Rad51 protein involved in repair and recombination in S. cerevisiae is a RecA-like protein

Phage T4 homologous strand exchange: a DNA helicase, not the strand transferase, drives polar branch migration

Homologous strand exchange is a central step in general genetic recombination. A multiprotein complex composed of five purified bacteriophage T4 proteins (the products of the uvsX, uvsY, 32, 41, and 59 genes) that mediates strand exchange under physiologically relevant conditions has been reconstituted. One of these proteins, the product of the uvsY gene, is required for homologous pairing but strongly inhibits branch migration catalyzed by UvsX protein, the phage RecA analog. Branch migration is completely dependent on the gene 41 protein, a DNA helicase that also functions in phage replication. The helicase is delivered to the strand exchange complex by the gene 59 accessory protein in a strand-specific fashion through direct interactions between the gene 59 and gene 32 proteins. These data suggest that strand transferases such as UvsX protein are essential for homologous pairing in vivo, but that a DNA helicase drives polar branch migration.

Expression of human RAD52 confers resistance to ionizing radiation in mammalian cells

Processing of mutagenic DNA damages by the double strand breaks (DSB) in eukaryotes is most likely achieved by multiple pathways, including homologous recombination. Although RAD52 has been shown to be important for DSB repair in yeasts, its role in DSB repair in mammalian cells has not been demonstrated. This study reports for the first time that the overexpression of human RAD52 confers enhanced resistance to gamma-rays and induces homologous intrachromosomal recombination in cultured monkey cells. Recombination frequency synergistically increased by the combination of overexpression of RAD52 and ionizing radiation. These observations suggest that homologous recombination mediated by RAD52 is involved in double-stranded break repair in mammalian cells.

Expression of Escherichia coli recA and ada genes in Saccharomyces cerevisiae using a vector with geneticin resistance

Construction of E. coli-yeast shuttle plasmids containing the neo selection gene is described. The protein-coding regions of the E. coli ada or recA genes under the control of the ADH1 promoter and terminator were ligated into the SphI unique site of pNF2 to produce pMSada and pMSrecA, respectively. The plasmids were used for transformation of the haploid and diploid pso4-1 strains of S. cerevisiae and their corresponding wild types. Transformants were obtained by selection for geneticin (G418) resistance. Crude protein samples were extracted from the individual transformants. Both the RecA and Ada proteins were present in all strains containing the recA and ada genes on plasmids, respectively. Thus the geneticin selection system was successfully used for the preparation of model yeast strains.

Pathway analysis of radiation-sensitive meiotic mutants of Coprinus cinereus

We have isolated 37 radiation-sensitive mutants of the basidiomycete Coprinus cinereus. Each mutation is recessive, and the collection defines at least ten complementation groups for survival of gamma irradiation. Four complementation groups define the genes rad3, rad9, rad11 and rad12, which are required both for survival of gamma irradiation and for meiosis. Mutants in each of these four groups fail to complete meiosis and produce mushrooms with greatly reduced numbers of viable spores. Propidium iodide staining of meiotic nuclei showed a characteristic terminal appearance for each mutant: few cells of any of the meiotic mutants progress beyond prophase I, and both condensation and fragmentation or dispersal of meiotic chromatin are frequently observed. Scanning electron micrographs showed that the meiotic mutants make varying numbers (0-6) of basidiospore initials and that few of these initials develop into mature spores. When initials are present they are always symmetrically arrayed on the basidium, regardless of initial number. In quantitative measurements of gamma ray sensitivity, double mutants of every tested combination of rad3, rad9, rad11 and rad12 consistently showed the same gamma ray sensitivity as the more sensitive single mutant parent of the cross. Therefore, these four genes are in the same pathway for the repair of gamma radiation damage, and this pathway also represents one or more functions essential for meiosis.

Localization of RecA-like recombination proteins on chromosomes of the lily at various meiotic stages

The Rad51 and Lim15 proteins of lily, which are homologs of the bacterial RecA protein, were found on chromosomes in various stages of meiotic prophase 1. The presence of both Rad51 and Lim15 proteins as discrete foci on leptotene and zygotene chromosomes and their colocalization suggest that meiotic recombination begins at the leptotene stage with the cooperation of these proteins and continues in zygotene. Localization of the foci on or adjacent to the chromosomes suggests that these proteins bind to the chromatin loops that extend from the axial cores. The proteins in these foci may participate in the searching and pairing of homologous DNA sequences, as the RecA protein does. The different pattern of localization of the Rad51 protein between the leptotene and pachytene stages and the absence of the Lim15 protein in the pachytene stage suggest that the Rad51 protein plays different roles in these stages.

RAD1 and RAD10, but not other excision repair genes, are required for double-strand break-induced recombination in Saccharomyces cerevisiae

HO endonuclease-induced double-strand breaks (DSBs) in the yeast Saccharomyces cerevisiae can be repaired by the process of gap repair or, alternatively, by single-strand annealing if the site of the break is flanked by directly repeated homologous sequences. We have shown previously (J. Fishman-Lobell and J. E. Haber, Science 258:480-484, 1992) that during the repair of an HO-induced DSB, the excision repair gene RAD1 is needed to remove regions of nonhomology from the DSB ends. In this report, we present evidence that among nine genes involved in nucleotide excision repair, only RAD1 and RAD10 are required for removal of nonhomologous sequences from the DSB ends. rad1 delta and rad10 delta mutants displayed a 20-fold reduction in the ability to execute both gap repair and single-strand annealing pathways of HO-induced recombination. Mutations in RAD2, RAD3, and RAD14 reduced HO-induced recombination by about twofold. We also show that RAD7 and RAD16, which are required to remove UV photodamage from the silent HML, locus, are not required for MAT switching with HML or HMR as a donor. Our results provide a molecular basis for understanding the role of yeast nucleotide excision repair gene and their human homologs in DSB-induced recombination and repair.

Interaction of Tyr103 and Tyr264 of the RecA protein with DNA and nucleotide cofactors. Fluorescence study of engineered proteins

To obtain structural insight on the interaction of the RecA protein with nucleotide cofactors (ATP and ADP) and DNA, we have made two engineered RecA proteins, in which either Tyr103 or Tyr264 was replaced with tryptophan. The fluorescence of tryptophan residues (two/subunit) of wild-type RecA is not significantly altered upon the binding of cofactor or DNA. Therefore, any detectable fluorescence change of the engineered proteins could be directly related to interaction with the particular inserted tryptophan residue. The fluorescence of Trp103 is almost completely quenched upon ADP binding, supporting a stacking interaction of adenine base of ADP with Tyr103. By contrast, with ATP the quenching of fluorescence of Trp103 is not complete (75%), possibly indicating that there is no stacking interaction with ATP. Such a difference could explain the antagonistic effects of ATP and ADP. Both nucleotides partially quench the fluorescence of Trp264 (about 70%), confirming that this residue is in the vicinity of the cofactor-binding site. The binding of ssDNA also decreases the fluorescence of both Trp103 and Trp264, the degree of quenching depending upon base composition and decreasing in the following order: poly(dT) > poly(dI) > M13 ssDNA > poly(dA). This order coincides with that of the binding affinities of these polynucleotides to RecA reported by Cazenave et al. [Cazenave, C., Chabbert, M., Toulmé, J. J. & Hélène, C. (1984) Biochim. Biophys. Acta 781, 7-13]. This correlation supports the finding that a region very close to Tyr103 interacts with DNA.

Nuclear foci of mammalian Rad51 recombination protein in somatic cells after DNA damage and its localization in synaptonemal complexes

Rad51 protein of Saccharomyces cerevisiae is a structural homolog of the Escherichia coli recombination enzyme RecA. In yeast, the Rad51 protein is required for mitotic and meiotic recombination and for repair of double-strand breaks in DNA. We have used antibodies raised against the homologous human protein, HsRad51, expressed in E. coli, to visualize the spatial distribution of the protein in mammalian somatic and meiotic cells. In cultured human cells, the HsRad51 protein is concentrated in multiple discrete foci in the nucleoplasm; it is largely absent from cytoplasm and nucleoli. After treatment of cells with methyl methanesulfonate, ultraviolet irradiation, or 137Cs irradiation, the percentage of cells with HsRad51 protein immunofluorescence increases; the same cells show unscheduled DNA synthesis. Induction of Rad51 foci is blocked by inhibitors of transcription. In mouse pachytene spermatocytes, the mouse homolog of Rad51 protein is highly enriched in synaptonemal complexes that are formed between the paired homologous chromosomes during meiotic prophase. We conclude that the mammalian proteins homologous to yeast Rad51 are involved in repair of DNA damage and recombinational repair during meiosis.

A novel allele of Saccharomyces cerevisiae RFA1 that is deficient in recombination and repair and suppressible by RAD52

To understand the mechanisms involved in homologous recombination, we have performed a search for Saccharomyces cerevisiae mutants unable to carry out plasmid-to-chromosome gene conversion. For this purpose, we have developed a colony color assay in which recombination is induced by the controlled delivery of double-strand breaks (DSBs). Recombination occurs between a chromosomal mutant ade2 allele and a second plasmid-borne ade2 allele where DSBs are introduced via the site-specific HO endonuclease. Besides isolating a number of new alleles in known rad genes, we identified a novel allele of the RFA1 gene, rfa1-44, which encodes the large subunit of the heterotrimeric yeast single-stranded DNA-binding protein RPA. Characterization of rfa1-44 revealed that it is, like members of the RAD52 epistasis group, sensitive to X rays, high doses of UV, and HO-induced DSBs. In addition, rfa1-44 shows a reduced ability to undergo sporulation and HO-induced gene conversion. The mutation was mapped to a single-base substitution resulting in an aspartate at amino acid residue 77 instead of glycine. Moreover, all radiation sensitivities and repair defects of rfa1-44 are suppressed by RAD52 in a dose-dependent manner, and one RAD52 mutant allele, rad52-34, displays nonallelic noncomplementation when crossed with rfa1-44. Presented is a model accounting for this genetic interaction in which Rfa1, in a complex with Rad52, serves to assemble other proteins of the recombination-repair machinery at the site of DSBs and other kinds of DNA damage. We believe that our findings and those of J. Smith and R. Rothstein (Mol. Cell. Biol. 15:1632-1641, 1995) are the first in vivo demonstrations of the involvement of a eukaryotic single-stranded binding protein in recombination and repair processes.

A mutation in the gene encoding the Saccharomyces cerevisiae single-stranded DNA-binding protein Rfa1 stimulates a RAD52-independent pathway for direct-repeat recombination

In the yeast Saccharomyces cerevisiae, recombination between direct repeats is synergistically reduced in rad1 rad52 double mutants, suggesting that the two genes define alternate recombination pathways. Using a classical genetic approach, we searched for suppressors of the recombination defect in the double mutant. One mutation that restores wild-type levels of recombination was isolated. Cloning by complementation and subsequent physical and genetic analysis revealed that it maps to RAF1. This locus encodes the large subunit of the single-stranded DNA-binding protein complex, RP-A, which is conserved from S. cerevisiae to humans. The rfa1 mutation on its own causes a 15-fold increase in direct-repeat recombination. However, unlike most other hyperrecombination mutations, the elevated levels in rfa1 mutants occur independently of RAD52 function. Additionally, rfa1 mutant strains grow slowly, are UV sensitive, and exhibit decreased levels of heteroallelic recombination. DNA sequence analysis of rfa1 revealed a missense mutation that alters a conserved residue of the protein (aspartic acid 228 to tyrosine [D228Y]). Biochemical analysis suggests that this defect results in decreased levels of RP-A in mutant strains. Overexpression of the mutant subunit completely suppresses the UV sensitivity and partially suppresses the recombination phenotype. We propose that the defective complex fails to interact properly with components of the repair, replication, and recombination machinery. Further, this may permit the bypass of the recombination defect of rad1 rad52 mutants by activating an alternative single-stranded DNA degradation pathway.

Genetic evidence for different RAD52-dependent intrachromosomal recombination pathways in Saccharomyces cerevisiae

Mutations in the RecA-like genes RAD51 and RAD57 reduce the frequency of gene conversion/reciprocal exchange between inverted repeats 7-fold. However, they enhance the frequency of deletions between direct repeats 5-12-fold. These induced deletions are RAD1- and RAD52-dependent. On the basis of these results it is proposed that there are several RAD52-dependent pathways of recombination: the recombinational repair pathway of gene conversion/reciprocal exchange dependent on RAD51 and RAD57; a RAD1- and RAD52-dependent pathway exclusively responsible for deletions that are induced in rad51 and rad57 mutants; and finally, it is possible that the gene conversion/reciprocal exchange events observed in rad51 and rad57 strains represent another RAD52-dependent recombination pathway of gene conversion/reciprocal exchange that does not require Rad51 and Rad57 functions. It is also shown that the RAD10 excision-repair gene is involved in long gene conversion tracts in homologous recombination between inverted repeats, as previously observed for RAD1. Finally, an analysis of meiotic recombination reveals that deletions are induced in meiosis 100-fold above mitotic levels, similar to intrachromosomal gene conversion/reciprocal exchange, and that, in contrast to intrachromosomal meiotic gene conversion (50% association), intrachromosomal meiotic gene conversion is not preferentially associated with reciprocal exchange (12-30% of association).

Cloning and characterisation of the Schizosaccharomyces pombe rad32 gene: a gene required for repair of double strand breaks and recombination

A new Schizosaccharomyces pombe mutant (rad32) which is sensitive to gamma and UV irradiation is described. Pulsed field gel electrophoresis of DNA from irradiated cells indicates that the rad32 mutant, in comparison to wild type cells, has decreased ability to repair DNA double strand breaks. The mutant also undergoes decreased meiotic recombination and displays reduced stability of minichromosomes. The rad32 gene has been cloned by complementation of the UV sensitive phenotype. The gene, which is not essential for cell viability and is expressed at a moderate level in mitotically dividing cells, has significant homology to the meiotic recombination gene MRE11 of Saccharomyces cerevisiae. Epistasis analysis indicates that rad32 functions in a pathway which includes the rhp51 gene (the S.pombe homologue to S.cerevisiae RAD51) and that cells deleted for the rad32 gene in conjunction with either the rad3 deletion (a G2 checkpoint mutation) or the rad2 deletion (a chromosome stability and potential nucleotide excision repair mutation) are not viable.

Genetic control of intrachromosomal recombination

Intrachromosomal recombination between direct repeats can occur either as gene conversion events, which maintain exactly the number of repeat units, or as deletions, which reduce the number of repeat units. Gene conversions are classical recombination events that utilize the standard chromosome recombination machinery. Spontaneous deletions between direct repeats are generally recA-independent in E. coli and RAD52-independent in S. cerevisiae. This independence from the major recombination genes does not mean that deletions form through a nonrecombinational process. Deletions have been suggested to result from sister chromatid exchange at the replication fork in a recA-independent process. The same type of exchange is proposed to be RAD52-independent in Saccharomyces cerevisiae. RAD52-dependent events encompass all events that involve the initial steps of a recombination reaction, which include strand invasion to form a heteroduplex intermediate.

The initiation and control of homologous recombination in Escherichia coli

The chromosome of Escherichia coli recombines at low frequency when it is an intact circle but recombines at high frequency when it is broken, for example by X-rays, or when a linear DNA fragment is introduced into the cell during conjugation or transduction. The high recombinogenicity of double-strand (ds) DNA ends is attributable to RecBCD enzyme, which acts on ds DNA ends and is essential for recombination and ds DNA break repair. RecBCD enzyme initiates DNA unwinding at ds DNA ends, and its nuclease activity is controlled by Chi sites (5' G-C-T-G-G-T-G-G 3') in such a way that the enzyme produces a potent single-stranded DNA substrate for homologous pairing by RecA and single-stranded DNA binding proteins. We discuss a unifying model for recombination and ds DNA break repair, based upon the enzymic activities of these and other proteins and upon the behaviour of E. coli mutants altered in these proteins.

DNA structure-dependent requirements for yeast RAD genes in gene conversion

In Saccharomyces cerevisiae, HO endonuclease-induced mating-type (MAT) switching is a specialized mitotic recombination event in which MAT sequences are replaced by those copied from a distant, unexpressed donor (HML or HMR). The donors have a chromatin structure inaccessible for both transcription and HO cleavage. Here we use physical monitoring of DNA to show that MAT switching is completely blocked at an early step in recombination in strains deleted for the DNA repair genes RAD51, RAD52, RAD54, RAD55 or RAD57. We find, however, that only RAD52 is required when the donor sequence is simultaneously not silenced and located on a plasmid. RAD51, RAD54, RAD55 and RAD57 are still required when the same transcribed donor is on the chromosome. We conclude that recombination in vivo occurs between DNA molecules in chromatin, whose structure significantly influences the outcome. RAD51, RAD54, RAD55 and RAD57 are all required to facilitate strand invasion into otherwise inaccessible donor sequences.

Molecular mechanisms of Holliday junction processing in Escherichia coli

Recent genetic and biochemical studies revealed the mechanisms of late stage of homologous recombination in E. coli. A central intermediate of recombination called "Holliday structure", in which two homologous duplex DNA molecules are linked by a single-stranded crossover, is formed by the functions of RecA and several other proteins. The products of the ruvA and ruvB genes, which constitute an SOS regulated operon, form a functional complex that promotes migration of Holliday junctions by catalyzing strand exchange reaction, thus enlarging the heteroduplex region. RuvA is a DNA-binding protein specific for these junctions, and RuvB is a motor molecule for branch migration providing energy by hydrolyzing ATP. The product of the ruvC gene, which is not regulated by the SOS system, resolves Holiday junctions by introducing nicks at or near the crossover junction in strands with the same polarity at the same sites. The recombination reaction is completed by sealing the nicks with DNA ligase, resulting in spliced or patched recombinants. The product of the recG gene provides an alternative route for resolving Holliday junctions. RecG has been proposed to promote branch migration in the opposite direction to that promoted by RecA protein. The atomic structure of RuvC protein revealed by crystallographic study, when combined with mutational analysis of RuvC, provides mechanistic insights into the interactions of RuvC with Holliday junction.

Hotspots of homologous recombination in mouse meiosis

The molecular mapping of recombinational breakpoints in the proximal region of the mouse MHC has revealed four hotspots at which breakpoints are clustered. A direct comparison of the nucleotide sequences of two independent hotspots revealed common molecular elements: a consensus sequence of the middle-repetitive MT-family, a repeat of tetramer sequences and a sequence homologous to a solitary LTR of mouse retroviruses. Extremely high frequency of recombination is observed at these hotspots when particular MHC haplotypes are used in genetic crosses. Wild mouse-derived wm7 haplotype instigates recombination at the hotspot located at the 3'-end of the Lmp-2 gene only during female meiosis. Fine genetic analysis demonstrated that the wm7 haplotype carries a genetic factor to instigate recombination and another factor to suppress recombination specifically during male meiosis. In addition, there is no dose effect of the hotspot on frequency of recombination. Finally, we described an attempt to establish an efficient in vitro assay system for monitoring recombination using plasmid DNAs that contain the Lmp-2 hotspot and nuclear extracts prepared from mouse testis.

Analysis of the DNA binding site of Escherichia coli RecA protein

To investigate the DNA binding site of RecA protein, we constructed 15 recA mutants having alterations in the regions homologous to the other ssDNA binding proteins. The in vivo analyses showed that the mutational change at Arg243, Lys248, Tyr264, or simultaneously at Lys6 and Lys19, or Lys6 and Lys23 caused severe defects in the recA functions, while other mutational changes did not. Purified RecA-K6A-K23A (Lys6 and Lys23 changed to Ala and Ala, respectively) protein was indistinguishable from the wild-type RecA protein in its binding to DNA. However, the RecA-R243A (Arg243 changed to Ala) and RecA-Y264A (Tyr264 changed to Ala) proteins were defective in binding to both ss- and ds-DNA. In self-oligomerization property, RecA-R243A was proficient but RecA-Y264A was deficient, suggesting that the RecA-R243A protein had a defect in DNA binding site and the RecA-Y264A protein was defective in its interaction with the adjacent RecA molecule. The region of residues 243-257 including the Arg243 is highly homologous to the DNA binding motif in the ssDNA binding proteins, while the eukaryotic RecA homologues have a similar structure at the amino-terminal side proximal to the nucleotide binding core. The region of residues 243-257 would be a part of the DNA binding site. The other parts of this site would be the Tyr103 and the region of residues 178-183, which were cross-linked to ssDNA. These three regions lie in a line in the crystal structure.

The human and mouse homologs of the yeast RAD52 gene: cDNA cloning, sequence analysis, assignment to human chromosome 12p12.2-p13, and mRNA expression in mouse tissues

The yeast Saccharomyces cerevisiae RAD52 gene is involved in DNA double-strand break repair and mitotic/meiotic recombination. The N-terminal amino acid sequence of yeast S. cerevisiae, Schizosaccharomyces pombe, and Kluyveromyces lactis and chicken is highly conserved. Using the technology of mixed oligonucleotide primed amplification of cDNA (MOPAC), two mouse RAD52 homologous cDNA fragments were amplified and sequenced. Subsequently, we have cloned the cDNA of the human and mouse homologs of yeast RAD52 gene by screening cDNA libraries using the identified mouse cDNA fragments. Sequence analysis of cDNA derived amino acid revealed a highly conserved N-terminus among human, mouse, chicken, and yeast RAD52 genes. The human RAD52 gene was assigned to chromosome 12p12.2-p13 by fluorescence in situ hybridization, R-banding, and DNA analysis of somatic cell hybrids. Unlike chicken RAD52 and mouse RAD51, no significant difference in mouse RAD52 mRNA level was found among mouse heart, brain, spleen, lung, liver, skeletal muscle, kidney, and testis. In addition to an approximately 1.9-kb RAD52 mRNA band that is present in all of the tested tissues, an extra mRNA species of approximately 0.85 kb was detectable in mouse testis.

Expression of the E.coli ada gene in S.cerevisiae provides cellular resistance to N-methyl-N'-nitro-N-nitrosoguanidine in rad6 but not in rad52 mutants

The Escherichia coli ada gene protein coding region under the control of the yeast alcohol dehydrogenase promoter in the extrachromosomally replicating yeast expression vectors pADHO6C and pVT103LO6C was introduced into the wild-type yeast strains, YNN-27 and FF-18733, and the repair deficient mutants LN-1 (rad1-1), VV-5 (rad6-1), C5-6 (rad52-1) and FF-18742 (rad52::URA3). This resulted in the expression of 3950, 1900, 1870, 1620, 1320 and 1420 fmol ada-encoded ATase/mg protein respectively: transformation with the parent vectors resulted in ATase activities of 3-17 fmol/mg protein. The wild-types, rad1-1 and rad6-1 yeast expressing the bacterial ATase showed increased resistance to the toxic and mutagenic effects of N-methyl-N'-nitro-N- nitrosoguanidine (MNNG). Expression of ATase in the rad52-1 and rad52::URA3 mutants neither complemented their sensitivity, nor reduced the mutagenic effects of this agent. These results suggest that whilst a portion of the toxic and mutagenic lesions induced by MNNG can be repaired in yeast by the E.coli Ada protein in a RAD1- and RAD6-independent manner, the RAD52 gene product may be essential for the complete functioning of the Ada ATase. This is the first suggestion of a possible cofactor requirement for ATase.

Transcriptional induction of Ty recombination in yeast

Families of repeated sequences are present in the genomes of all eukaryotes. Little is known about the mechanism(s) that prevents recombination between repeated sequences. In the yeast Saccharomyces cerevisiae, recombination between homologous sequences placed at nonhomologous locations in the genome (ectopic recombination) has been shown to occur at high frequencies for artificially created repeats, but at relatively low frequencies for a natural family of repeated sequences, the Ty family. We have previously shown that a high level of Ty cDNA in the cell causes an increase in the rate of nonreciprocal recombination (gene conversion) of a marked Ty element. In the present study, we show that it is also possible to elevate the rate of recombination of a marked Ty by increasing its transcription. This induction is different from, and acts synergistically to, the one seen upon increased levels of donor Ty cDNA. We show that the induction by transcription does not require the products of the RAD50, RAD51, and RAD57 genes. In contrast, cDNA-mediated recombination is dependent on the product of the RAD51 gene but not on products of the genes RAD50 or RAD57.

RecA homologs Dmc1 and Rad51 interact to form multiple nuclear complexes prior to meiotic chromosome synapsis

Dmc1 and Rad51, yeast homologs of the E. coli RecA protein, are shown by immunostaining to localize to as many as 64 sites within spread meiotic nuclei. Genetic requirements for this punctate pattern suggest it represents recombination intermediates. Dmc1 and Rad51 colocalize and are therefore likely to act together during recombination. Despite their similarities, the two proteins have specialized functions: Dmc1 complexes do not form in rad51 mutants, while Rad51 complexes are retained indefinitely in dmc1 mutants. Dmc1 and, by inference, Rad51 form complexes before synapsis as monitored by immunostaining for Zip1 protein. Analysis of zip1 mutants shows that Zip1 promotes dissociation of Dmc1 complexes. Colocalization of Dmc1 and Zip1 raises the possibility that Dmc1 and Rad51 are components of recombination nodules.

Cloning of the cDNA and genomic DNA that correspond to the recA-like gene of Drosophila melanogaster

We have isolated a cDNA homologous to the yeast DMC1 and RAD51 genes from Drosophila melanogaster. The DMC1 and RAD51 genes of Saccharomyces cerevisiae are known to play crucial roles during meiosis and during both meiosis and mitosis, respectively, and their gene products are homologous to each other and to the RecA protein of Escherichia coli. The cDNA cloned here contains an open reading frame that encodes 336 amino acids. Sequence analysis of the corresponding genomic DNA fragment showed one short intron existing in the coding region as in the DMC1 gene, but not in the RAD51 gene. By in situ hybridization to the salivary gland chromosomes, the recA-like gene was cytologically mapped to 99D of the third chromosome.

The REC2 gene encodes the homologous pairing protein of Ustilago maydis

Amino acid sequence analysis has established that the homologous pairing protein of Ustilago maydis, known previously in the literature as rec1, is encoded by REC2, a gene essential for recombinational repair and meiosis with regional homology to Escherichia coli RecA. The 70-kDa rec1 protein is most likely a proteolytic degradation product of REC2, which has a predicted mass of 84 kDa but which runs anomalously during sodium dodecyl sulfate-gel electrophoresis with an apparent mass of 110 kDa. To facilitate purification of the protein product, the REC2 gene was overexpressed from a vector that fused a hexahistidine leader sequence onto the amino terminus, enabling isolation of the REC2 protein on an immobilized metal affinity column. The purified protein exhibits ATP-dependent DNA renaturation and DNA-dependent ATPase activities, which were reactions characteristic of the protein as purified from cell extracts of U. maydis. Homologous pairing activity was established in an assay that measures recognition via non-Watson-Crick bonds between identical DNA strands. A size threshold of about 50 bp was found to govern pairing between linear duplex molecules and homologous single-stranded circles. Joint molecule formation with duplex DNA well under the size threshold was efficiently catalyzed when one strand of the duplex was composed of RNA. Linear duplex molecules with hairpin caps also formed joint molecules when as few as three RNA residues were present.

Homotypic and heterotypic protein associations control Rad51 function in double-strand break repair

Rad51 is essential for efficient repair of DNA double-strand breaks (DSBs) and recombination in Saccharomyces cerevisiae. Here, we examine Rad51 protein-protein interactions and their biological significance. GAL4 two-hybrid fusion analysis demonstrated that the amino-terminal region of Rad51 mediates both a strong Rad51:Rad51 self-association and a Rad51:Rad52 interaction. Several Rad51 variants were characterized that imparted DSB repair defects; these defects appear to result from Rad51 protein-protein interactions. First, a rad51 allele bearing a missense mutation in the consensus ATP-binding sequence disrupted DSB repair in wild-type yeast. The effect of this allele was dependent on the presence of wild-type Rad51 because MMS sensitivity of rad51 delta strains were not increased by its expression. Second, we identified a highly conserved RAD51 homolog from Kluyveromyces lactis (KlRAD51) that only partially complemented rad51 delta strains and impaired DSB repair in wild-type S. cerevisiae. Third, fusions of Gal4 domains to Rad51 disrupted DSB repair in a manner that required the presence of either Rad51 or Rad52. Because K. lactis RAD51 and RAD52 did not complement a S. cerevisiae rad51 delta rad52 delta strain, Rad51-Rad52 functions appear to be mediated through additional components. Thus, multiple types of Rad51 protein interactions, including self-association, appear to be important for DSB repair.

Cloning of human and mouse genes homologous to RAD52, a yeast gene involved in DNA repair and recombination

The RAD52 gene of Saccharomyces cerevisiae is required for recombinational repair of double-strand breaks. Using degenerate oligonucleotides based on conserved amino acid sequences of RAD52 and rad22, its counterpart from Schizosaccharomyces pombe, RAD52 homologs from man and mouse were cloned by the polymerase chain reaction. DNA sequence analysis revealed an open reading frame of 418 amino acids for the human RAD52 homolog and of 420 amino acid residues for the mouse counterpart. The identity between the two proteins is 69% and the overall similarity 80%. The homology of the mammalian proteins with their counterparts from yeast is primarily concentrated in the N-terminal region. Low amounts of RAD52 RNA were observed in adult mouse tissues. A relatively high level of gene expression was observed in testis and thymus, suggesting that the mammalian RAD52 protein, like its homolog from yeast, plays a role in recombination. The mouse RAD52 gene is located near the tip of chromosome 6 in region G3. The human equivalent maps to region p13.3 of chromosome 12. Until now, this human chromosome has not been implicated in any of the rodent mutants with a defect in the repair of double-strand breaks.

The REC2 gene encodes the homologous pairing protein of Ustilago maydis

Amino acid sequence analysis has established that the homologous pairing protein of Ustilago maydis, known previously in the literature as rec1, is encoded by REC2, a gene essential for recombinational repair and meiosis with regional homology to Escherichia coli RecA. The 70-kDa rec1 protein is most likely a proteolytic degradation product of REC2, which has a predicted mass of 84 kDa but which runs anomalously during sodium dodecyl sulfate-gel electrophoresis with an apparent mass of 110 kDa. To facilitate purification of the protein product, the REC2 gene was overexpressed from a vector that fused a hexahistidine leader sequence onto the amino terminus, enabling isolation of the REC2 protein on an immobilized metal affinity column. The purified protein exhibits ATP-dependent DNA renaturation and DNA-dependent ATPase activities, which were reactions characteristic of the protein as purified from cell extracts of U. maydis. Homologous pairing activity was established in an assay that measures recognition via non-Watson-Crick bonds between identical DNA strands. A size threshold of about 50 bp was found to govern pairing between linear duplex molecules and homologous single-stranded circles. Joint molecule formation with duplex DNA well under the size threshold was efficiently catalyzed when one strand of the duplex was composed of RNA. Linear duplex molecules with hairpin caps also formed joint molecules when as few as three RNA residues were present.

The SAD1/RAD53 protein kinase controls multiple checkpoints and DNA damage-induced transcription in yeast

Inhibition of DNA synthesis prevents mitotic entry through the action of the S-phase checkpoint. We have isolated S-phase arrest-defective (sad) mutants that show lethality in the presence of the DNA synthesis inhibitor hydroxyurea (HU). Several of these mutants show phenotypes consistent with inappropriate mitotic entry in the presence of unreplicated DNA, indicating a defect in the S-phase checkpoint. sad1 mutants are additionally defective for the G1 and G2 DNA damage checkpoints, and for DNA damage-induced transcription of RNR2 and RNR3. The transcriptional response to DNA damage requires activation of the Dun1 protein kinase. Activation of Dun1 in response to replication blocks or DNA damage is blocked in sad1 mutants. The HU sensitivity of sad1 mutants is suppressed by mutations in CKS1, a subunit of the p34CDC28 kinase, further establishing a link between cell cycle progression and lethality. sad1 mutants are allelic to rad53, a radiation-sensitive mutant. SAD1 encodes an essential protein kinase. The observation that SAD1 controls three distinct checkpoints suggests a common mechanism for cell cycle arrest at these points. Together, these observations implicate protein phosphorylation in the cellular response to DNA damage and replication blocks.

DNA replication triggered by double-stranded breaks in E. coli: dependence on homologous recombination functions

Homologous recombination-dependent DNA replication (RDR) of a lambda cos site-carrying plasmid is demonstrated in E. coli cells when the cells express lambda terminase that introduces a double-stranded break into the cos site. RDR occurs in normal wild-type cells if the plasmid also contains the recombination hotspot chi. Chi is dispensable when cells are induced for the SOS response or contain a recD mutation. recBC sbcA mutant cells are also capable of RDR induction. A recN mutation greatly reduces RDR in normal cells, but not in SOS-induced cells. RDR proceeds by the theta mode or rolling circle mode of DNA synthesis, yielding covalently closed circular plasmid monomers or linear plasmid multimers, respectively. Previously described inducible stable DNA replication is considered to be a special type of RDR that starts exclusively from specific sites (oriMs) on the chromosome.

Atomic structure of the RuvC resolvase: a holliday junction-specific endonuclease from E. coli

The crystal structure of the RuvC protein, a Holliday junction resolvase from E. coli, has been determined at 2.5 A resolution. The enzyme forms a dimer of 19 kDa subunits related by a dyad axis. Together with results from extensive mutational analyses, the refined structure reveals that the catalytic center, comprising four acidic residues, lies at the bottom of a cleft that nicely fits a DNA duplex. The structural features of the dimer, with a 30 A spacing between the two catalytic centers, provide a substantially defined image of the Holliday junction architecture. The folding topology in the vicinity of the catalytic site exhibits a striking similarity to that of RNAase H1 from E. coli.

Structure of REC2, a recombinational repair gene of Ustilago maydis, and its function in homologous recombination between plasmid and chromosomal sequences

Mutation in the REC2 gene of Ustilago maydis leads to defects in DNA repair, recombination, and meiosis. Analysis of the primary sequence of the Rec2 protein reveals a region with significant homology to bacterial RecA protein and to the yeast recombination proteins Dmc1, Rad51, and Rad57. This homologous region in the U. maydis Rec2 protein was found to be functionally sensitive to mutation, lending support to the hypothesis that Rec2 has a functional RecA-like domain essential for activity in recombination and repair. Homologous recombination between plasmid and chromosomal DNA sequences is reduced substantially in the rec2 mutant following transformation. The frequency can be restored to a level approaching, but not exceeding, that observed in the wild-type strain if transformation is performed with cells containing multiple copies of REC2.

Structure of REC2, a recombinational repair gene of Ustilago maydis, and its function in homologous recombination between plasmid and chromosomal sequences

Mutation in the REC2 gene of Ustilago maydis leads to defects in DNA repair, recombination, and meiosis. Analysis of the primary sequence of the Rec2 protein reveals a region with significant homology to bacterial RecA protein and to the yeast recombination proteins Dmc1, Rad51, and Rad57. This homologous region in the U. maydis Rec2 protein was found to be functionally sensitive to mutation, lending support to the hypothesis that Rec2 has a functional RecA-like domain essential for activity in recombination and repair. Homologous recombination between plasmid and chromosomal DNA sequences is reduced substantially in the rec2 mutant following transformation. The frequency can be restored to a level approaching, but not exceeding, that observed in the wild-type strain if transformation is performed with cells containing multiple copies of REC2.

Biochemistry of homologous recombination in Escherichia coli

Homologous recombination is a fundamental biological process. Biochemical understanding of this process is most advanced for Escherichia coli. At least 25 gene products are involved in promoting genetic exchange. At present, this includes the RecA, RecBCD (exonuclease V), RecE (exonuclease VIII), RecF, RecG, RecJ, RecN, RecOR, RecQ, RecT, RuvAB, RuvC, SbcCD, and SSB proteins, as well as DNA polymerase I, DNA gyrase, DNA topoisomerase I, DNA ligase, and DNA helicases. The activities displayed by these enzymes include homologous DNA pairing and strand exchange, helicase, branch migration, Holliday junction binding and cleavage, nuclease, ATPase, topoisomerase, DNA binding, ATP binding, polymerase, and ligase, and, collectively, they define biochemical events that are essential for efficient recombination. In addition to these needed proteins, a cis-acting recombination hot spot known as Chi (chi: 5'-GCTGGTGG-3') plays a crucial regulatory function. The biochemical steps that comprise homologous recombination can be formally divided into four parts: (i) processing of DNA molecules into suitable recombination substrates, (ii) homologous pairing of the DNA partners and the exchange of DNA strands, (iii) extension of the nascent DNA heteroduplex; and (iv) resolution of the resulting crossover structure. This review focuses on the biochemical mechanisms underlying these steps, with particular emphases on the activities of the proteins involved and on the integration of these activities into likely biochemical pathways for recombination.

Catalysis of ATP-dependent homologous DNA pairing and strand exchange by yeast RAD51 protein

The RAD51 gene of Saccharomyces cerevisiae is required for genetic recombination and DNA double-strand break repair. Here it is demonstrated that RAD51 protein pairs circular viral single-stranded DNA from phi X 174 or M13 with its respective homologous linear double-stranded form. The product of synapsis between these DNA partners is further processed by RAD51 to yield nicked circular duplex DNA, which indicates that RAD51 can catalyze strand exchange. The pairing and strand exchange reaction requires adenosine triphosphate, a result consistent with the presence of a DNA-dependent adenosine triphosphatase activity in RAD51 protein. Thus, RAD51 is a eukaryotic recombination protein that can catalyze the strand exchange reaction.

The yeast Saccharomyces cerevisiae DNA polymerase IV: possible involvement in double strand break DNA repair

We identified and purified a new DNA polymerase (DNA polymerase IV), which is similar to mammalian DNA polymerase beta, from Saccharomyces cerevisiae and suggested that it is encoded by YCR14C (POLX) on chromosome III. Here, we provided a direct evidence that the purified DNA polymerase IV is indeed encoded by POLX. Strains harboring a pol4 deletion mutation exhibit neither mitotic growth defect nor a meiosis defect, suggesting that DNA polymerase IV participates in nonessential functions in DNA metabolism. The deletion strains did not exhibit UV-sensitivity. However, they did show weak sensitivity to MMS-treatment and exhibited a hyper-recombination phenotype when intragenic recombination was measured during meiosis. Furthermore, MAT alpha pol4 delta segregants had a higher frequency of illegitimate mating with a MAT alpha tester strain than that of wild-type cells. These results suggest that DNA polymerase IV participates in a double-strand break repair pathway. A 3.2kb of the POL4 transcript was weakly expressed in mitotically growing cells. During meiosis, a 2.2 kb POL4 transcript was greatly induced, while the 3.2 kb transcript stayed at constant levels. This induction was delayed in a swi4 delta strain during meiosis, while no effect was observed in a swi6 delta strain.

Effect of mutations in genes affecting homologous recombination on restriction enzyme-mediated and illegitimate recombination in Saccharomyces cerevisiae

Restriction enzyme-mediated events (REM events; integration of transforming DNA catalyzed by in vivo action of a restriction enzyme) and illegitimate recombination events (IR events; integration of transforming DNA that shares no homology with the host genomic sequences) have been previously characterized in Saccharomyces cerevisiae. This study determines the effect of mutations in genes that are involved in homologous recombination and/or in the repair of double-stranded DNA breaks on these recombination events. Surprisingly, REM events are completely independent of the double-strand-break repair functions encoded by the RAD51, RAD52, and RAD57 genes but require the RAD50 gene product. IR events are under different genetic control than homologous integration events. In the rad50 mutant, homologous integration occurred at wild-type frequency, whereas the frequency of IR events was 20- to 100-fold reduced. Conversely, the rad52 mutant was grossly deficient in homologous integration (at least 1,000-fold reduced) but showed only a 2- to 8-fold reduction in IR frequency.

DNA repair in the extremely radioresistant bacterium Deinococcus radiodurans

Deinococcus radiodurans and other members of the same genus share extraordinary resistance to the lethal and mutagenic effects of ionizing and u.v. radiation and to many other agents that damage DNA. While it is known that this resistance is due to exceedingly efficient DNA repair, the molecular mechanisms responsible remain poorly understood. Following very high exposures to u.v. irradiation (e.g. 500 J m-2, which is non-lethal to D. radiodurans), this organism carries out extremely efficient excision repair accomplished by two separate nucleotide excision repair pathways acting simultaneously. One pathway requires the uvrA gene and appears similar to the UvrABC excinuclease pathway defined in Escherichia coli. The other excision repair pathway is specific for u.v. dimeric photoproducts, but is not mediated by a pyrimidine dimer DNA glycosylase. Instead, it is initiated by a second bona fide endonuclease that may recognize both pyrimidine dimers and pyrimidine-(6-4)pyrimidones. After high doses of ionizing-radiation (e.g. 1.5 Mrad), D. radiodurans can mend > 100 double-strand breaks (dsb) per chromosome without lethality or mutagenesis. Both dsb mending and survival are recA-dependent, indicating that efficient dsb mending proceeds via homologous recombination. D. radiodurans contains multiple chromosomes per cell, and it is proposed that dsb mending requires extensive recombination amongst these chromosomes, a novel phenomenon in bacteria. Thus, D. radiodurans may serve as an easily accessible model system for the double-strand-break-initiated interchromosomal recombination that occurs in eukaryotic cells during mitosis and meiosis.

Effect of mutations in genes affecting homologous recombination on restriction enzyme-mediated and illegitimate recombination in Saccharomyces cerevisiae

Restriction enzyme-mediated events (REM events; integration of transforming DNA catalyzed by in vivo action of a restriction enzyme) and illegitimate recombination events (IR events; integration of transforming DNA that shares no homology with the host genomic sequences) have been previously characterized in Saccharomyces cerevisiae. This study determines the effect of mutations in genes that are involved in homologous recombination and/or in the repair of double-stranded DNA breaks on these recombination events. Surprisingly, REM events are completely independent of the double-strand-break repair functions encoded by the RAD51, RAD52, and RAD57 genes but require the RAD50 gene product. IR events are under different genetic control than homologous integration events. In the rad50 mutant, homologous integration occurred at wild-type frequency, whereas the frequency of IR events was 20- to 100-fold reduced. Conversely, the rad52 mutant was grossly deficient in homologous integration (at least 1,000-fold reduced) but showed only a 2- to 8-fold reduction in IR frequency.

Formation of base triplets by non-Watson-Crick bonds mediates homologous recognition in RecA recombination filaments

Whereas complementary strands of DNA recognize one another by forming Watson-Crick base pairs, the way in which RecA protein enables a single strand to recognize homology in duplex DNA has remained unknown. Recent experiments, however, have shown that a single plus strand in the RecA filament can recognize an identical plus strand via bonds that, by definition, are non-Watson-Crick. In experiments reported here, base substitutions had the same qualitative and quantitative effects on the pairing of two identical strands in the RecA filament as on the recognition of duplex DNA by a third strand, indicating that similar non-Watson-Crick interactions govern both reactions.

Homologous pairing is reduced but not abolished in asynaptic mutants of yeast

In situ hybridization was used to examine chromosome behavior at meiotic prophase in the rad50S, hop1, rad50, and spo11 mutants of Saccharomyces cerevisiae, which are defective in chromosome synapsis and meiotic recombination. Painting of chromosomes I and III revealed that chromosome condensation and pairing are reduced in these mutants. However, there is some residual pairing in meiosis, suggesting that homologue recognition is independent of synaptonemal complex formation and recombination. Association of homologues was observed in the rad50, rad50S, and spo11 mutants, which are defective in the formation or processing of meiotic double-strand breaks. This indicates that double-strand breaks are not an essential component of the meiotic homology searching mechanism or that there exist additional or alternative mechanisms for locating homologues.

The Sep1 strand exchange protein from Saccharomyces cerevisiae promotes a paranemic joint between homologous DNA molecules

Strand exchange protein 1 (Sep1) from the yeast Saccharomyces cerevisiae promotes the transfer of one strand of a linear duplex DNA to a homologous single-stranded DNA circle. Using a nitrocellulose filter binding assay and electron microscopy, we find that Sep1 promotes the pairing of homologous DNA molecules via a paranemic joint. In this joint there is no net intertwining of the parental DNA molecules, as in the standard plectonemic double helix. The paranemic joints form with as little as 41 bp of homology between the parental DNA molecules. The substrates used were a circular molecule (either single-stranded DNA or duplex supercoiled DNA) and a linear duplex with heterologous regions at both ends to bar duplex plectonemic intertwining. We excluded the possibility that the exonuclease activity of Sep1 exposes complementary single-stranded regions that constitute the joint. The paranemic joint is the key intermediate in the search for homologous DNA by the RecA protein of Escherichia coli. Our results imply that the search process in a eukaryote such as yeast can be mechanistically similar.

Mutations in XRS2 and RAD50 delay but do not prevent mating-type switching in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, a large number of genes in the RAD52 epistasis group has been implicated in the repair of chromosomal double-strand breaks and in both mitotic and meiotic homologous recombination. While most of these genes are essential for yeast mating-type (MAT) gene switching, neither RAD50 nor XRS2 is required to complete this specialized mitotic gene conversion process. Using a galactose-inducible HO endonuclease gene to initiate MAT switching, we have examined the effect of null mutations of RAD50 and of XRS2 on intermediate steps of this recombination event. Both rad50 and xrs2 mutants exhibit a marked delay in the completion of switching. Both mutations reduce the extent of 5'-to-3' degradation from the end of the HO-created double-strand break. The steps of initial strand invasion and new DNA synthesis are delayed by approximately 30 min in mutant cells. However, later events are still further delayed, suggesting that XRS2 and RAD50 affect more than one step in the process. In the rad50 xrs2 double mutant, the completion of MAT switching is delayed more than in either single mutant, without reducing the overall efficiency of the process. The XRS2 gene encodes an 854-amino-acid protein with no obvious similarity to the Rad50 protein or to any other protein in the database. Overexpression of RAD50 does not complement the defects in xrs2 or vice versa.

Cloning and sequence analysis of rhp51+, a Schizosaccharomyces pombe homolog of the Saccharomyces cerevisiae RAD51 gene

A homology (rhp51+) of the RAD51 gene in Schizosaccharomyces pombe was cloned by screening a Sz. pombe genomic library using the 3'-end of RAD51 from Saccharomyces cerevisiae as a probe. As in S. cerevisiae, the sequence of rhp51+ showed two MluI cell-cycle boxes and a putative DNA damage-responsive element in its upstream region. The open reading frame codes for a 365-amino-acid (aa) polypeptide with an estimated molecular mass of 40,555 Da. The deduced aa sequence shows 27, 66, 75 and 80% identity with Escherichia coli RecA, S. cerevisiae Rad51 and the Rad51 homologs from chicken and humans, respectively. The aa sequence encoded by rhp51+ contains A- and B-type nucleotide-binding consensus sequences, as found in other RAD51 homologs. Northern blot analysis showed that rhp51+ encodes a 1.7-kb transcript. Methyl methanesulfonate treatment increased the level of this transcript three- to fivefold. Southern hybridization analysis suggests that a single copy of rhp51+ exists in the Sz. pombe genome.

Efficient UV stimulation of yeast integrative transformation requires damage on both plasmid strands

The nature of UV-induced pre-recombinational structures was studied using transformation of Saccharomyces cerevisiae cells with non-replicative plasmids. Transformation by double-stranded plasmids irradiated with UV was stimulated up to 50-fold, and both plasmid integration and conversion of the mutated chromosomal selective gene were found to be equally increased. The stimulation observed with such 'totally' irradiated plasmids was not found with plasmids bearing lesions in only one strand. This effect is attributed to the formation by excision repair of recombinogenic structures consisting of a pyrimidine dimer opposite a gap. When single-stranded integrative plasmids were irradiated, their transforming potential was decreased but the proportion of transformants that arose by gene conversion, rather than by plasmid integration, was increased from 8% to 49% as a function of the UV dose. Possible reasons why single-strand UV lesions favour gene conversion are discussed.

Sequence of the RAD55 gene of Saccharomyces cerevisiae: similarity of RAD55 to prokaryotic RecA and other RecA-like proteins

The RAD55 gene is required for radiation resistance and meiotic viability and presumably acts in recombination and recombinational DNA repair pathways. The nucleotide (nt) sequence of RAD55 from Saccharomyces cerevisiae was determined. The amino-acid sequence predicted from the nt sequence showed similarity to the RecA protein of bacteria and the RecA-like proteins from yeast: RAD51, RAD57 and DMC1. Similarity was strongest in the region of RecA that interacts with ATP cofactor.

The Escherichia coli recA gene increases UV-induced mitotic gene conversion in Saccharomyces cerevisiae

The effect of the Escherichia coli RecA protein on mitotic recombination in the diploid D7 strain of Saccharomyces cerevisiae damaged by UV radiation was investigated. The D7 strain was transformed by two modified versions of the pNF2 plasmid: one, containing the ADH-1 promoter, and the other containing the recA gene tandemly arranged behind the ADH-1 promoter region. Immunological analysis proved the presence of the 38-kDa RecA protein in D7/pNF2ADHrecA transformants. We observed a positive effect of recA gene expression on mitotic gene conversion, mainly at higher doses of UV radiation. The results indicate that a RecA-like activity could participate in steps preceding mitotic conversion events in yeast.

The E. coli recA gene can restore the defect in mutagenesis of the pso4-1 mutant of S. cerevisiae

The E. coli recA gene was introduced into the pso4-1 mutant of S. cerevisiae and transformants were treated with 8-MOP+UVA and 254-nm UV light. The results showed that the recA gene increased the resistance to the toxic effect of 8-MOP+UVA and restored the frequency of reversion of the pso4-1 mutants after both treatments. The presence of the recA gene stimulated expression of the small subunit of the ribonucleotide reductase (Rnr2) in the pso4-1 mutants. Thus the E. coli recA gene is functional in yeast. Moreover, it was shown that the pso4-1 mutant is epistatic to pso1-1 and rad6-1, which belong to a mutagenic repair pathway. We propose here that the PSO4 gene has some role in the control of mutagenic repair in yeast.