Abstract:
In one aspect of the invention, single stranded DNA binding proteins (SSB) may be used to modulate homologous recombination in mitosis or meiosis. Methods are provided for modulating homologous recombination comprising transforming a cell with a nucleic acid encoding a single stranded DNA binding protein; and, allowing the cell to undergo mitosis or meiosis. Similarly, methods are provided for modulating homologous recombination comprising transforming a cell with an inhibitor of expression of a single stranded DNA binding protein, such as an antisense or co-suppressive nucleic acid; and, allowing the cell to undergo mitosis or meiosis.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is in the field of genetic manipulation of homologous recombination in meiosis and mitosis.  
         BACKGROUND OF THE INVENTION  
         [0002]    It has been suggested that recombinases may be used to stimulate mitotic homologous recombination between sister chromatids in eukaryotes, which has been proposed as a mechanism to promote gene targeting in vegetative/somatic cells. Gene targeting generally involves the directed alteration of a specific DNA sequence in its genomic locus in vivo. Problems have however been reported with mitotic gene targeting. It has for example been found that overexpression of RecA-homologues in mitotic cells may cause cell cycle arrest. International Patent Publication WO 97/08331 dated Mar. 6, 1997 summarizes a range of difficulties with earlier suggestions that the  E. coli  RecA recombinase would be useful for stimulating homologous mitotic recombination (as for example had been suggested in International Patent Publications WO 93/22443, WO 94/04032 and WO 93/06221). Utilization of  E. coli  RecA in eukaryotic cells is potentially problematic because the direction of strand transfer catalysed by RecA is the opposite to the direction of strand transfer catalysed by eukaryotic RecA homologues. Nevertheless, overexpression of  E. coli  RecA has been reported to promote gene targeting approximately 10-fold in mouse cells and less than two-fold in plants. However, this latter result in plants also demonstrated a very low overall frequency of gene targeting, which would tend to cast doubt on the statistical significance of the result.  
           [0003]    In the face of difficulties associated with the use of  E. coli  RecA in mitotic gene targeting, alternative enzymes have been used to catalyse homologous sister chromatid exchange in mitotic cells. For example, U.S. Pat. Nos. 5,780,296 and 5,945,339 disclose methods to promote homologous recombination using Rec2 as an alternative recombinase to overcome problems with the use of RecA. It has been reported that overexpression of human RAD51 (homologous recombination AD51) can increase gene targeting frequency by 2-3 fold.  
           [0004]    In applications other than gene targeting in mitotic cells, other studies have suggested that increased expression of  E. coli  RecA or RAD51 may increase the resistance of cells to radiation or other DNA damaging agents, and enhance the frequency of intrachromosomal recombination and sister-chromatid exchange.  
         SUMMARY OF THE INVENTION  
         [0005]    In one aspect of the invention, single stranded DNA binding proteins (SSB) may be used to modulate recombination in mitosis or meiosis. Methods are provided for modulating homologous recombination comprising transforming a cell with a nucleic acid encoding a single stranded DNA binding protein; and, allowing the cell to undergo mitosis or meiosis. Similarly, methods are provided for modulating homologous recombination comprising transforming a cell with an inhibitor of expression of a single stranded DNA binding protein, such as an antisense or co-suppressive nucleic acid; and, allowing the cell to undergo mitosis or meiosis.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0006]    In one embodiment of this invention, meiotic or mitotic homologous recombination frequency may be modulated by modifying the activity or functional level of proteins involved in ssDNA binding. This may for example be achieved by 1) modifying the activity or level of endogenous ssDNA-binding proteins or 2) expression of heterologous ssDNA-binding proteins. In eukaryotes, a heterotrimeric complex known as RPA facilitates the ssDNA-binding activity. Therefore, in some aspects of the invention, the function of this complex may be modified by modifying the level or action of one or more of the subunits. Homologous recombination subunits typically possess DNA-binding domains. Accordingly, in some embodiments, by increasing the expression or abundance of one or more of such subunits, a greater amount of ssDNA-binding potential may be provided in the cell. In some embodiments, this may be utilized to increase conversion of ssDNA ends to a recombinogenic form suited to be a substrate for RecA-homologue proteins, such as DMC1 and RAD51, thereby increasing the frequency of homologous recombination. RPA complex is known to interact with RAD51 and DMC 1. Accordingly, in alternative aspects of the invention, methods of increasing the abundance of RPA may be used to assist in increasing homologous recombination frequency, putatively by recruiting RecA-homologues and increasing conversion of ssDNA into recombinogenic ends.  
           [0007]    Alternatively to enhance homologous recombination frequency, methods of the invention may be used to modify the abundance or function of ssDNA binding proteins, such as RPA, or subunits thereof, to reduce homologous recombination frequency. For example, by increasing expression or function of RPA, or subunits thereof, ssDNA ends may be titrated by the ssDNA-binding proteins thereby making the ssDNA ends unavailable as substrate for RecA-homologues, thus decreasing the frequency of homologous recombination. In accordance with alternative aspects of the invention, homologous recombination frequency may be decreased by reducing the expression or function of RPA or its subunits. Such embodiments may be adapted to reduce the levels of RPA subunits to decrease conversion of ssDNA ends to a recombinogenic form, thereby decreasing the functional activity of RecA-homologues. Alternatively, RPA subunits may be engineered to have reduced function (such as reduced DNA-binding ability or protein-protein interaction ability). In some embodiments, reduced homologous recombination frequency may be achieved by the expression of such an altered form of protein adapted to have a dominant-negative effect, for example by reducing the formation of functional RPA complexes, or reducing the recruitment of RecA-homologues, such as by engineering the domain responsible for interaction with RAD51 and DMC1.  
           [0008]    In alternative embodiments, the effect of modifying ssDNA-binding protein level and function on increasing or decreasing homologous recombination frequency may vary depending upon the altered loci in the genome.  
           [0009]    In some embodiments, expression of heterologous ssDNA-binding proteins in eukaryotic cells may be adapted to modulate the frequency of homologous recombination.  E. coli  ssDNA-binding protein (EcSSB) is thought to bind ssDNA as a single subunit up to a homotetramer. Eukaryotic RPA is believed to have arisen by duplication of an ancestral SSB. Accordingly, the prokaryotic and eukaryotic ssDNA-binding proteins are apparently structural and functional homologues. In alternative embodiments of the invention, ssDN-binding proteins from bacteria, such as EcSSB or  Agrobacterium tumefaciens  VirE2, and other bacterial structural or functional homologues, or from prokaryotic or eukaryotic viruses, such as phi 29 gene 5 product, T4 phage gene 32 product, adenovirus, tobacco mosaic virus or other viral structural or functional homologues, can be used to modify homologous recombination frequency in eukaryotes. In alternative embodiments, this may be achieved by overexpression of the heterologous ssDNA-binding protein, such as EcSSB, in eulcaryotes. In one aspect, EcSSB may be expressed to bind to ssDNA ends created by MRE11 and associated proteins thereby titrating the ssDNA an making it less available for interaction with endogenous RPA or RecA-homologues such as RA 51 or DMC 1. In alternative embodiments, by reducing the binding of RPA to ssDNA, the recruitment of RecA homologues to the ssDNA ends may be reduced (RPA is known to physically interact with RAD51 and DMC1). In some embodiments, this may for example result in a reduction of conversion of ssDNA ends into recombinogenic ends, thereby reducing homologous recombination frequency. Alternatively, the expression of an abundance of SSB may be adapted to maintain ssDNA ends in a topology that is favourable for the action of RecA-homologues. In some embodiments, this may be adapted to result in an increase in homologous recombination frequency (for example because of increased conversion of ssDNA ends into recombinogenic ends).  
           [0010]    In some embodiments, the effect of heterologous expression of ssDNA-binding proteins on increasing or decreasing homologous recombination frequency may vary depending upon the relevant expression locus (or loci) in the genome. In some embodiments, the action of heterologous ssDNA-binding proteins in eukaryotes may be enhanced by inclusion of a nuclear localization sequence to promote accumulation of the protein in the nucleus.  
           [0011]    In other embodiments, expression of EcSSB in eukaryotic cells may be adapted to promote the activity of EcRecA expressed in eukaryotic cells. In some embodiments, for example, the activity of EcRecA in promoting homologous recombination and gene targeting in eukaryotic cells may be increased by coexpression of EcSSB alone or with a nuclear localization sequence attached to it.  
           [0012]    In other embodiments, expression of EcSSB may be adapted to increase gene targeting frequency in eukaryotes, for example by conferring a favourable topology for homologous recombination to ssDNA gene targeting substrates delivered to the cell, or double-stranded DNA gene targeting substrates delivered to the cell and partially or entirely converted to ssDNA. In other embodiments, expression of EcSSB may be adapted to promote localization of gene targeting substrate to the nucleus of eukaryotic cells. In some instances, only a fraction of gene targeting substrate delivered to cells may actually reach the nucleus to WO 02/22798 PCT/CA01/01315 interact with the target genomic locus. In one aspect of the invention, to increase the effective concentration of gene targeting substrate, the substrate may be coated in vitro with EcSSB engineered to encode a nuclear localization sequence (i.e. NLS-SSB). The substrate may then be transferred more efficiently to the nucleus resulting in a higher nuclear concentration of the substrate leading to higher frequency of gene targeting. NLS-SSB expressed in the host cell may for example be adapted to bind in vivo to linear or circular ssDNA or dsDNA substrates delivered to the cell, or T-DNA transferred by  Agrobacterium tumefaciens, to promote transfer of the substrate to the nucleus to promote gene-targeting frequency. In the case of T-DNA from Agrobacterium, in one aspect of the invention, methods of coating the T-DNA with NLS-SSB instead of VirE 2 (the normal protein coating T-DNA) may be used to promote gene targeting. This may for example be useful in embodiments where VirE2 would otherwise interact with host proteins that promote illegitimate recombination resulting in integration of the gene targeting substrate at random sites in the genome instead of the target genomic locus. In such embodiments, the replacement of VirE2 with NLS-SSB may be adapted so that the T-DNA is less affected be these host proteins and therefore has a greater likelihood of integrating at the correct targeted genomic locus.  
           [0013]    In other embodiments, EcSSB may be used to target proteins or peptides to DNA, particularly where those proteins or peptides normally have no or little ability to bind to DNA molecules in vitro and in vivo. Our data demonstrate that EcSSB can tolerate some additions and modifications to its structure and still bind to DNA. This has been illustrated by engineering EcSSB to encode foreign amino acids encoding a nuclear localization sequence (i.e. NLS-SSB) in embodiments wherein this modified protein remains functional in its ability to interact with DNA (as illustrated by results showing NLS-SSB has a greater effect on modifying homologous recombination than SSB). Therefore, in some aspects of the invention, SSB or NLS-SSB may be engineered to comprise additional peptides or proteins, for example to target such molecules to interact with DNA molecules. This may for example be used to target proteins such as transcription factors, nucleases, recombinases, DNA-repair proteins, antigenic peptides, antibodies, cytotoxic agents, reporter proteins, and others.  
           [0014]    In alternative embodiments, an anti-sense molecule or a co-suppressive nucleic acid may be used to modulate the expression of SSB to modulate homologous recombination. In general, a co-suppressive nucleic acid is a nucleic acid that suppresses the expression of another nucleic acid by means of co-suppression. Anti-sense oligonucleotides, including anti-sense RNA molecules and anti-sense DNA molecules, generally act to block the translation of mRNA by binding to targeted mRNA and inhibiting protein translation from the bound mRNA. For example, anti-sense oligonucleotides complementary to regions of a DNA sequence encoding an enzyme involved in homologous recombination, such as DMC1, may be expressed in transformed cells (such as plant cells) during the appropriate developmental stage to down-regulate the enzyme. Alternative methods of down-regulating protein expression may include the use of ribozymes or other enzymatic RNA molecules (such as hammerhead RNA structures) that are capable of catalysing the cleavage of RNA (as for example disclosed in U.S. Pat. Nos. 4,987,071 and 5,591,610, incorporated herein by reference). The mechanism of ribozyme action generally involves sequence specific hybridization of the ribozyme molecule to complementary target RNA, followed by endonucleolytic cleavage. Additionally, antibodies or peptides which inhibit the activity of the target protein may be introduced or expressed in cells to suppress activity of the target protein.  
           [0015]    It is well known in the art that some modifications and changes can be made in the structure of a polypeptide without substantially altering the biological function of that peptide; to obtain a biologically equivalent polypeptide. In one aspect of the invention, SSB proteins that modulate meiotic or mitotic recombination may differ from a portion of the corresponding native sequence by conservative amino acid substitutions. As used herein, the term “conserved amino acid substitutions” refers to the substitution of one amino acid for another at a given location in the peptide, where the substitution can be made without loss of function. In making such changes, substitutions of like amino acid residues can be made, for example, on the basis of relative similarity of side-chain substituents, for example, their size, charge, hydrophobicity, hydrophilicity, and the like, and such substitutions may be assayed for their effect on the function of the peptide by routine testing. In some embodiments, conserved amino acid substitutions may be made where an amino acid residue is substituted for another having a similar hydrophilicity value (e.g., within a value of plus or minus 2.0), where the following hydrophilicity values are assigned to amino acid residues (as detailed in U.S. Pat. No. 4,554,101, incorporated herein by reference): Arg (+3.0); Lys (+3.0); Asp (+3.0); Glu (+3.0); Ser (+0.3); Asn (+0.2); Gln (+0.2); Gly (0); Pro (−0.5); Thr (−0.4); Ala (−0.5); His (−0.5); Cys (−1.0); Met (−1.3); Val (−1.5); Leu (−1.8); Ile (−1.8); Tyr (−2.3); Phe (−2.5); and Trp (−3.4). In alternative embodiments, conserved amino acid substitutions may be made where an amino acid residue is substituted for another having a similar hydropathic index (e.g., within a value of plus or minus 2.0). In such embodiments, each amino acid residue may be assigned a hydropathic index on the basis of its hydrophobicity and charge characteristics, as follows: Ile (+4.5); Val (+4.2); Leu (+3.8); Phe (+2.8); Cys (+2.5); Met (+1.9); Ala (+1.8); Gly (−0.4); Thr (−0.7); Ser (−0.8); Trp (−0.9); Tyr (−1.3); Pro (−1.6); His (−3.2); Glu (−3.5); Gln (−3.5); Asp (−3.5); Asn (−3.5); Lys (−3.9); and Arg (−4.5). In alternative embodiments, conserved amino acid substitutions may be made where an amino acid residue is substituted for another in the same class, where the amino acids are divided into non-polar, acidic, basic and neutral classes, as follows: non-polar: Ala, Val, Leu, ile, Phe, Trp, Pro, Met; acidic: Asp, Glu; basic: Lys, Arg, His; neutral: Gly, Ser, Thr, Cys, Asn, Gln, Tyr.  
           [0016]    Various aspects of the present invention encompass nucleic acid or amino acid sequences that are homologous to other sequences. As the term is used herein, an amino acid or nucleic acid sequence is “homologous” to another sequence if the two sequences are substantially identical and the functional activity of the sequences is conserved (for example, both sequences function as or encode a selected enzyme or promoter function; as used herein, the term “homologous” does not infer evolutionary relatedness). Nucleic acid sequences may also be homologous if they encode substantially identical amino acid sequences, even if the nucleic acid sequences are not themselves substantially identical, a circumstance that may for example arise as a result of the degeneracy of the genetic code.  
           [0017]    Two nucleic acid or protein sequences are considered substantially identical if, when optimally aligned, they share at least about 25% sequence identity in protein domains essential for conserved function. In alternative embodiments, sequence identity may for example be at least 50%, 70%, 75%, 90%, 95% or any value from 25% to 99%. Optimal alignment of sequences for comparisons of identity may be conducted using a variety of algorithms, such as the local homology algorithm of Smith and Waterman, 1981 , Adv. Appl. Math  2: 482, the homology alignment algorithm of Needleman and Wunsch, 1970  J. Mol. Biol.  48:443, the search for similarity method of Pearson and Lipman, 1988 , Proc. Natl. Acad. Sci. USA  85: 2444, and the computerised implementations of these algorithms (such as GAP, BESTFIT, FASTA and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, Madison, Wis., U.S.A.). Sequence alignment may also be carried out using the BLAST algorithm, described in Altschul et al., 1990 , J. Mol. Biol.  215:403-10 (using the published default settings). Software for performing BLAST analysis may be available through the National Center for Biotechnology Information (through the internet at http://www.ncbi.nlm.nih.gov). The BLAST algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence that either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighbourhood word score threshold. Initial neighbourhood word hits act as seeds for initiating searches to find longer HSPs. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Extension of the word hits in each direction is halted when the following parameters are met: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W. T and X determine the sensitivity and speed of the alignment. The BLAST programs may use as defaults a word length (W) of 11, the BLOSUM62 scoring matrix (Henikoff and Henikoff, 1992 , Proc. Natl. Acad. Sci. USA  89: 10915-10919) alignments (B) of 50, expectation (E) of 10 (which may be changed in alternative embodiments to 1 or 0.1 or 0.01 or 0.001 or 0.0001; although E values much higher than 0.1 may not identify functionally similar sequences, it is useful to examine hits with lower significance, E values between 0.1 and 10, for short readings of similarity), M=5, N=4, for nucleic acids a comparison of both strands. For protein comparisons, BLASTP may be used with defaults as follows: G=11 (cost to open a gap); E=1 (cost to extend a gap); E=10 (expectation value, at this setting, 10 hits with scores equal to or better than the defined alignment score, S, are expected to occur by chance in a database of the same size of the one being searched; the E value can be increased or decreased to alter the stringency of the search); and W=3 (word size, default is 11 for BLASTN, 3 for other blast programs). The BLOSUM matrix assigns a probability score for each position in an alignment that is based on the frequency with which that substitution is known to occur among consensus blocks within related proteins. The BLOSUM62 (gap existence cost=11; per residue gap cost=1; lambda ratio=0.85) substitution matrix is used by default in BLAST 2.0. A variety of other matrices may be used as alternatives to BLOSUM62, including: PAM30 (9,1,0.87); PAM70 (10,1,0.87) BLOSUM80 (10,1,0.87); BLOSUM62 (11,1,0.82) and BLOSUM45 (14,2,0.87). One measure of the statistical similarity between two sequences using the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. In alternative embodiments of the invention, nucleotide or amino acid sequences are considered substantially identical if the smallest sum probability in a comparison of the test sequences is less than about 1, preferably less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001.  
           [0018]    An alternative indication that two nucleic acid sequences are substantially identical is that the two sequences hybridize to each other under moderately stringent, or preferably stringent, conditions. Hybridization to filter-bound sequences under moderately stringent conditions may, for example, be performed in 0.5 M NaHPO 4 , 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.2×SSC/0.1% SDS at 42° C. (see Ausubel, et al. (eds), 1989 , Current Protocols in Molecular Biology , Vol. 1, Green Publishing Associates, Inc., and John Wiley &amp; Sons, Inc., New York, at p. 2.10.3). Alternatively, hybridization to filter-bound sequences under stringent conditions may, for example, be performed in 0.5 M NaHPO 4 , 7% SDS, 1 mM EDTA at 65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. (see Ausubel, et al. (eds), 1989, supra.). Hybridization conditions may be modified in accordance with krnown methods depending on the sequence of interest (see Tijssen, 1993 , Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes , Part I, Chapter 2 “Overview of principles of hybridization and the strategy f nucleic acid probe assays”, Elsevier, N.Y.). Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point for the specific sequence at a defined ionic strength and pH. 
       
    
    
     EXAMPLE 1  
       [0019]    Template for amplifying SSB was genomic DNA from  E. coli  strain CC 106. Genomic DNA from the strain was isolated as follows: 1) cells were cultured to mid-log phase in TYS liquid medium at 37° C.; 2) cells were pelleted by centrifugation and washed with sterile distilled water; 3) 20 ml of cell culture vas centrifuged and the cell pellet resuspended in 0.5 ml TE/Tween buffer (10 mM Tris-HCl (pH 8.0), 1 mM EDTA, 0.01% (w/v) Tween 20); 4) cells were incubated at 85° C. for 20-30 min and then pelleted by microcentrifugation for 5-10 min; 5) the supernatant was collected at 50 μl of TE-RNase (RNase A 20 micrograms/ml) was added before incubation at room temperature for 30 min; 6) the supernatant was extracted with phenol and chloroform and precipitated with ethanol as per standard procedure. 7) the DNA was resuspended in 20 μl of LTE (TE) diluted 1:10 with distilled water).  
         [0020]    The SSB gene was amplified with two primer sets to create two clones of the gene with restrictions sites at the 5′ end. PCR reactions were performed with 4 μl of genomic DNA, 1.0 pmol SSB-5′Bam oligo and 1.0 pmol SSB-3′Pst oligo, or 1.0 pmol SSB-Sma oligo and 1.0 pmol SSB-3′Pst oligo, plus 0.2 mM dNTP&#39;s, 2.5 U Pfu (Stratagene) and Pfu buffer constituents recommended by the manufacturer in a volume of 50 μl. The PCR conditions were 5 min @94° C., followed by 25 cycles of 30 s @94° C., 30 s @55° C. and 1.0 min @72° C., followed by 10 min @72° C. and storage at 4° C. or −20° C. The amplified DNA from the PCR reactions using SSB-5′Bam oligo and SSB-3′Pst oligo or SSB-5′Sma oligo and SSB-3′Pst oligo was digested with BamHI and PstI or SmaI and PstI, respectively. The corresponding plasmid cloning vector pBluescript II KS (Stratagene) was also digested with BamHI and PstI or SmaI and PstI, respectively. DNA fragments of interest corresponding to SSB (˜0.55 kb) and the vector (˜3 kb) were purified by agarose gel electrophoresis and recovered from the agarose as described above. The fragments were ligated together, transformed into  E. coli  and putative clones of the SSB gene identified as described above. The DNA sequence of the resultant clones, pTK27 and PTK28, were determined to confirm they encoded SSB with flanking restriction sites of BamHI and PstI or SmaI and PstI, respectively.  
         [0021]    A SSB derivative was also created so that the resultant protein would encode a nuclear localization sequence (i.e. NLS-SSB). A synthetic oligonucleotide was created which encoded the nuclear localization sequence (NLS) corresponding to that found in simian virus 40 T-antigen. The nucleotide sequence (GGATCCAAAAAAATGGCTCCTAAGAAGAAG-AGAAAGGTTGGAGGAGGACCCGGG) encodes a BamHI site, in-frame start codon, and SmaI site (underlined). A plasmid containing this cloned NLS sequence and derived from pBluescript II KS-(Stratagene) was digested with SmaI and PstI and the DNA fragment corresponding to the vector (˜3 kb) was gel purified. pTK28 was also digested with SmaI and PstI and the fragement pTK28 was also digested with SmaI and PstI and the DNA fragment corresponding to the SSB gene (˜0.55 kb) was also gel purified. The DNA fragments were recovered from agarose, ligated together, transformed into  E. coli  and putative clones of the NLS-SSB gene identified as described above. The DNA sequence of the resultant clone, pTK29, was determined to confirm it encoded NLS-SSB.  
         [0022]    EcSSB  
         [0023]    To demonstrate the effect of increased ssDNA-binding protein on homologous recombination frequency, SSB and NLS-SSB were cloned and expressed in  S. cerevisiae  cells undergoing meiosis. SSB and NLS were cloned individually into pCM190 by first digesting this vector with BamHI and PstI. pTK27 (SSB) and pTK29 (NLS-SSB) were also digested with BamHI and PstI. DNA fragments of interest corresponding to SSB or NLS-SSB (˜0.6 kb) and pCM190 (˜8 kb) were purified by agarose gel electrophoresis and recovered from the agarose as described above. The SSB and NLS-SSB DNA fragments were ligated independently to the DNA fragment corresponding to pCM190, transformed into  E. coli  and putative clones of the genes in the expression vector were identified. The resultant clones of SSB and NLS-SSB in pCM190 were denoted pTK35 and pTK36, respectively.  
         [0024]    Biological Assay  
         [0025]    To demonstrate the effect of SSB on homologous recombination frequency, plasmids containing the candidate genes were introduced into  S. cerevisiae  BR2495 to create different strains. These strains were the grown, induced to undergo meiosis and the resultant progeny scored for phenotypic markers to determine homologous recombination frequency. Comparison of the homologous recombination frequency in the various strains to control strains containing parental vector containing no test gene enabled assessment of the genetic and biological effects of the test genes.  
         [0026]    Expression vectors containing the test genes were introduced into  S. cerevisiae . Exemplified genes and the corresponding expression plasmids are outlined in TABLE 1. Cell lines carrying plasmid constructs were selected by culturing cells in minimal medium lacking uracil (i.e. SC-URA; [1]): BR2495 is homozygous for the defective ura3-1 allele [2] and therefore cannot synthesize this essential metabolite; expression plasmids based on pCM188, pCM189 and pCM190 have a functional URA3 gene and therefore BR2495 cells possessing such plasmids will be able to synthesize uracil and be able to grow on medium lacking uracil. Cells were cultured in the presence of doxycycline (10 μg/ml for solid media; 5 μg/ml for liquid media) to suppress expression of test genes until desired growth stages.  
         [0027]    To assay homologous recombination frequency, single colonies from each test strain were used to first inoculate 3 ml of SC-URA+DOX (i.e. SC-URA containing doxycycline at 5 μg/ml) in a 15 ml tube (Falcon) which was then incubated at 30° C. with shaking (200 RPM) for ˜1.5 d. Ten cultures were prepared for each test strain, including BR2495 possessing the parental expression vector without a test gene and BR2495 possessing the various expression plasmids containing the test genes. Cells from 1 ml of culture were pelleted by centrifugation at 9000 RPM for 2 min in a standard microcentrifuge (Brinkman) and resuspended in 1 ml of sterile-distilled water (SDW). The cells were used to inoculate 5 ml of SC-A pre-meiosis medium (per litre: 1.7 g yeast nitrogen free base (Difco), 5 g ammonium acetate (Sigma), 20 g potassium acetate (Sigma), 2 g amino acid drop out mix; and in some experiments doxycycline at 5 μg/ml) in a 50 ml tube (Falcon) at a 1:50 dilution. The cultures were then incubated at 30° C. with shaking (225 RPM) for 2 d. Aliquots of cells from each culture were then collected to assay for mitotic homologous recombination frequency occurring during vegetative growth. Dilutions of these cells were plated on YPD medium (per litre: 10 g Bacto-yeast extract, 20 g Bacto-peptone, 20 g glucose, 20 g Bacto-agar; [1]) to determine viable cell number, and plated on minimal media lacking particular amino acids so as to examine homologous recombination at different test genomic loci in BR2495 (i.e. SC minus histidine (SC-his), leucine (SC-leu), threonine (SC-thr), or tryptophan (SC-trp) [1]). These plates were incubated at 30° C. for 24 d and then colonies were counted. The remaining cells in each culture in pre-meiosis medium were pelleted by centrifugation at 4000 RPM for 10 min at 4° C. For cultures containing doxycyline in the pre-meiosis medium, the pellet was resuspended in 5 ml of SC-A pre-meiosis medium and incubated at room temperature for 3 hours. These cells, and those cells incubated in pre-meiosis medium without doxycycline, were then pelleted by centrifugation at 4000 RPM for 10 min at 4° C. and resuspended in 4 ml SPM meiosis-induction medium (0.3% (w/v) potassium acetate, 0.02% (w/v) raffinose 5 μg/ml histidine, 25 μg/ml leucine, 5 μg/ml tryptophan, 50 μg/ml threonine, 5 μg/ml adenine). The cells were again pelleted by centrifugation at 4000 RPM for 10 min at 4° C. and resuspended in 3.5 ml SPM meiosis-induction medium. The cultures were then incubated at 30° C. with shaking (225 RPM) for 2 d (days) to enable cells to undergo meiosis. Dilutions of the cells were made using SDW and cells were then plated on YPD to determine viable cell number, and on minimal media lacking particular amino acids so as to examine homologous recombination at different test genomic loci in BR2495, as described above. Duplicate dilutions and plating of each culture were performed. Plates were incubated at 30° C. for 2-4 d and then colonies were counted. Frequency of recombinants for each culture was determined by dividing the number of prototrophs conferred by restoration of function for a particular test locus heteroallele by the viable cell number, taking into consideration the dilution factors. Meiotic homologous recombination frequency for each culture was corrected when necessary for background recombinants resulting during vegetative growth by subtraction of the mitotic homologous recombination frequency determined prior to placing the cells in SPM meiosis-induction medium. Mean values for the 10 replicates of each test strain were determined using the corrected values. Inclusion of the values from all 10 replicates in determining the mean was evaluated by the Q-test and values from individual replicates were excluded from the final mean if the statistic indicted a significant deviation from the values of other replicates. Comparison of means of meiotic homologous recombination frequency from test genes to that from control strains was done to determine the effect of the test gene. Statistical significance of the differences between these values was confirmed by evaluation using the t-test [3].  
         [0028]    Results  
         [0029]    As shown in TABLE 1, results from the exemplified embodiments demonstrate modification of homologous recombination frequency (increases and decreases) through modifying the expression and activity of SSB.  
         [0030]    Heterologous expression of a ssDNA-binding protein, SSB, is shown to decrease or increase homologous recombination frequency depending upon the genomic locus. A dominant-negative effect results at some loci to suppress homologous recombination, as shown at the leu2 locus where a reduction of homologous recombination frequency by ˜40% was shown. In contrast, a stimulation of homologous recombination occurs at some loci, as shown by the his4 locus where homologous recombination frequency was enhanced by 13%. In both cases, the action of SSB in eukaryotes was promoted ˜10% by addition of a nuclear localization sequence to the protein (i.e. NLS-SSB). In alternative embodiments, modifying the function or expression of endogenous native ssDNA-binding proteins may also be used to modify meiotic homologous recombination frequency.  
       REFERENCES  
       [0031]    1. Adams, A., et al.,  Methods in Yeast Genetics , Cold Spring Harbour Laboratory Press, 1987.  
         [0032]    2. Ross-MacDonald, P. et al., “Mutation of a meiosis specific MutS homologue decreases crossing over but not mismatch correction”,  Cell,  79: 1069-1080, 1994.  
         [0033]    3. Devore, J. L.,  Probability and Statistics , Duxbury Press, 1995.  
                                                                                                                                                                                             TABLE 1                                       His4   Leu2   Trp1                Expression Plasmid       Mean           Mean   Photo-       Mean                                Copy       Ratio   Ratio       Ratio   Ratio   troph   Ratio   Ratio               Plasmid       Pro-   Num-   Phototroph   of   of   Phototroph   of   of   Fre-   of   of       Gene   Experiment   Construct   Vector   moter b     ber c     Frequency d     HR e     HR   Frequency   HR   HR   quency   HR   HR                    SSB   1   control a     pCM190   Strong   High   6.96 × 10 −3     1.13   1.13   1.58 × 10 −4     0.61   0.61                   pTK35   pCM190   Strong   High   7.89 × 10 −3             9.58 × 10 −5         NLS-SSB   1   control   pCM190   Strong   High   6.96 × 10 −3     1.29   1.29   1.58 × 10 −4     0.54   0.54               pTK36   pCM190   Strong   High   9.00 × 10 −3             8.61 × 10 −5                                                            
 
         [0034]    [0034] 
     
       
       
         1 
         
           
             1  
           
           
             1  
             54  
             DNA  
             Artificial Sequence  
             
               Nuclear localization sequence (NLS) 
      cirresponding to that found in simian virus 40 T-antigen  
             
           
            1 

ggatccaaaa aaatggctcc taagaagaag agaaaggttg gaggaggacc cggg           54