Unless otherwise indicated, nucleic acids are written left to right in 5′ to 3′ orientation, respectively. The headings provided herein are not limitations of the various aspects or embodiments of the invention which can be had by reference to the specification as a whole. Accordingly, the terms defined immediately below are more fully defined by reference to the specification as a whole.
As used herein, the terms “amplify” or “amplified” as used interchangeably herein refer to the construction of multiple copies of a nucleic acid sequence or multiple copies complementary to the nucleic acid sequence using at least one of the nucleic acid sequences as a template. Amplification methods include the polymerase chain reaction (hereinafter “PCR”; described in U.S. Pat. Nos. 4,683,195 and 4,683,202), the ligase chain reaction (hereinafter “LCR”; described in EP-A-320,308 and EP-A-439,182), the transcription-based amplification system (hereinafter “TAS”), nucleic acid sequence based amplification (hereinafter “NASBA”, Cangene, Mississauga, Ontario; described in Proc. Natl. Acad. Sci., USA, 87:1874-1878 (1990); Nature, 350 (No. 6313): 91-92 (1991)), Q-Beta Replicase systems, and strand displacement amplification (hereinafter “SDA”). The product of amplification is referred to as an amplicon.
As used herein, the term “antibody” includes reference to an immunoglobulin molecule obtained by in vitro or in vivo generation of a humoral response, and includes both polyclonal and monoclonal antibodies. The term also includes genetically engineered forms such as chimeric antibodies (e.g., humanized murine antibodies), heteroconjugate antibodies (e.g., bispecific antibodies), and recombinant single chain Fc fragments (hereinafter “scFc”). The term “antibody” also includes antigen binding forms of antibodies (e.g., Fab¹, F(ab¹)₂, Fab, Fc, and, inverted IgG (See, Pierce Catalog and Handbook, (1994-1995) Pierce Chemical Co., Rockford, Ill.)). An antibody immunologically reactive with a particular antigen can be generated in vivo or by recombinant methods such as by the selection of libraries of recombinant antibodies in phage or similar vectors (See, e.g. Huse et al., Science, 246:1275-1281 (1989); and Ward, et al., Nature, 341:544-546 (1989); and Vaughan et al., Nature Biotechnology, 14:309-314 (1996)).
As used herein, the term “antisense RNA” means an RNA sequence which is complementary to a sequence of bases in the mRNA in question in the sense that each base (or the majority of bases) in the antisense sequence (read in the 3′ to 5′ sense) is capable of pairing with the corresponding base (G with C, A with U) in the mRNA sequence read in the 5′ to 3′ sense.
As used herein, the term “conservatively modified variants” applies to both amino acid and nucleic acid sequences. With respect to particular nucleic acid sequences, conservatively modified variants refers to those nucleic acids which encode identical or conservatively modified variants of the amino acid sequences. Because of the degeneracy of the genetic code, a large number of functionally identical nucleic acids encode any given protein. For example, the codons GCA, GCC, GCG and GCU all encode the amino acid alanine. Thereupon, at every position where an alanine is specified by a codon, the codon can be altered to any of the corresponding codons described without altering the encoded polypeptide. Such nucleic acid variations are “silent variations” and represent one species of conservatively modified variation. Every nucleic acid sequence herein which encodes a polypeptide also describes every possible “silent variation” of the nucleic acid. It is known by persons skilled in the art that each codon in a nucleic acid (except AUG, which is the only codon for the amino acid, methionine; and UGG, which is the only codon for the amino acid tryptophan) can be modified to yield a functionally identical molecule. Therefore, each silent variation of a nucleic acid which encodes a polypeptide of the present invention is implicit in each described polypeptide sequence.
With respect to amino acid sequences, persons skilled in the art will recognize that individual substitutions, deletions or additions to a nucleic acid, peptide, polypeptide, or protein sequence which alters, adds or deletes a single amino acid or a small percentage of amino acids in the encoded sequence is a “conservatively modified variant” where the alteration results in the substitution of an amino acid with a chemically similar amino acid. Conservative substitution tables providing functionally similar amino acids are well known in the art.
The following six groups each contain amino acids that are conservative substitutions for one another:

1) Alanine (A), Serine (S), Threonine (T);
2) Aspartic acid (D), Glutamic acid (E);
3) Asparagine (N), Glutamine (Q);
4) Arginine (R), Lysine (K);
5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V); and
6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W). See also, Creighton (1984) Proteins W. H. Freeman and Company.

As used herein, the term “constitutive promoter” refers to a promoter which is active under most environmental conditions.
As used herein, the term “full length” when used in connection with a specified polynucleotide or encoded protein refers to having the entire amino acid sequence of, a native (i.e. non-synthetic), endogenous, catalytically active form of the specified protein. Methods for determine whether a sequence is full length are well known in the art. Examples of such methods which can be used include Northern or Western blots, primer extension, etc. Additionally, comparison to known full-length homologous sequences can also be used to identify full length sequences of the present invention.
As used herein, the term “heterologous” when used to describe nucleic acids or polypeptides refers to nucleic acids or polypeptides that originate from a foreign species, or, if from the same species, are substantially modified from their original form. For example, a promoter operably linked to a heterologous structural gene is from a species different from that from which the structural gene was derived, or, if from the same species, is different from any naturally occurring allelic variants.
The term “immunologically reactive conditions” as used herein, includes reference to conditions which allow an antibody, generated to a particular epitope of an antigen, to bind to that epitope to a detectably greater degree than the antibody binds to substantially all other epitopes, generally at least two times above background binding, preferably at least five times above background. Immunologically reactive conditions are dependent upon the format of the antibody binding reaction and typically are those utilized in immunoassay protocols.
As used herein, the term “inducible promoter” refers to a promoter which is under environmental control. Examples of environmental conditions that may effect transcription by inducible promoters include anaerobic conditions or the presence of light.
As used herein, the term “isolated” includes reference to material which is substantially or essentially free from components which normally accompany or interact with it as found in its naturally occurring environment. The isolated material optionally comprises material not found with the material in its natural environment. However, if the material is in its natural environment, the material has been synthetically, (e.g. non-naturally) altered by deliberate human intervention to a composition and/or placed in a locus in a cell (e.g., genome or subcellular organelle) not native to a material found in that environment.
Two polynucleotides or polypeptides are said to be “identical” if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned (either manually for visual inspection or via the use of a computer algorithm or program) for maximum correspondence as described below. The terms “identical” or “percent identity” when used in the context of two or more polynucleotide or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence over a comparison window, as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. With respect to polypeptides or proteins having a “percent identity” or “percentage of sequence identity” one skilled in the art would recognize that residue positions that are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues possessing similar chemical and/or physical properties such as charge or hydrophobicity and therefore do not change the functional properties of the molecule. Where sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well-known to persons skilled in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity.
As used herein, the term “comparison window” includes reference to a contiguous and specified segment of a polynucleotide sequence, wherein the polynucleotide sequence may be compared to a reference sequence and wherein the portion of the polynucleotide sequence in the comparison window may comprise additions or deletions (e.g., gaps) compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Generally, the comparison window is at least 20 contiguous nucleotides in length, and can be 30, 40, 50, 100, or even longer. Persons skilled in the art will recognize that to avoid a high similarity to a reference sequence due to inclusion of gaps in the polynucleotide sequence a gap penalty is typically introduced and is subtracted from the number of matches.
The alignment of polynucleotide and/or polypeptide sequences for the purposes of determine sequence identity and similarity can be by either manual alignment and visual inspection or via the use of some type of computer program or algorithm. In fact, a number of computer programs are available which can be used to align polynucleotide and/or polypeptide sequences are known in the art. For example, the programs available in the Wisconsin Sequence Analysis Package, Version 9 (available from the Genetics Computer Group, Madison, Wis., 52711), such as GAP, BESTFIT, FASTA and TFASTA. For example, the GAP program is capable of calculating both the identity and similarity between two polynucleotide or two polypeptide sequences. Specifically, the GAP program uses the homology alignment algorithm of Needleman and Wunsch (J. Mol. Biol., 48:443-453 (1970)). Another example of a useful computer program is PILEUP. PILEUP creates a multiple sequence alignment from a group of related sequences using progressive, pairwise alignments to show relationship and percent sequence identity. It also plots a tree or dendogram showing the clustering relationships used to create the alignment. PILEUP uses a simplification of the progressive alignment method of Feng & Doolittle, J. Mol. Evol., 35:351-360 (1987). Yet another example of a useful computer program that can be used for determine percent sequence identity and sequence similarity is the BLAST algorithm (Altsuchul et al., J. Mol. Biol., 215:403-410 (1990)). The software for performing BLAST analysis is publicly available through the National Center for Biotechnology Information (http:\\www.ncbi.nlm.nih.gov/).
With respect to polynucleotide sequences, the term “substantial identity” means that a polynucleotide comprises a sequence that has at least 60% sequence identity, preferably at least 70% sequence identity, more preferably at least 80% sequence identity, even more preferably 90% sequence identity and most preferably at least 90% sequence identity, compared to a reference sequence using one of the alignment programs described herein conducted according to standard parameters. One skilled in the art will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning and the like. Substantial identity of amino acid sequences for these purposes normally means sequence identity of at least 60%, more preferably at least 70%, 80%, 90% identity, and most preferably at least 95% identity.
Polynucleotide sequences can also be considered to be substantially identical if two molecules hybridize to each other under stringent conditions. However, polynucleotides which do not hybridize to each other under stringent conditions are still substantially identical if the polypeptides which they encode are substantially identical. This can occur when a copy of a polynucleotide is created using the maximum codon degeneracy permitted by the genetic code. One indication that two polynucleotide sequences are substantially identical if the polypeptide encoded by the first nucleic acid encodes is immunologically cross reactive with the polypeptide encoded by the second polynucleotide.
With peptides, the term “substantial identity” as used herein means that a peptide comprises a sequence having at least 60% sequence identity to a reference sequence, preferably 70% sequence identity, more preferably 80% sequence identity, even more preferably 90% sequence identity, and most preferably at least 95% sequence identity to the reference sequence over a specified comparison window. Preferably, optimal alignment is conducted using the homology alignment algorithm (GAP program discussed previously) of Needleman and Wunsch, J. Mol. Biol., 48: 443-453 (1990). An indication that two peptide sequences are substantially identical is that one peptide is immunologically reactive with antibodies raised against the second peptide. Thereupon, a peptide is substantially identical to a second peptide where the two peptides differ only by a conservative substitution. Peptides which are “substantially similar” share sequences as described above except that any residue positions which are not identical differ only by conservative amino acid changes.
As used herein, the term “Mez1 gene” refers to a gene of the present invention, specifically, the heterologous genomic form of a full length Mez1 polynucleotide.
As used herein, the term “Mez1 nucleic acid” refers to a nucleic acid of the present invention, specifically, a nucleic acid comprising a polynucleotide of the present invention encoding a Mez1 polypeptide (hereinafter “Mez1 polynucleotide”). An example of a Mez1 polynucleotide (cDNA) is shown in SEQ ID NO:1.
As used herein, the terms “Mez1 polypeptide”, “Mez1 peptide” or “Mez1 protein” as used interchangeable herein refer to a polypeptide shown in SEQ ID NO:2. The term also includes fragments, variants, homologs, alleles or precursors (e.g., preproproteins or proproteins) thereof.
As used herein, the term “Mez2 gene” refers to a gene of the present invention, specifically, the heterologous genomic form of a full length Mez2 polynucleotide.
As used herein, the term “Mez2 nucleic acid” refers to a nucleic acid of the present invention, specifically, a nucleic acid comprising a polynucleotide of the present invention encoding a Mez2 polypeptide (hereinafter a “Mez2 polynucleotide”). An example of a Mez2 polynucleotide (cDNA) is shown in SEQ ID NO:3.
As used herein, the terms “Mez2 polypeptide”, “Mez2 peptide” or “Mez2 protein” as used interchangeably herein refer to a polypeptide shown in SEQ ID NO:4. The term also includes fragments, variants, homologs, alleles or precursors (e.g., preproproteins or proproteins) thereof. A “Mez2 protein” is a protein of the present invention and comprises a Mez2 polypeptide.
As used herein, the term “nucleic acid” refers to a deoxyribonucleotide or ribonucleotide polymer in either single- or double-stranded form, and unless otherwise limited, encompasses known analogues having the essential nature of natural nucleotides in that they hybridize to single-stranded nucleic acids in a manner similar to naturally occurring nucleotides (e.g., peptide nucleic acids).
As used herein, the term “nucleotide(s)” refers to a macromolecule containing a sugar (either a ribose or deoxyribose), a phosphate group and a nitrogenous base.
As used herein, the term “operably linked” includes reference to a functional linkage between a promoter and a second sequence, wherein the promoter sequence initiates and mediates transcription of the DNA sequence corresponding to the second sequence. Generally, operably linked means that the polynucleotide sequences being linked are contiguous and, where necessary to joint two protein coding regions, contiguous and in the same reading frame.
As used herein, the term “plant” includes reference to whole plants, plant organs (e.g., leaves, stems, flowers, roots, etc.), seeds and plant cells and progeny of the same. Plant cell, as used herein, includes, but is not limited to, suspension cultures, embryos, meristematic regions, callus tissue, shoots, gametophytes, sporophytes, pollen and microspores. The class of plants which can be used in the methods of the present invention are generally as broad as the class of higher plants amenable to transformation techniques, including angiosperms (monocotyledonous and dicotyledonous plants) as well as gymnosperms (e.g. Coniferophyta (conifers, Cycadophyta (cycads), Ginkgophyta (maidenhair tree) and Gnetophyta (gnetophytes)). The term “plant” as used herein also includes plants of a variety of ploidy levels, such as polyploid, diploid, haploid and hemizygous.
As used herein, the term “plant promoter” refers to a promoter capable of initiating transcription in plant cells.
As used herein, the term “polymorphic variant” in connection with a polynucleotide sequence refers to a variation in the polynucleotide sequence of a particular gene between individuals of a given species. Polymorphic variants may also encompass “single nucleotide polymorphisms” (SNPs) in which the polynucleotide sequence varies by one base. The presence of SNPs may be indicative of a certain population for a disease state or propensity for a disease state.
As used herein, the term “polynucleotide” refers to a deoxyribopolynucleotide, ribopolynucleotide, or analogs thereof that have the essential nature of a natural ribonucleotide in that they hybridize, under stringent hybridization conditions, to substantially the same nucleotide sequence as naturally occurring nucleotides and/or allow translation into the same amino acid(s) as the naturally occurring nucleotide(s). A polynucleotide can be full length or a subsequence of a native or heterologous structural or regulatory gene. Unless otherwise indicated, the term includes reference to the specified sequence as well as the complementary sequence thereof. Thereupon, DNAs or RNAs with backbones modified for stability or for other reasons are “polynucleotides” as that term is intended herein. Moreover, DNAs or RNAs comprising unusual bases, such as inosine, or modified bases, such as tritylated bases, to name just two examples, are polynucleotides as the term is used herein. As used herein, the term polynucleotide includes such chemically, enzymatically or metabolically modified forms of polynucleotides, as well as the chemical forms of DNA and RNA characteristic of viruses and cells, including, but not limited to, simple and complex cells.
As used herein, the terms “polypeptide”, “peptide” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residue is an artificial chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers. The essential nature of such analogues of naturally occurring amino acids is that, when incorporated into a protein, that protein is specifically reactive to antibodies elicited to the same protein but consisting entirely of naturally occurring amino acids. The terms “polypeptide”, “peptide” and “protein” are also inclusive of modifications including, but not limited to, glycosylation, lipid attachment, sulfation, gamma-carboxylation of glutamic acid residues, hydroxylation and ADP-ribosylation.
As used herein, the term “promoter” refers to a region of DNA upstream from the start of transcription and involved in recognition and binding of RNA polymerase and other proteins to initiate transcription. A promoter can optionally include distal enhancers or repressor elements which can be located several thousand base pairs from the start site of transcription.
As used herein, the term “recombinant” includes reference to a cell, or nucleic acid, or vector, that has been modified by the introduction of a heterologous nucleic acid or the alteration of a native nucleic acid to a form not native to that cell, or that the cell is derived from a cell so modified. For example, recombinant cells express genes that are not found within the native (non-recombinant) form of the cell or express native genes that are otherwise abnormally expressed, under expressed or not expressed at all.
As used herein, the term “recombinant expression cassette” is a nucleic acid construct, generated recombinantly or synthetically, with a series of specified nucleic acid elements which permit transcription of a particular nucleic acid in a target cell. The expression vector can be part of a plasmid, virus, or nucleic acid fragment. Typically, the recombinant expression cassette portion of the expression vector includes a nucleic acid to be transcribed, and a promoter.
As used herein, the terms “residue” or “amino acid” or “amino acid residue” are used interchangeably herein to refer to an amino acid that is incorporated into a protein, polypeptide or peptide. The amino acid may be a naturally occurring amino acid, and unless otherwise limited, may encompass known analogs of natural amino acids that can function in a similar manner as naturally occurring amino acids.
As used herein, the term “selective hybridization” or “selectively hybridizes” are used interchangeably herein includes reference to hybridization, under stringent hybridization conditions, of a nucleic acid sequence to a specified nucleic acid target sequence to a detectably greater degree (e.g., at least 2-fold over background) than its hybridization to non-target nucleic acid sequences and to the substantial exclusion of non-target nucleic acids. Selectively hybridizing sequences typically have about at least 80% sequence identity, preferably 90% sequence identity, and most preferably 100% sequence identity (e.g., complementary) with each other.
As used herein, the term, “specifically binds” includes reference to the preferential association of a ligand, in whole or part, with a particular target molecule (i.e., “binding partner” or “binding moiety” relative to compositions lacking that target molecule). It is, of course, recognized that a certain degree of non-specific interaction may occur between a ligand and a non-target molecule. Nevertheless, specific binding, may be distinguished as mediated through specific recognition of the target molecule. Typically, specific binding results in a much stronger association between the ligand and the target molecule than between the ligand and non-target molecule. Specific binding by an antibody to a protein under such conditions requires an antibody that is selected for its specificity for a particular protein. The affinity constant of the antibody binding site for its cognate monovalent antigen is at least 10⁷, usually at least 10⁹, more preferably at least 10¹⁰, and most preferably at least 10¹¹liters/mole.
As used herein, the terms “stringent hybridization” conditions or “stringent conditions” refers to conditions under which a probe will hybridize to its target subsequence, typically in a complex mixture of nucleic acid, but to no other sequences. Stringent conditions are sequence dependent and are different under different environmental parameters. An extensive guide to hybridization of nucleic acids is found in Tijssen (1993) Laboratory Techniques in Biochemistry and Molecular Biology-Hybridization with Nucleic Acid Probes Part 1, Chapter 2 “Overview of Principles of Hybridization and the Strategy of Nucleic Acid Probe Assays” Elsevier, N.Y. Generally, highly stringent conditions are selected to be about 5° C. -10° C. lower than the thermal melting point (T_m) for the specific sequence at a defined ionic strength and pH. The T_mis the temperature (under defined ionic strength and pH and nucleic concentration) at which 50% of the target sequence hybridizes to a perfectly matched probe. Stringent conditions are those in which the salt concentration is less than about 1.0M sodium ion, typically about 0.01 to 1.0M sodium ion concentration (or other salts) at a pH of 7.0 to 8.3 and at a temperature of at least about 30° C. for short probes (such as those having a length between about 10 to 50 nucleotides) and at least about 60° C. for long probes (such as those having a length greater than 50 nucleotides). In contrast, low stringency conditions are at about 15-30° C. below the T_m. Stringent hybridization conditions are sequence-dependent and will be different in different circumstances. Longer sequences hybridize at higher temperatures.
As used herein, the term “tissue-specific promoter” includes reference to a promoter in which expression of an operably linked gene is limited to a particular tissue or tissues.
As used herein, the term “transgenic plant” includes reference to a plant modified by introduction of a heterologous polynucleotide. Generally, the heterologous polynucleotide is a Mez1 or Mez2 structural or regulatory gene or subsequences thereof.

Sequence Listings

The present application also contains a sequence listing that contains twenty (20) sequences. The sequence listing contains nucleotide sequences and amino acid sequences. For the nucleotide sequences, the base pairs are represented by the following base codes:



	Symbol	Meaning

	A	A; adenine
	C	C; cytosine
	G	G; guanine
	T	T; thymine
	U	U; uracil
	M	A or C
	R	A or G
	W	A or T/U
	S	C or G
	Y	C or T/U
	K	G or T/U
	V	A or C or G; not T/U
	H	A or C or T/U; not G
	D	A or G or T/U; not C
	B	C or G or T/U; not A
	N	(A or C or G or T/U)

The amino acids shown in the application are in the L-form and are represented by the following amino acid-three letter abbreviations:



	Abbreviation	Amino acid name

	Ala	L-Alanine
	Arg	L-Arginine
	Asn	L-Asparagine
	Asp	L-Aspartic Acid
	Asx	L-Aspartic Acid or Asparagine
	Cys	L-Cysteine
	Glu	L-Glutamic Acid
	Gln	L-Glutamine
	Glx	L-Glutamine or Glutamic Acid
	Gly	L-Glycine
	His	L-Histidine
	Ile	L-Isoleucine
	Leu	L-Leucine
	Lys	L-Lysine
	Met	L-Methionine
	Phe	L-Phenylalanine
	Pro	L-Proline
	Ser	L-Serine
	Thr	L-Threonine
	Trp	L-Tryptophan
	Tyr	L-Tyrosine
	Val	L-Valine
	Xaa	L-Unknown or other

Introduction

The present invention is based, at least in part, on the discovery and cloning of two (2) PcG genes from Zea mays L. (maize) termed the Mez1 gene and the Mez2 gene. The protein encoded by the Mez1 gene has been mapped to chromosome 6 (bin 6.01-6.02) and the protein for the Mez2 gene has been mapped to chromosome 9 (bin 9.04).
The present invention is applicable to a broad range of types of plants, including, but not limited to, Zea mays L., Oryza sativa, Secale cereale, Triticum aestivum, Daucus carota, Brassica oleracea, Cucumis melo, Cucumis sativus, Latuca sativa, Solanum tubersoum, Lycopersicon esculentum, Phaseolus vulgaris, and Brassica napus.
Nucleic Acids
In one embodiment, the present invention relates to isolated nucleic acids of DNA, RNA, and analogs and/or chimeras thereof comprising a polynucleotide, wherein said polynucleotide is a Mez1 or Mez2 polynucleotide which encodes a polypeptide of SEQ ID NO:2 (a Mez1 polypeptide) or SEQ ID NO:4 (a Mez2 polypeptide), and conservatively modified variants thereof. It is known in the art that the degeneracy of the genetic code allows for a plurality of polynucleotides to encode for the identical amino acid sequence. These “silent variations”, as they are common referred to, can be used to selectively hybridize and detect polymorphic variants of the polynucleotides of the present invention.
An example of a Mez1 polynucleotide which encodes the Mez1 polypeptide of SEQ ID NO:2 is shown in SEQ ID NO:1. The polynucleotide of SEQ ID NO:1 is 3180 base pairs in length.
An example of a Mez2 polynucleotide which encodes the Mez2 polypeptide of SEQ ID NO:4 is shown in SEQ ID NO:3. The polynucleotide of SEQ ID NO:3 is 3030 base pairs in length.
The Mez2 polynucleotide of SEQ ID NO:3, in addition to encoding for the Mez2 polypeptide, contains two (2) alternative splice sites. These alternative splice sites are referred to herein as Mez2 alternative splice 1 (“Mez2^as1”) (SEQ ID NO:5) and Mez2 alternative splice 2 (“Mez2^as2”) (SEQ ID NO:6). The polynucleotide sequence of Mez2^as1(hereinafter Mez2^aslpolynucleotide”) is identical to the Mez2 polynucleotide of SEQ ID NO:3 except that Mez2^as1polynucleotide is missing a fragment of 659 basepairs in length. Specifically, this deleted fragment corresponds to 1016 to 1676 in the Mez2 polynucleotide of SEQ ID NO:3. The Mez2^as1polynucleotide deletion causes a frameshift and a truncated protein of 341 amino acids which is missing the SANT, nuclear localization signal, cysteine rich region and SET domains (See FIG. 7).
The polynucleotide sequence of Mez2^as2(hereinafter Mez2^as2polynucleotide”) is identical to the Mez2 polynucleotide of SEQ ID NO:3 except that Mez2^as2polynucleotide is missing a fragment of 810 basepairs in length. Specifically, this deleted fragment corresponds to 1016 to 1827 in the Mez2 polynucleotide of SEQ ID NO:3. The Mez2^as2polynucleotide deletion does not result in a frameshift. The deletion in Mez2^as2results in a 624 amino acid protein that is missing the SANT domain (See FIG. 7).
In another embodiment, the present invention also provides isolated of nucleic acids comprising polynucleotides encoding conservatively modified variants of a Mez1 or Mez2 polypeptides of SEQ ID NOS:2 and 4. Such conservatively modified variants can be used for a number of useful purposes, such as, but not limited to, the generation or selection of antibodies immunoreactive to the non-variant polypeptide. Also, in yet another embodiment, the present invention also relates to isolated nucleic acids comprising polynucleotides encoding one or more polymorphic variants of polypeptides/polynucleotides. Polymorphic variants are used to follow the segregation of chromosome regions and are typically used in marker assisted selection methods for crop improvement.
In another embodiment, the present invention relates to the isolation nucleic acids comprising polynucleotides of the present invention which selectively hybridize, under selective hybridization conditions (i.e. stringent hybridization conditions), to the Mez1 or Mez2 polynucleotide. The isolation of such nucleic acids can be accomplished by a nutnber of techniques. For example, oligonucleotide probes based upon the Mez1 and Mez2 polynucleotides described herein can be used to identify, isolate or amplify partial or full length clones in a deposited library (such as a cDNA or genomic DNA library). For example, a cDNA or genomic library can be screened using a probe based upon the sequence of the Mez1 or Mez2 polynucleotides described herein. These probes can be used to hybridize with genomic DNA or cDNA sequences to isolate homologous genes in the same or different plant species.
Alternatively, nucleic acids of interest can be amplified from nucleic acid samples using various amplification techniques known in the art. For example, PCR can be used to amplify the sequences of the Mez1 or Mez2 genes directly from genomic DNA, from cDNA, from genomic libraries or cDNA libraries. PCR and other in vitro amplification methods (such as LCR, etc.) can be used to clone nucleic acid sequences that code for proteins to be expressed, to make nucleic acids for use as probes for detecting the presence of the desired mRNA in samples, for nucleic acid sequencing or for other purposes.
In yet another embodiment, the present invention relates to isolated nucleic acid comprising polynucleotides, wherein the polynucleotides of said nucleic acid have a specified identity at the nucleotide level to the previously described Mez1 or Mez2 polynucleotides. The percentage of identity is at least 60%, preferably 70%, more preferably 80%, even more preferably 90% and most preferably 95%.
In yet another embodiment, the present invention relates to isolated nucleic acids comprising polynucleotides complementary to the previously described Mez1 or Mez2 polynucleotides. One skilled in the art will recognize that complementary sequences will base pair throughout their entire length with the previously described Mez1 or Mez2 polynucleotides (meaning that they have 100% sequence identity over their entire length). Complementary bases associate through hydrogen bonding in double stranded nucleic acids. Base pairs known to be complementary include the following: adenine and thymine, guanine and cytosine and adenine and uracil.
In yet another embodiment, the present invention relates to isolated nucleic acids comprising polynucleotides which comprise at least 15 contiguous bases from the previously described Mez1 or Mez2 polynucleotides. More specifically, the length of the polynucleotides can be from about 15 continguous bases to the length of the Mez1 or Mez polynucleotide from which the polynucleotide is a subsequence of. For example, such polynucleotides can be 15, 35, 55, 75, 95, 100, 200, 400, 500, 750, etc. continguous nucleotides in length from the previously described Mez1 or Mez2 polypeptide. In addition, such subsequences can optionally comprise or lack certain structural characteristics from the Mez1 or Mez2 polynucleotides from which it is derived.
Polypeptides
In one embodiment, the present invention relates to a Mez1 polypeptide of SEQ ID NO:2. The Mez1 polypeptide is 931 amino acids in length, has a molecular weight of about 103.75 kDa and an isoelectric point of 8.91.
In a second embodiment, the present invention relates to a Mez2 polypeptide of is SEQ ID NO:4. The Mez2 polypeptide is 893 amino acids in length, has a molecular weight of about 100.01 kDa and an isoelectric point of 8.47.
The Mez1 and Mez2 polypeptides contain a number of domains. These domains are: EZD1, EZD2, SANT domain, cysteine rich region and SET domain (See, FIG. 5). The EZD1 and EZD2 regions are conserved domains specific to the E(z) family. EZD1 is a highly conserved acidic region of 74 amino acids in the N-terminal region. The EZD 1 domain contains a significant proportion of charged residues (34-39%) with seven more acidic residues than basic residues. The function of this domain is presently not known. The EZD1 is highly conserved between Mez1, Mez2, clf and EZA 1. EZD2 is a small, highly conserved region of 44 amino acids near amino acid 250 of the plant and animal E(z)-like proteins. The EZD2 region is composed primarily of polar or charged residues. There are two (2) regions near the C-terminus of these protein are well conserved among all E(z) proteins (See FIG. 5). These are the cysteine rich region and the SET domain. The Cys-rich region has fiften invariant cysteine residues with a conserved spacing pttem in all E(z) homologs. The spacing of the cystein residues in all E(z) homologs is unique and is different from other Cys-rich zinc finger domains involved in DNA binding. The function of the cysteine rich domain is not known but it is highly conserved among all E(z)-like genes. The SET domain is also highly conserved and is believed to be involved in mediating protein-protein interactions (Cui et al., Nat. Genet., 18:331-337 (1998); Huang et al., J Biol. Chem., 273:15933-15939 (1998)). The SANT binding domain is often invovled in non-specific DNA binding (Aasland, R., et al., Trends Biochem. Sci., 21(3):8-88 (1996)).
In another embodiment, the present invention relates to a pol peptide having a specified percentage of sequence identity with the Mez1 or Mez2 polypeptide of the present invention. The percentage of sequence identity is at least 60%, preferably 70%, more preferably 80%, even more preferably 90% and most preferably 95%.
The present invention also provides antibodies which specifically react with the Mez1 or Mez2 polypeptides of the present invention under immunologically reactive conditions. An antibody immunologically reactive with a particular antigen can be generated in vivo or by recombinant methods such as by selection of libraries of recombinant antibodies in phage or similar vectors.
Many methods of making antibodies are known to persons skilled in the art. A number of immunogens can be used to produce antibodies specifically reactive to the isolated Mez1 or Mez2 polypeptides of the present invention under immunologically reactive conditions. An isolated recombinant, synthetic, or native isolated Mez1 or Mez2 polypeptide of the present invention is the preferred immunogens (antigen) for the production of monoclonal or polyclonal antibodies.
The Mez1 or Mez2 polypeptide can be injected into an animal capable of producing antibodies. Either monoclonal or polyclonal antibodies can be generated for subsequent use in immunoassays to measure the presence and quantity of the Mez1 or Mez2 polypeptide. Methods of producing monoclonal or polyclonal antibodies are known to persons skilled in the art (See, Coligan, Current Protocols in Immunology Wiley/Greene, N.Y. (1991); Harlow and Lane, Antibodies: A Laboratory Manual Cold Spring Harbor Press, NY (1989)); and Goding Monoclonal Antibodies: Principles and Practice (2d ed.) Academic Press, New York, N.Y. (1986)).
The Mez1 or Mez2 polypeptides and antibodies can be labeled by joining, either covalently or non-covalently, a substance which provides for a detectable signal. A wide variety of labels and conjugation techniques are known to persons skilled in the art. Suitable labels include radionucleotides, enzymes, substrates, cofactors, inhibitors, fluorescent moieties, chemiluminescent moieties, magnetic particles, and the like. Patents teaching the use of such labels include U.S. Pat. Nos. 3,817,837; 3,850,752; 3,939,350; 3,996,345; 4,277,437; 4,275,149; and 4,366,241.
The antibodies of the present invention can be used to screen plants for the expression of the Mez1 or Mez2 polypeptides of the present invention. The antibodies of the present invention can also be used for affinity chromatography for the purpose of isolating Mez1 or Mez2 polypeptides.
The present invention further provides Mez1 or Mez2 polypeptides that specifically bind, under immunologically reactive conditions, to an antibody generated against a defined immunogen, such as an immunogen consisting of the Mez1 or Mez2 polypeptides. Immunogens will generally have a length of at least 10 contiguous amino acids from the Mez1 or Mez2 polypeptides of the present invention, respectively.
A variety of immunoassay formats are appropriate for selecting antibodies specifically reactive with a particular protein. For example, solid-phase ELISA immunoassays are routinely used to select monoclonal antibodies specifically reactive with a protein (See Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Publications, New York (1988), for a description of immunoassay formats and conditions that can be used to determine specific reactivity). The antibody may be polyclonal but preferably is monoclonal. Generally, antibodies cross-reactive to Mez1 or Mez2 polypeptides are removed by immunoabsorbtion.
Immunoassays in the competitive binding format are typically used for cross-reactivity determinations. For example, an immunogenic Mez1 or Mez2 polypeptide can be immobilized to a solid support. Polypeptides added to the assay compete with the binding of the antisera to the immobilized antigen. The ability of the above polypeptides to compete with the binding of the antisera to the immobilized Mez1 or Mez2 polypeptide is compared to the immunogenic Mez1 or Mez2 polypeptide. The percent cross-reactivity for the above proteins is calculated, using standard calculations known to persons skilled in the art.
The immunoabsorbed and pooled antisera are then used in a competitive binding immunoassay to compare a second “target” polypeptide to the immunogenic polypeptide. In order to make this comparison, the two polypeptides are each assayed at a wide range of concentrations and the amount of each polypeptide required to inhibit 50% of the binding of the antisera to the immobilized protein is determined using standard techniques. If the amount of the target polypeptide required is less than twice the amount of the immunogenic polypeptide that is required, then the target polypeptide is said to specifically bind to an antibody generated to the immunogenic protein. As a final determination of specificity, the pooled antisera is fully immunoabsorbed with the immunogenic polypeptide until no binding to the polypeptide used in the immunoabsorbtion is detectable. The fully immunoabsorbed antisera is then tested for reactivity with the test polypeptide. If no reactivity is observed, then the test polypeptide is specifically bound by the antisera elicited by the immunogenic protein.
Production of Recombinant Expression Cassettes
Isolated nucleic acids of the present invention can be used in recombinant expression cassettes. One of ordinary skill in the art will recognize that a nucleic acid used in the recombinant expression cassettes described herein encoding a functional Mez1 or Mez2 polypeptide need not have a sequence identical to the exemplified nucleic acids disclosed herein and does not need to be full length, so long as the desired functional domain of the Mez1 or Mez2 protein is expressed.
A nucleic acid comprising a polynucleotide coding for the desired functional Mez1 or Mez2 polypeptide, for example a cDNA or a genomic sequence encoding a full length protein, can be used to construct a recombinant expression cassette which can be introduced into a desired plant. An expression cassette will typically comprise the functional Mez1 or Mez2 nucleic acid operably linked in either the sense or antisense direction to transcriptional and translational initiation regulatory sequences which will direct the transcription of the sequence from the functional Mez1 or Mez2 nucleic acid in the intended tissues for the transformed plant. Examples of transcriptional and translational initiation regions that can be used in the recombinant expression cassette are well known in the art.
The recombinant expression cassette will contain a promoter which is used to direct expression of the polynucleotides of the present invention in one, more than one, or in all of the tissues of a regenerated plant. For example, a constitutive plant promoter may be employed which will direct expression of the functional Mez1 or Mez2 polypeptide in all tissues of a regenerated plant. Examples of constitutive promoters includes, but is not limited to, the cauliflower mosaic virus (hereinafter “CaMV”) 35S transcription initiation region, the NOS promoter, the RUBISCO promoter, the 1′ or 2′-promoter derived from T-DNA of Agrobacterium tumefaciens, etc. The determination of a suitable constitutive plant promoter to be used in the recombinant expression cassette can readily be determined by persons skilled in the art.
Alternatively, an inducible plant promoter can be used. An inducible plant promoter may direct expression of the Mez1 or Mez2 nucleic acid in specific tissue or under more precise environmental or developmental control in a regenerated plant. Examples of environmental conditions that may effect transcription by inducible promoters include pathogen attack, anaerobic conditions, or the presence of light. Examples of inducible promoters include, but are not limited to, the Hsp70 promoter (which is inducible by heat stress), the PPDK promoter (which is inducible by light), etc.
Promoters derived from the Mez1 or Mez2 genes can be used to direct expression. These promoters can also be used to direct expression of heterologous sequences. The promoters can be used, for example, in recombinant expression cassettes to drive expression of the Mez1 or Mez2 nucleic acids of the present invention or heterologous sequences.
Such promoters can be identified as follows. The 5′ portions of the Mez1 or Mez2 genes described herein are analyzed for sequences characteristic of promoter sequences. For instance, promoter sequence elements include the TATA box consensus sequence (TATAAT), which is usually 20 to 30 base pairs upstream of the transcription start site. In plants, further upstream from the TATA box, at positions −80 to −100, there is typically a promoter element with a series of adenines surrounding the trinucleotide G (or T) N G. (See, J. Messing et al., in Genetic Engineering in Plants, pp. 221-227 (Kosage, Meredith and Hollaender, eds. 1983)).
If proper polypeptide expression is desired, a polyadenylation region at the 3′-end of the Mez1 or Mez2 polynucleotide coding region should be included. The polyadenylation region can be derived from a natural gene, from a variety of other plant genes, or from T-DNA. For example, polyadenylation regions can be derived from the nopaline synthase or octopine synthase genes.
The expression cassette comprising the Mez1 or Mez2 nucleic acids will typically comprise one or more marker genes which confers a selectable phenotype on plant cells. For example, the marker gene can encode biocide resistance, particularly antibiotic resistance, such as resistance to kanamycin, G418, bleomycin, hygromycin, or herbicide resistance, such as resistance to chlorosulforon.
As discussed briefly above, the Mez1 or Mez2 nucleic acids can be inserted into a recombinant expression cassette in the antisense direction. Expression of the Mez1 or Mez2 nucleic acid in antisense direction will result in the production of antisense RNA. It is well known to persons skilled in the art that a cell manufactures protein by transcribing the DNA of the gene encoding a protein to produce RNA, which is then processed to messenger RNA (hereinafter “mRNA”) (e.g., by the removal of introns) and finally translated by ribosomes into protein. This process may be inhibited in the cell by the presence of antisense RNA. It is believed that this inhibition takes place by formation of a complex between the two complementary strands of RNA, thus preventing the formation of protein. It is presently unclear how this mechanism works. However, it is believed that the complex may interfere with further translation, degrade the mRNA, or have more than one of these effects. This antisense RNA can be produced in the cell by transformation of the cell with an appropriate recombinant expression cassette designed to transcribe the non-template strand (as opposed to the template strand) of the relevant gene (or of a nucleic acid sequence showing substantial identity therewith).
The use of antisense RNA to downregulate the expression of specific plant genes is well known. Reduction in gene expression has been determined to led to changes in the phenotype of a plant, either at the level of gross visible phenotypic difference (see van der Krol et al., Nature, 333:866-869 (1988)), or at a more subtle biochemical level (Smith et al., Nature, 334:724-726 (1988)). Another method for inhibiting gene expression in transgenic plants involves the use of sense RNA transcribed from an exogenous template to downregulate the expression of specific plant genes (See, Jorgensen, Keystone Symposium “Improved Crop and Plant Products through Biotechnology”, Abstract X1-022 (1994)). Thereupon, both antisense and sense RNA can be used to achieve downregulation of gene expression in plants, which are encompassed by the present invention.
Production of Transgenic Plants
Techniques for transforming a wide variety of higher plant species using the recombinant expression cassettes hereinbefore described are well known and described in the technical and scientific literature (See, for example, Weising et al., Ann. Rev. Genet., 22:421-477 (1988)).
The hereinbefore described recombinant expression cassettes can be introduced into the genome of a desired plant host by a variety of conventional techniques which are well known to persons skilled in the art. For example, the recombinant expression cassette can be introduced directly into the genomic DNA of the plant cell using techniques such as electroporation, PEG poration, particle bombardment, silicon fiber delivery, and microinjection of plant cell protoplasts or embryogenic callus, or the expression cassettes can be introduced directly to plant tissue using ballistic methods, such as DNA particle bombardment. Alternatively, the expression cassettes may be combined with suitable T-DNA flanking regions and introduced into a conventional Agrobacterium tumefaciens or Agrobacterium rhizogenes host vector. The virulence functions of the Agrobacterium host will direct the insertion of the expression cassette and adjacent marker gene into the plant cell DNA when the cell is infected by the bacteria.
Plants which can be transformed with the recombinant expression cassette of the present invention include, but are not limited to, Zea mays L., Oryza sativa, Secale cereale, Triticum aestivum, Daucus carota, Brassica oleracea, Cucumis melo, Cucumis sativus, Latuca sativa, Solanum tubersoum, Lycopersicon esculentum, Phaseolus vulgaris, Brassica napus, etc.
Transformation techniques are well known to persons skilled in the art. For example, the introduction of expression cassettes using polyethylene glycol precipitation is described in Paszkowski et al., EMBO J, 3:2712-2722 (1984). Electroporation techniques are described in Fromm et al., Proc. Natl. Acad. Sci. USA, 82:5824 (1985). Biolistic transformation techniques are described in Klein et al., Nature, 327:70-73 (1987).
Agrobacterium tumefaciens-mediated transformation techniques are well known to persons skilled in the art (See, for example Horsch et al., Science 233:496-498 (1984), and Fraley et al., Proc. Natl. Acad. Sci. USA, 80:4803 (1983)). Although Agrobacterium is useful primarily in dicots, certain monocots can be transformed by Agrobacterium. U.S. Pat. No. 5,550,318 describes Agrobacterium transformation of maize.
Moreover, the following methods of transfection or transformation can also be used: (a) Agrobacterium rhizogenes-mediated transformation (See, Lichtenstein and Fuller In Genetic Engineering, vol. 6, PWJ Rigby, Ed., London, Academic Press, (1987)); (b) liposome-mediated DNA uptake (See, Freeman et al., Plant Cell Physiol., 25:1353 (1984)); and (3) the vortexing method (See, Kindle, Proc. Natl. Acad. Sci. USA, 87:1228 (1990)).
Transformed plant cells which are derived by any of the above transformation techniques can be cultured to regenerate a whole plant which possesses the transformed genotype. Such regeneration techniques rely on manipulation of certain phytohormones in a tissue culture growth medium, typically relying on a biocide and/or herbicide marker which has been introduced together with the Mez1 or Mez2 nucleic acid. Plant regeneration from cultured protoplasts is described in Evans et al., Protoplasts Isolation and Culture, Handbook ofPlant Cell Culture, pp. 124-176, MacMillian Publishing Company, New York, 1983; and Binding; Regeneration of Plants, Plant Protoplasts, pp. 21-73, CRC Press, Boca Raton, 1985. Regeneration can also be obtained from plant callus, explants, organs, or parts thereof. Such regeneration techniques are described generally in Klee et al., Ann. Ref ofPlant Phys. 38:467-486 (1987).
One of ordinary skill in the art will recognize that after the expression cassette is stably incorporated in transgenic plants and confirmed to be operable, it can be introduced into other plants by sexual crossing. Any of a number of standard breeding techniques can be used, depending upon the species to be crossed.
Transgenic plants containing the expression cassettes described herein can be identified by using restriction enzymes or High Performance Liquid Chromatography. Techniques for restriction enzymes and High Performance Liquid Chromatography are well known to persons skilled the art. Transgenic plants containing the expression cassettes described herein can be identified by using a Northern Blot analysis which is well known to persons skilled in the art.
Synthetic Polypeptides and Purification of Polypeptides
In addition to being produced recombinantly, the polypeptides of the present invention can also be produced synthetically, using techniques known in the art. For example, polypeptides having a length of about 50 amino acids can be synthesized using solid phase synthesis techniques, such as those described by Barany and Merrifield, Solid-Phase Peptide Synthesis, pp. 3-284 in The Peptides. Analysis, Synthesis, Biology. Vol. 2: Special Methods in Peptide Synthesis, Part A.; Merrifield et al., J. Am. Chem. Soc. 85:2149-2156 (1963). Polypeptides having a length greater than about 50 amino acids can be synthesized by condensation of the amino and carboxy termini of shorter fragments, a technique which is well known to persons skilled in the art.
Polypeptides of the present invention produced either recombinantly or synthetically, can be purified using standard techniques known to those persons skilled in the art, including, but not limited to, column chromatography, selective precipitation with ammonium sulfate, affinity chromatography, etc.
Methods for Repressing the Expression or Inhibiting the Repression of Expression of a Target Gene In Vivo
The Mez1 and Mez2 proteins belongs to the E(z) group of Polycomb proteins. As discussed previously, it is known in the art that the esc and esc-like (homologs) proteins interact with the E(z) and E(z)-like proteins in vivo to form complexes. The E(z) and esc proteins interact with each other, but are not known to physically interact with any other characterized PcG proteins. While C. elegans and plants contain homologs of the proteins in the E(z)/esc complex, they do not contain the PRC1 complex. The E(z)/esc complex has been found to repress the expression of a gene during a specific developmental stage and in a specific tissue in plants and C. elegans which lack the PRC1 complex (see Goodrich et al, Nature, 386(6620):44-51 (1997), Holdeman et al., Development, 125(13):2457-67 (1998), Korf et al., Development, 125(13):2469-78 (1998), Kelly and Fire, Development, 125(13):2451-6 (1998)).
The Mez1 and Mez2 nucleic acids and proteins of the present invention can be used for a number of useful purposes. First, the Mez1 and/or Mez2 proteins can be used in a method to repress the expression of a desired target gene in specific tissue in a plant in vivo. The gene targeted for silencing would either be in cells expressing endogenous or introduced Mez1 and/or Mez2 and ZmFIE proteins. The ZmFIE2 protein is an esc-like protein isolated from Zea mays L. and is described in copending application U.S. Ser. No. 09/______ filed on Jul. 16, 2001 and entitled, “Polycomb Gene from Maize -ZmFIE2”, hereby incorporated by reference. The Mez1 and/or Mez2 nucleic acids and ZmFIE2 nucleic acids could be constitutively expressed in these cells or introduced into a plant containing the cells by crossing. The gene targeted for silencing may have any of a number of different promoters, but would also contain DNA sequence motifs or contexts to which the Mez1 and/or Mez2 ZmFIE2 complex is targeted. This would allow silencing of a gene in specific tissues or at specific times in development. For example, immature roots contain a non-functional Mez2 protein, but a functional ZmFIE2 protein. Therefore, these cells would not silence an introduced or endogenous gene containing DNA sequences which attract the Mez2/ZmFIE2 complex. Alternatively, developing leaf tissues contain a functional Mez2 and ZmFIE2 protein. Therefore, an introduced or endogenous gene containing DNA sequences which attract the Mez2/ZmFIE2 complex would be silenced.
Alternatively, the Mez1 and Mez2 proteins of the present invention can be used in a method to prevent the repression of a particular desired target gene in vivo in a plant. One mechanism by which this could be accomplished is by producing dominant negative mutant forms of said Mez1 and Mez2 protein which fail to form a complex with any esc or esc-like proteins. In this approach, the recombinant expression cassette encodes a mutant Mez1 and/or Mez2 polypeptide (the mutant polypeptide contain various substitutions, deletions, additions, etc.) which fails to bind to any esc or esc-like proteins properly. Thereupon, the complex would not form.
A second mechanism by which this could be accomplished is through the use of antisense RNA. In this approach, recombinant expression cassettes containing the Mez1 and/or Mez nucleic acids in the antisense direction can be inserted into a plant. Preferably, the recombinant expression casettes contain a tissue-specific promoter which will direct expression to the tissues containing the desired target gene of interest. The antisense RNA produced by the expression cassette will hybridize with the endogenous mRNA produced from the Mez1 or Mez2 genes within the plant, thus preventing the expression of any Mez1 or Mez2 protein. Because there will be no Mez1 or Mez2 protein, the complex between the Mez1 and/or Mez2 proteins and any esc or esc-like proteins will fail to form.
The use of the Mez1 and Mez2 proteins of the present invention to repress the expression or prevent the repression of the expression of a target gene in specific tissue in a plant in vivo could be used to regulate homeotic gene expression in plants to create novel plants having improved agronomic traits (see Goodrich et al, Nature, 386(6620):44-51 (1997)).
The following Examples are offered by way of illustration, not limitation.

EXAMPLE 1

Cloning and Characterization of the Mez1 and Mez2 Genes

Cloning of Mez1 and Mez2: Drosophila E(z) (AAC46462) was used in a TBLASTN search of the Pioneer Hi-Bred EST database. Two contigs with significant similarity were discovered, and named Maize E(z)-like 1 (Mez1) and Maize E(z)-like 2 (Mez2). Other contigs containing a SET domain were also present but displayed more similarity to trithorax than to E(z). The ctsbp19 clone contained the 3′ 801 bp of Mez1. The Mez2 contig originating from the cbmfe16 clone contained the 3′ 1144 bp of the Mez2 cDNA. To obtain full-length clones and sequence for the 5′ region of both genes, Random Amplification of cDNA Ends (RACE) was performed. Additionally the 3′ end of Mez1 and Mez2 were obtained by RACE to verify the EST sequence. RACE reactions were performed on one-week seedling Mo17 cDNA using the Marathon cDNA kit (Clontech, Palo Alto Calif.) using Advantage2 polymerase (Clontech, Palo Alto Calif.). The primers used were as follows: Mez1F1—GGG TGT GGT GAT GGT ACA TTG G (SEQ ID NO:7), Mez1R2—CAG CTT GTC ACC CAT TCT GTA TGC G (SEQ ID NO:8), Mez2R3—TGC CTC GTC CTT CTT TGA TCC TTC G (SEQ ID NO:9)and Mez2F3—CTC ACA AGG AAG CAG ACA AAC GCG G (SEQ ID NO:10). RACE products were gel purified and cloned into pGEM-T Easy (Promega, Madison Wis.).
Sequencing: The plasmids were sequenced using BigDye terminator cycle sequencing on an ABI sequencer (Perkin-Elmer Applied Biosystems). Sequencing reactions were done in a 10 μl volume with 320 ng DNA and 10 pg of primer. Primers used were as follows: T7 (Promega), SP6 (Promega), Mez1F1, Mez1F2—TAC CTT GGT GAG TAC ACT GGG GAA C (SEQ ID NO:11), Mez1F4—CCA TTT CGT GTA TCA GAC CTA AGC (SEQ ID NO:12), Mez1F5—CAT CAA CGC CCT CCA AGC (SEQ ID NO:13), Mez1R6—TGC CAC ATT CTT GAA CTG TCA TCC G (SEQ ID NO:14), Mez1R4—GCA CAG TGA CAT CCT CGA AAA CG (SEQ ID NO:15), Mez1R5—GTC CCT GCT CAA TTG CC (SEQ ID NO:16), Mez2F4—GCG GAC AAT TGT GCG GTT CG (SEQ ID NO:17), Mez2F5—GGT TGT TCA CAG AAT TTG G (SEQ ID NO:18), Mez2R4—CTT CCT AAC AAA ATC CTT TGC TGT TG (SEQ ID NO:19) and Mez2R5—TTG CTC CAT GTA GTC TTG (SEQ ID NO:20).
Sequence analysis: The sequences were assembled through the contig assembly program (http://gcg.tigem.it/ASSEMBLY/assemble.html). Reverse complement, translation and ClustalW were all accessed from the ABCC sequence analysis page (http://biosci.cbs.umn.edu/seqanal/). ClustalW alignments were processed using Boxshade (http://www.ch.embnet.org/software/BOX_form.html) All BLAST searches were performed using the NCBI BLAST feature. For some searches the advanced BLAST feature was used and a target organism was specified. Targeting signals and putative localization were predicted using PSORT (http://psort.nibb.ac.jp/). Domains were identified using SMART (http://smart.embl-heidelberg.de/).
Phylogenetic analysis: The SET domains from all E(z)-like proteins were aligned using ClustalW. This alignment was then submitted to the PHYLIP server at http://bioweb.pasteur.fr/seqanal/phylogeny/phylip-uk.html. The protpars feature was used with bootstrapping performed before analysis. One hundred replicates were examined to determine bootstrap values. The consensus tree was then displayed with bootstrap values.
RT-PCR analysis: Total RNA was extracted from tissues including embryo, leaf, immature ear, immature tassel, 3-day root, pollen and BMS (Black Mexican Sweet) suspension cultures using TRIzol (Life Technologies Gibco/BRL). PolyA+ RNA, isolated using PolyAtract (Promega) was used to make cDNA with Marathon cDNA Amplification Kit (Clontech). 2 ng of cDNA was used in each PCR reaction. The primers used were: Mez1F1, Mez1R1—CGG GAC CTA ACT CTA CGG ATG G (SEQ ID NO:21), Mez2F6—CGC AGC TGA TAC GGC AAG TCC AAT CG (SEQ ID NO:22) and Mez2R2—GTA TCA TCC GGA GCG ACT CTT CAG C (SEQ ID NO:23). Cycling conditions were as follows: 94° 2′, 5 cycles of 94° for 30″, 70° for 30″, 72° for 1′, 5 cycles of 94° for 30″, 67.5° for 30″, 72° for 1′, then 25 cycles of 94° for 30″, 65° for 30″, 72° for 1′ followed by 72° for 7′. Each 25 μl reaction contained 1 μl of a 10 μM primer solution for each primer, 2 ng cDNA, 2.5 μl 10× buffer, 2 μl 25 mM MgCl₂, 0.3 μl 25 mM dNTP's (Promega), 0.2 μl Taq polymerase (Promega) and 17 μl ddH₂O.
Sequence analysis: The sequences were assembled through the contig assembly program (http://gcg.tigem.it/ASSEMBLY/assemble.html). Reverse complement, translation and ClustalW were all accessed from the ABCC sequence analysis page (http://biosci.cbs.umn.edu/seqanal/). ClustalW alignments were processed using Boxshade (http://www.ch.embnet.org/software/BOX_form.html). All BLAST searches were performed using the NCBI BLAST feature. For some searches the advanced BLAST feature was used and a target organism was specified. Targeting signals and putative localization were predicted using PSORT (http://psort.nibb.ac.jp/). Domains were identified using SMART (http://smart.embl-heidelberg.de/).
Results:
Mez1 and Mez2:

Two contigs with significant similarity to the Drosophila E(z) were discovered in the Pioneer Hi-Bred EST database. These contigs were named Maize E(z)-like 1 (Mez1) and Maize E(z)-like 2 (Mez2). To test for the presence of Mez1 ESTs in the public maize database the Mez1 cDNA was used in a BLASTN search (www.zmdb.iastate.edu). No Mez1 ESTs were found, but two putative trithorax hits were detected due to similarity of the E(z) and trithorax SET domains.
Mez1 was mapped to the short arm of chromosome 6 (bin 6.01-6.02). The Mez2 sequence was placed to the short arm of chromosome 9 (bin 9.04). Mutants with the phenotypes similar to the Arabidopsis clf or medea have not been mapped to these regions.
Alignment of Mez1 and Mez2:
The amino acid sequences of Mez1 and Mez2 were aligned using ClustalW (FIG. 3). The sequences are 42% identical and 56% similar over their entire lengths. The nucleotide sequences of Mez1 and Mez2 are 52% identical. In maize, it is common to find two closely related sequences due to the ancient tetraploid nature of maize. Often the two sequences that arose from the tetraploid fusion display greater than 70% nucleotide identity (Gaut and Doebley, PNAS, U.S.A., 94:6809-6814 (1997)). The lower identity of the Mez1 and Mez2 nucleotide sequences indicates that these genes were probably duplicated prior to the formation of the maize tetraploidy event. In addition the map positions of these two sequences do not correspond to colinear regions of the maize genome (Helentjaris, T., Maize Newsletter, 69:67-81 (1995)).
Characteristics of Mez1 and Mez2:
A putative bipartite nuclear localization signal is found in both Mez1 and Mez2 (See, FIGS. 4 and 5). Mez2 and Mez1 were aligned with the other characterized E(z)-like proteins using ClustalW (FIG. 4).
There are two regions near the C-terminal of the protein that are well conserved among all E(z) proteins (FIG. 4 a). These are the Cys-rich region and the SET domain. The Cys-rich region has a number of highly conserved cysteine residues. The spacing of the cysteine residues is unlike other Cys-rich zinc finger domains involved in DNA binding. The function of this domain is not known but it is highly conserved among all E(z) like genes. Mez1 is 45% identical to E(z) in this region while Mez2 is 46% identical. The SET (Su(var)3-9, Enhancer-of-zeste, Trithorax) domain found at the C-terminal end of the protein is also highly conserved. The SET domain of Mez1 is 55% identical to the E(z) SET domain (Mez2 is 54% identical). SET domains appear to be involved in mediating protein-protein interactions (Cui et al., Nat. Genet., 18:331-337 (1998); Huang et al., J. Biol. Chem., 273:15933-15939 (1998)). Interestingly, the nonspecific transcriptional activator, trithorax, also contains a SET domain indicating that SET domains alone are not responsible for transcriptional repression.
The Mez1 and Mez2 sequences were submitted to the SMART server to identify other domains within these proteins (Schultz et al., PNAS USA, 95:5857-5864 (1998); Schultz et al., Nucl. Acids Res., 28:231-234 (2000)). In addition to the SET domain, a SANT (SWI3, ADA2, N-CoR and TFIIIB”DNA-binding domains) domain was identified (FIGS. 4 and 5). The myb-DNA binding domain is a SANT domain as well. This indicates that plant E(z)-like genes have a domain that may facilitate DNA binding. The SMART program also predicts the presence of a SANT domain in the animal E(z)-like proteins.
An acidic region is present in E(z)-like proteins near the N-terminal region (FIGS. 4 and 5). The function of this domain is not known. This acidic region is conserved in all E(z)-like proteins. A small region near amino acid 250 of the plant E(z)-like proteins is highly conserved. This region, named CRRC region, is not recognized by the SMART program. The CRRC region is composed primarily of polar or charged residues.
Evolution of E(z) Sequences:
Arabidopsis contains at least three E(z)-like genes that perform distinct functions. The low degree of nucleotide similarity between Mez1 and Mez2 indicates that these genes may have distinct evolutionary origins. The SET domain sequences of all E(z)-like proteins were aligned using ClustalW. This alignment was then processed using PHYLIP and a parsimonious tree was constructed (FIG. 5). The tree shows grouping of the Arabidopsis clf and the maize Mez1. When the full-length protein sequences were used for the alignments, the same tree was produced. The results indicate that Mez1 is a clf-like gene in maize while Mez2 is likely EZA1homolog.
Alternative Splicing of Mez2:
In an attempt to generate a full length Mez2 clone, PCR primers in the 5′ and 3′ UTR region were used to amplify B73 ear cDNA. In addition to a major product of the expected size, two smaller products were observed (FIG. 6 a). These two products were excised and used for PCR reactions with primers from various regions of the gene to detect where the difference in size was arising. A region near the middle of Mez2 was identified and the PCR products from the two isoforms, Mez2 alternative splice 1 (Mez2^as1) and Mez2 alternative splice 2 (Mez2^as2), were sequenced. Sequencing revealed that the smaller products were identical to Mez2 except for the missing 659 base pairs in Mez2^as1and 810 base pairs in Mez2^as2. The deleted fragment in Mez2^as1corresponds to base pairs 1016 to basepairs 1676 of Mez2. The Mez2^as1deletion will cause a frameshift and a truncated protein of 341 amino acids (FIG. 6). The deletion in the Mez2^as2corresponds to basepairs 1016 to basepairs 1827 of Mez2 and does not result in a frameshift. The deletion in Mez2^as2results in a 624 amino acid protein that is missing the SANT domain. It is possible that the presence of multiple products in these PCR reactions is due to secondary structure of the RNA or aberrant PCR products. The presence of the products displaying identical size shifts in PCR reactions using multiple primers sets makes it unlikely that these are the result of mispriming events. No significant secondary structure was identified in these regions using secondary structure prediction programs. Together, these findings indicate that the presence of multiple products is most likely due to alternative splicing of Mez2 mRNA.
Expression of Mez1 and Mez2:
cDNA from various maize tissues was tested for the presence of Mez1 and Mez2 transcripts. Abundant Mez1 transcripts were detected in embryo, ear and root tissues (FIG. 7 a). Transcripts were also present in leaf, BMS cell culture, and pollen tissues. There were no tissues tested that did not contain Mez1 transcripts.
The same tissues were tested for the presence of Mez2 transcripts (FIG. 7 b). The primers used to test for Mez2 expression flank the site of alternative splicing documented in cDNA ear tissue. Amplification from ear cDNA revealed the presence of the three transcripts observed previously. In the lane amplified from embryo cDNA, a doublet of Mez2^as2and a smaller fragment is observed. The sequence of this smaller fragment has not been analyzed. No Mez2 or Mez2^asltranscripts are observed in embryo tissue. Mez2 transcripts are the predominant form in leaf tissue, with very faint Mez2^as1and Mez2^as2products. An intense Mez2 product is amplified from immature tassel cDNA. In addition, a Mez2^as2and two uncharacterized products are present. Only Mez2^as1transcripts are detected in 3-day root cDNA. Faint Mez2 and Mez2^as2products are observed from the BMS cell culture cDNA.
Mutator Insertions into Mez2:
The Mez1 and Mez2 sequences were submitted to the Pioneer Hi-Bred Int'l TUSC system. The TUSC system is designed to find Mutator (Mu) insertions in a sequence of interest. Difficulties were encountered in designing primers to amplify the Mez1 sequence. Mez2 primers were designed and used to screen the DNA pools. Four independent insertions were found. The location of the four Mu insertions and five of the Mez2 introns are shown in FIG. 2 a. Mez2-Mu1 is an intron insertion while Mez2-Mu2, Mez2-Mu3 and Mez2-Mu4 are all exon insertions.
All references, patents and patent applications referred to herein are hereby incorporated by reference.
The present invention is illustrated by way of the foregoing description and examples. The foregoing description is intended as a non-limiting illustration, since many variations will become apparent to those skilled in the art in view thereof. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.
Changes can be made to the composition, operation and arrangement of the method of the present invention described herein without departing from the concept and scope of the invention as defined in the following claims.

Claims

1. An isolated nucleic acid for repressing the expression of or inhibiting the repression of a target gene comprising a polynucleotide selected from the group consisting of SEQ ID NO:3 and a polynucleotide having at least 95% sequence identity to SEQ ID NO:3.

2. (canceled)

3. (canceled)

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. (canceled)

9. (canceled)

10. (canceled)

11. (canceled)

12. (canceled)

13. (canceled)

14. An expression cassette comprising a promoter sequence operably linked to the nucleic acid of claim 1.

15. The expression cassette of claim 14 further comprising a polyadenylation signal operably linked to the nucleic acid.

16. The expression cassette of claim 14 wherein the promoter is a constitutive or tissue specific promoter.

17. A bacterial cell comprising the expression cassette of claim 14.

18. The bacterial cell of claim 17 wherein the bacterial cell is an Agrobacterium tumefaciens cell or an Agrobacterium rhizogenes cell.

19. A plant cell transformed with the expression cassette of claim 14.

20. A transformed plant containing the plant cell of claim 19.

21. The transformed plant of claim 20 wherein the plant is Zea mays.

22. A seed that contains the expression cassette of claim 14.

23. A transformed seed of the transformed plant of claim 20.