Yeast MPH1 gene functions in an error-free DNA damage bypass pathway that requires genes from Homologous recombination, but not from postreplicative repair

Interplay between the Smc5/6 complex and the Mph1 helicase in recombinational repair

The evolutionarily conserved Smc5/6 complex is implicated in recombinational repair, but its function in this process has been elusive. Here we report that the budding yeast Smc5/6 complex directly binds to the DNA helicase Mph1. Mph1 and its helicase activity define a replication-associated recombination subpathway. We show that this pathway is toxic when the Smc5/6 complex is defective, because mph1Delta and its helicase mutations suppress multiple defects in mutants of the Smc5/6 complex, including their sensitivity to replication-blocking agents, growth defects, and inefficient chromatid separation, whereas MPH1 overexpression exacerbates some of these defects. We further demonstrate that Mph1 and its helicase activity are largely responsible for the accumulation of potentially deleterious recombination intermediates in mutants of the Smc5/6 complex. We also present evidence that mph1Delta does not alleviate sensitivity to DNA damage or the accumulation of recombination intermediates in cells lacking Sgs1, which is thought to function together with the Smc5/6 complex. Thus, our results reveal a function of the Smc5/6 complex in the Mph1-dependent recombinational subpathway that is distinct from Sgs1. We suggest that the Smc5/6 complex can counteract/modulate a pro-recombinogenic function of Mph1 or facilitate the resolution of recombination structures generated by Mph1.

Role of double-stranded DNA translocase activity of human HLTF in replication of damaged DNA

Unrepaired DNA lesions can block the progression of the replication fork, leading to genomic instability and cancer in higher-order eukaryotes. In Saccharomyces cerevisiae, replication through DNA lesions can be mediated by translesion synthesis DNA polymerases, leading to error-free or error-prone damage bypass, or by Rad5-mediated template switching to the sister chromatid that is inherently error free. While translesion synthesis pathways are highly conserved from yeast to humans, very little is known of a Rad5-like pathway in human cells. Here we show that a human homologue of Rad5, HLTF, can facilitate fork regression and has a role in replication of damaged DNA. We found that HLTF is able to reverse model replication forks, a process which depends on its double-stranded DNA translocase activity. Furthermore, from analysis of isolated dually labeled chromosomal fibers, we demonstrate that in vivo, HLTF promotes the restart of replication forks blocked at DNA lesions. These findings suggest that HLTF can promote error-free replication of damaged DNA and support a role for HLTF in preventing mutagenesis and carcinogenesis, providing thereby for its potential tumor suppressor role.

The role of the Fanconi anemia network in the response to DNA replication stress

Fanconi anemia is a genetically heterogeneous disorder associated with chromosome instability and a highly elevated risk for developing cancer. The mutated genes encode proteins involved in the cellular response to DNA replication stress. Fanconi anemia proteins are extensively connected with DNA caretaker proteins, and appear to function as a hub for the coordination of DNA repair with DNA replication and cell cycle progression. At a molecular level, however, the raison d'être of Fanconi anemia proteins still remains largely elusive. The thirteen Fanconi anemia proteins identified to date have not been embraced into a single and defined biological process. To help put the Fanconi anemia puzzle into perspective, we begin this review with a summary of the strategies employed by prokaryotes and eukaryotes to tolerate obstacles to the progression of replication forks. We then summarize what we know about Fanconi anemia with an emphasis on biochemical aspects, and discuss how the Fanconi anemia network, a late acquisition in evolution, may function to permit the faithful and complete duplication of our very large vertebrate chromosomes.

The MPH1 gene of Saccharomyces cerevisiae functions in Okazaki fragment processing

Saccharomyces cerevisiae MPH1 was first identified as a gene encoding a 3' to 5' DNA helicase, which when deleted leads to a mutator phenotype. In this study, we isolated MPH1 as a multicopy suppressor of the dna2K1080E helicase-negative lethal mutant. Purified Mph1 stimulated the endonuclease activities of both Fen1 and Dna2, which act faithfully in the processing of Okazaki fragments. This stimulation required neither ATP hydrolysis nor the helicase activity of Mph1. Multicopy expression of MPH1 also suppressed the temperature-sensitive growth defects in cells expressing dna2Delta405N, which lacks the N-terminal 405 amino acids of Dna2. However, Mph1 did not stimulate the endonuclease activity of the Dna2Delta405N mutant protein. The stimulation of Fen1 by Mph1 was limited to flap-structured substrates; Mph1 hardly stimulated the 5' to 3' exonuclease activity of Fen1. Mph1 binds to flap-structured substrate more efficiently than to nicked duplex structures, suggesting that the stimulatory effect of Mph1 is exerted through its binding to DNA substrates. In addition, we found that Mph1 reversed the inhibitory effects of replication protein A on Fen1 activity. Our biochemical and genetic data indicate that the in vivo suppression of Dna2 defects observed with both dna2K1080E and dna2Delta405N mutants occur via stimulation of Fen1 activity. These findings suggest that Mph1 plays an important, although not essential, role in processing of Okazaki fragments by facilitating the formation of ligatable nicks.

Esc2 and Sgs1 act in functionally distinct branches of the homologous recombination repair pathway in Saccharomyces cerevisiae

Esc2 is a member of the RENi family of SUMO-like domain proteins and is implicated in gene silencing in Saccharomyces cerevisiae. Here, we identify a dual role for Esc2 during S-phase in mediating both intra-S-phase DNA damage checkpoint signaling and preventing the accumulation of Rad51-dependent homologous recombination repair (HRR) intermediates. These roles are qualitatively similar to those of Sgs1, the yeast ortholog of the human Bloom's syndrome protein, BLM. However, whereas mutation of either ESC2 or SGS1 leads to the accumulation of unprocessed HRR intermediates in the presence of MMS, the accumulation of these structures in esc2 (but not sgs1) mutants is entirely dependent on Mph1, a protein that shows structural similarity to the Fanconi anemia group M protein (FANCM). In the absence of both Esc2 and Sgs1, the intra-S-phase DNA damage checkpoint response is compromised after exposure to MMS, and sgs1esc2 cells attempt to undergo mitosis with unprocessed HRR intermediates. We propose a model whereby Esc2 acts in an Mph1-dependent process, separately from Sgs1, to influence the repair/tolerance of MMS-induced lesions during S-phase.

Yeast Mph1 helicase dissociates Rad51-made D-loops: implications for crossover control in mitotic recombination

Eukaryotes possess mechanisms to limit crossing over during homologous recombination, thus avoiding possible chromosomal rearrangements. We show here that budding yeast Mph1, an ortholog of human FancM helicase, utilizes its helicase activity to suppress spontaneous unequal sister chromatid exchanges and DNA double-strand break-induced chromosome crossovers. Since the efficiency and kinetics of break repair are unaffected, Mph1 appears to channel repair intermediates into a noncrossover pathway. Importantly, Mph1 works independently of two other helicases-Srs2 and Sgs1-that also attenuate crossing over. By chromatin immunoprecipitation, we find targeting of Mph1 to double-strand breaks in cells. Purified Mph1 binds D-loop structures and is particularly adept at unwinding these structures. Importantly, Mph1, but not a helicase-defective variant, dissociates Rad51-made D-loops. Overall, the results from our analyses suggest a new role of Mph1 in promoting the noncrossover repair of DNA double-strand breaks.

The FANCM ortholog Fml1 promotes recombination at stalled replication forks and limits crossing over during DNA double-strand break repair

The Fanconi anemia (FA) core complex promotes the tolerance/repair of DNA damage at stalled replication forks by catalyzing the monoubiquitination of FANCD2 and FANCI. Intriguingly, the core complex component FANCM also catalyzes branch migration of model Holliday junctions and replication forks in vitro. Here we have characterized the ortholog of FANCM in fission yeast Fml1 in order to understand the physiological significance of this activity. We show that Fml1 has at least two roles in homologous recombination-it promotes Rad51-dependent gene conversion at stalled/blocked replication forks and limits crossing over during mitotic double-strand break repair. In vitro Fml1 catalyzes both replication fork reversal and D loop disruption, indicating possible mechanisms by which it can fulfill its pro- and antirecombinogenic roles.

A role for DEAD box 1 at DNA double-strand breaks

DEAD box proteins are a family of putative RNA helicases associated with all aspects of cellular metabolism involving the modification of RNA secondary structure. DDX1 is a member of the DEAD box protein family that is overexpressed in a subset of retinoblastoma and neuroblastoma cell lines and tumors. DDX1 is found primarily in the nucleus, where it forms two to four large aggregates called DDX1 bodies. Here, we report a rapid redistribution of DDX1 in cells exposed to ionizing radiation, resulting in the formation of numerous foci that colocalize with gamma-H2AX and phosphorylated ATM foci at sites of DNA double-strand breaks (DSBs). The formation of DDX1 ionizing-radiation-induced foci (IRIF) is dependent on ATM, which was shown to phosphorylate DDX1 both in vitro and in vivo. The treatment of cells with RNase H prevented the formation of DDX1 IRIF, suggesting that DDX1 is recruited to sites of DNA damage containing RNA-DNA structures. We have shown that DDX1 has RNase activity toward single-stranded RNA, as well as ADP-dependent RNA-DNA- and RNA-RNA-unwinding activities. We propose that DDX1 plays an RNA clearance role at DSB sites, thereby facilitating the template-guided repair of transcriptionally active regions of the genome.

Mph1p promotes gross chromosomal rearrangement through partial inhibition of homologous recombination

Gross chromosomal rearrangement (GCR) is a type of genomic instability associated with many cancers. In yeast, multiple pathways cooperate to suppress GCR. In a screen for genes that promote GCR, we identified MPH1, which encodes a 3'-5' DNA helicase. Overexpression of Mph1p in yeast results in decreased efficiency of homologous recombination (HR) as well as delayed Rad51p recruitment to double-strand breaks (DSBs), which suggests that Mph1p promotes GCR by partially suppressing HR. A function for Mph1p in suppression of HR is further supported by the observation that deletion of both mph1 and srs2 synergistically sensitize cells to methyl methanesulfonate-induced DNA damage. The GCR-promoting activity of Mph1p appears to depend on its interaction with replication protein A (RPA). Consistent with this observation, excess Mph1p stabilizes RPA at DSBs. Furthermore, spontaneous RPA foci at DSBs are destabilized by the mph1Delta mutation. Therefore, Mph1p promotes GCR formation by partially suppressing HR, likely through its interaction with RPA.

The primary target of the killer toxin from Pichia acaciae is tRNA(Gln)

The Pichia acaciae killer toxin (PaT) arrests yeast cells in the S-phase of the cell cycle and induces DNA double-strand breaks (DSBs). Surprisingly, loss of the tRNA-methyltransferase Trm9 - along with the Elongator complex involved in synthesis of 5-methoxy-carbonyl-methyl (mcm(5)) modification in certain tRNAs - conferred resistance against PaT. Overexpression of mcm(5)-modified tRNAs identified tRNA(Gln)((UUG)) as the intracellular target. Consistently, toxin-challenged cells displayed reduced levels of tRNA(Gln) and in vitro the heterologously expressed active toxin subunit disrupts the integrity of tRNA(Gln)((UUG)). Other than Kluyveromyces lactis zymocin, an endonuclease specific for tRNA(Glu)((UUC)), affecting its target in a mcm(5)-dependent manner, PaT exerts activity also on tRNA(Gln) lacking such modification. As sensitivity is restored in trm9 elp3 double mutants, target tRNA cleavage is selectively inhibited by incomplete wobble uridine modification, as seen in trm9, but not in elp3 or trm9 elp3 cells. In addition to tRNA(Gln)((UUG)), tRNA(Gln)((CUG)) is also cleaved in vitro and overexpression of the corresponding gene increased resistance. Consistent with tRNA(Gln)((CUG)) as an additional TRM9-independent target, overexpression of PaT's tRNase subunit abolishes trm9 resistance. Most interestingly, a functional DSB repair pathway confers PaT but also zymocin resistance, suggesting DNA damage to occur generally concomitant with specific tRNA offence.

Mutants defective in Rad1-Rad10-Slx4 exhibit a unique pattern of viability during mating-type switching in Saccharomyces cerevisiae

Efficient repair of DNA double-strand breaks (DSBs) requires the coordination of checkpoint signaling and enzymatic repair functions. To study these processes during gene conversion at a single chromosomal break, we monitored mating-type switching in Saccharomyces cerevisiae strains defective in the Rad1-Rad10-Slx4 complex. Rad1-Rad10 is a structure-specific endonuclease that removes 3' nonhomologous single-stranded ends that are generated during many recombination events. Slx4 is a known target of the DNA damage response that forms a complex with Rad1-Rad10 and is critical for 3'-end processing during repair of DSBs by single-strand annealing. We found that mutants lacking an intact Rad1-Rad10-Slx4 complex displayed RAD9- and MAD2-dependent cell cycle delays and decreased viability during mating-type switching. In particular, these mutants exhibited a unique pattern of dead and switched daughter cells arising from the same DSB-containing cell. Furthermore, we observed that mutations in post-replicative lesion bypass factors (mms2Delta, mph1Delta) resulted in decreased viability during mating-type switching and conferred shorter cell cycle delays in rad1Delta mutants. We conclude that Rad1-Rad10-Slx4 promotes efficient repair during gene conversion events involving a single 3' nonhomologous tail and propose that the rad1Delta and slx4Delta mutant phenotypes result from inefficient repair of a lesion at the MAT locus that is bypassed by replication-mediated repair.

FANCM of the Fanconi anemia core complex is required for both monoubiquitination and DNA repair

In response to DNA damage, the Fanconi anemia (FA) core complex functions as a signaling machine for monoubiquitination of FANCD2 and FANCI. It remains unclear whether this complex can also participate in subsequent DNA repair. We have shown previously that the FANCM constituent of the complex contains a highly conserved helicase domain and an associated ATP-dependent DNA translocase activity. Here we show that FANCM also possesses an ATP-independent binding activity and an ATP-dependent bi-directional branch-point translocation activity on a synthetic four-way junction DNA, which mimics intermediates generated during homologous recombination or at stalled replication forks. Using an siRNA-based complementation system, we found that the ATP-dependent activities of FANCM are required for cellular resistance to a DNA-crosslinking drug, mitomycin C, but not for the monoubiquitination of FANCD2 and FANCI. In contrast, monoubiquitination requires the entire helicase domain of FANCM, which has both ATP dependent and independent activities. These data are consistent with participation of FANCM and its associated FA core complex in the FA pathway at both signaling through monoubiquitination and the ensuing DNA repair.

Compensatory role for Rad52 during recombinational repair in Ustilago maydis

A single Rad52-related protein is evident by blast analysis of the Ustilago maydis genome database. Mutants created by disruption of the structural gene exhibited few discernible defects in resistance to UV, ionizing radiation, chemical alkylating or cross-linking agents. No deficiency was noted in spontaneous mutator activity, allelic recombination or meiosis. GFP-Rad51 foci were formed in rad52 cells following DNA damage, but were initially less intense than normal suggesting a possible role for Rad52 in formation of the Rad51 nucleoprotein filament. A search for interacting genes that confer a synthetic fitness phenotype with rad52 after DNA damage by UV irradiation identified the genes for Mph1, Ercc1 and the Rad51 paralogue Rec2. Testing known mutants in recombinational repair revealed an additional interaction with the BRCA2 orthologue Brh2. Suppression of the rec2 mutant's UV sensitivity by overexpressing Brh2 was found to be dependent on Rad52. The results suggest that Rad52 serves in an overlapping, compensatory role with both Rec2 and Brh2 to promote and maintain formation of the Rad51 nucleoprotein filament.

Phosphorylation of Slx4 by Mec1 and Tel1 regulates the single-strand annealing mode of DNA repair in budding yeast

Budding yeast (Saccharomyces cerevisiae) Slx4 is essential for cell viability in the absence of the Sgs1 helicase and for recovery from DNA damage. Here we report that cells lacking Slx4 have difficulties in completing DNA synthesis during recovery from replisome stalling induced by the DNA alkylating agent methyl methanesulfonate (MMS). Although DNA synthesis restarts during recovery, cells are left with unreplicated gaps in the genome despite an increase in translesion synthesis. In this light, epistasis experiments show that SLX4 interacts with genes involved in error-free bypass of DNA lesions. Slx4 associates physically, in a mutually exclusive manner, with two structure-specific endonucleases, Rad1 and Slx1, but neither of these enzymes is required for Slx4 to promote resistance to MMS. However, Rad1-dependent DNA repair by single-strand annealing (SSA) requires Slx4. Strikingly, phosphorylation of Slx4 by the Mec1 and Tel1 kinases appears to be essential for SSA but not for cell viability in the absence of Sgs1 or for cellular resistance to MMS. These results indicate that Slx4 has multiple functions in responding to DNA damage and that a subset of these are regulated by Mec1/Tel1-dependent phosphorylation.

Homologous recombination and the yKu70/80 complex exert opposite roles in resistance against the killer toxin from Pichia acaciae

The linear plasmid (pPac1-2) encoded killer toxin (PaT) of the yeast Pichia acaciae arrests sensitive Saccharomyces cerevisiae cells in the S-phase of the cell cycle and induces mutations. Here we provide evidence for opposite effects in PaT resistance of homologous recombination (HR) and non-homologous end joining (NHEJ), the two alternative repair mechanisms acting on DNA double strand breaks (DSB). As mutants defective in genes of the RAD52 epistasis group react hypersensitive and cells lacking YKU70 or YKU80 are partially resistant, the yKu70/80 complex facilitates PaT toxicity, whereas HR is antagonistic. In contrast to yku70 and yku80, lif1 mutants, the latter being defective in the ligation step of NHEJ, are PaT sensitive, confining toxicity promoting effects of NHEJ to the DSB end binding Ku proteins. Since rad52 yku80 double mutants display strong hypersensitivity, yku80 mediated resistance depends on HR. Opposite effects of the yKu70/80 complex and HR are consistent with the occurrence of replication dependent (one sided) DSBs in PaT treated cells. Concordantly, two cellular markers signaling DSBs are induced during PaT mediated S-phase arrest, i.e. histone H2A phosphorylation and formation of subnuclear repair foci by GFP tagged recombination protein Rad52. As only moderate chromosome fragmentation could be detected by PFGE, transient occurrence and efficient in vivo repair of PaT induced DSBs is assumed. Consistent with replication dependent DSB formation induced by PaT, we demonstrate a protective function of the RecQ helicase Sgs1 and the structure specific endonuclease Mus81, both of which are considered to be involved in processing and restart of stalled replication forks.

Systematic pathway analysis using high-resolution fitness profiling of combinatorial gene deletions

Systematic genetic interaction studies have illuminated many cellular processes. Here we quantitatively examine genetic interactions among 26 Saccharomyces cerevisiae genes conferring resistance to the DNA-damaging agent methyl methanesulfonate (MMS), as determined by chemogenomic fitness profiling of pooled deletion strains. We constructed 650 double-deletion strains, corresponding to all pairings of these 26 deletions. The fitness of single- and double-deletion strains were measured in the presence and absence of MMS. Genetic interactions were defined by combining principles from both statistical and classical genetics. The resulting network predicts that the Mph1 helicase has a role in resolving homologous recombination-derived DNA intermediates that is similar to (but distinct from) that of the Sgs1 helicase. Our results emphasize the utility of small molecules and multifactorial deletion mutants in uncovering functional relationships and pathway order.

Transpositions and translocations induced by site-specific double-strand breaks in budding yeast

Much of what we know about the molecular mechanisms of repairing a broken chromosome has come from the analysis of site-specific double-strand breaks (DSBs). Such DSBs can be generated by conditional expression of meganucleases such as HO or I-SceI or by the excision of a DNA transposable element. The synchronous creation of DSBs in nearly all cells of the population has made it possible to observe the progress of recombination by monitoring both the DNA itself and proteins that become associated with the recombining DNA. Both homologous recombination mechanisms and non-homologous end-joining (NHEJ) mechanisms of recombination have been defined by using these approaches. Here I focus on recombination events that lead to alterations of chromosome structure: transpositions, translocations, deletions, DNA fragment capture and other small insertions. These rearrangements can occur from ectopic gene conversions accompanied by crossing-over, break-induced replication, single-strand annealing or non-homologous end-joining.

Fanconi Anemia (Cross)linked to DNA Repair

Fanconi anemia is characterized by hypersensitivity to DNA interstrand crosslinks (ICLs) and susceptibility to tumor formation. Despite the identification of numerous Fanconi anemia (FANC) genes, the mechanism by which proteins encoded by these genes protect a cell from DNA interstrand crosslinks remains unclear. The recent discovery of two DNA helicases that, when defective, cause Fanconi anemia tips the balance in favor of the direct involvement of the FANC proteins in DNA repair and the bypass of DNA lesions.

A human ortholog of archaeal DNA repair protein Hef is defective in Fanconi anemia complementation group M

Fanconi anemia is a genetic disease characterized by genomic instability and cancer predisposition. Nine genes involved in Fanconi anemia have been identified; their products participate in a DNA damage-response network involving BRCA1 and BRCA2 (refs. 2,3). We previously purified a Fanconi anemia core complex containing the FANCL ubiquitin ligase and six other Fanconi anemia-associated proteins. Each protein in this complex is essential for monoubiquitination of FANCD2, a key reaction in the Fanconi anemia DNA damage-response pathway. Here we show that another component of this complex, FAAP250, is mutant in individuals with Fanconi anemia of a new complementation group (FA-M). FAAP250 or FANCM has sequence similarity to known DNA-repair proteins, including archaeal Hef, yeast MPH1 and human ERCC4 or XPF. FANCM can dissociate DNA triplex, possibly owing to its ability to translocate on duplex DNA. FANCM is essential for monoubiquitination of FANCD2 and becomes hyperphosphorylated in response to DNA damage. Our data suggest an evolutionary link between Fanconi anemia-associated proteins and DNA repair; FANCM may act as an engine that translocates the Fanconi anemia core complex along DNA.

The vertebrate Hef ortholog is a component of the Fanconi anemia tumor-suppressor pathway

The helicase-associated endonuclease for fork-structured DNA (Hef) is an archaeabacterial protein that processes blocked replication forks. Here we have isolated the vertebrate Hef ortholog and investigated its molecular function. Disruption of this gene in chicken DT40 cells results in genomic instability and sensitivity to DNA cross-links. The similarity of this phenotype to that of cells lacking the Fanconi anemia-related (FA) tumor-suppressor genes led us to investigate whether Hef functions in this pathway. Indeed, we found a genetic interaction between the FANCC and Hef genes. In addition, Hef is a component of the FA nuclear protein complex that facilitates its DNA damage-inducible chromatin localization and the monoubiquitination of the FA protein FANCD2. Notably, Hef interacts directly with DNA structures that are intermediates in DNA replication. This discovery sheds light on the origins, regulation and molecular function of the FA tumor-suppressor pathway in the maintenance of genome stability.

Genome-wide requirements for resistance to functionally distinct DNA-damaging agents

The mechanistic and therapeutic differences in the cellular response to DNA-damaging compounds are not completely understood, despite intense study. To expand our knowledge of DNA damage, we assayed the effects of 12 closely related DNA-damaging agents on the complete pool of approximately 4,700 barcoded homozygous deletion strains of Saccharomyces cerevisiae. In our protocol, deletion strains are pooled together and grown competitively in the presence of compound. Relative strain sensitivity is determined by hybridization of PCR-amplified barcodes to an oligonucleotide array carrying the barcode complements. These screens identified genes in well-characterized DNA-damage-response pathways as well as genes whose role in the DNA-damage response had not been previously established. High-throughput individual growth analysis was used to independently confirm microarray results. Each compound produced a unique genome-wide profile. Analysis of these data allowed us to determine the relative importance of DNA-repair modules for resistance to each of the 12 profiled compounds. Clustering the data for 12 distinct compounds uncovered both known and novel functional interactions that comprise the DNA-damage response and allowed us to define the genetic determinants required for repair of interstrand cross-links. Further genetic analysis allowed determination of epistasis for one of these functional groups.

Low cytotoxicity of ecteinascidin 743 in yeast lacking the major endonucleolytic enzymes of base and nucleotide excision repair pathways

Ecteinascidin 743 (ET-743) is a promising antitumoral drug for the treatment of soft tissues sarcomas, becoming a good candidate for clinical trials. However, the molecular mechanism of how ET-743 induces cells death is poorly understood. The chemical structure of ET-743 suggests that it can form cytotoxic cross-links with proteins and DNA. Experiments with Escherichia coli and mammalian cells indicate that the nucleotide excision repair (NER) pathway promotes ET-743 cytotoxicity. We therefore analyzed cytotoxicity and tolerance to ET-743 in the yeast Saccharomyces cerevisiae, defective for NER and/or base excision repair (BER), either in single mutants or in combination with mutant alleles of genes encoding proteins involved in DNA translesion synthesis (TLS) and homologous recombination (HR). Treatment of haploid and diploid S. cerevisiae strains with ET-743 led to induced mutagenesis, mitotic gene conversion, and crossing-over. The results indicated that yeast strains lacking endonucleases of the NER and BER pathways are especially resistant for ET-743. The mutagenesis data points to a weak mutagenic activity of ET-743 in both WT and strains lacking BER/NER endonuclease, and that a mutant blocked in both BER and TLS totally lacks induced mutagenesis. The diploid strain shows an increase in the frequencies of crossing-over and mitotic recombination. These data lead us to propose a model for ET-743 action in eukaryotic cells, where the presence of BER and NER endonucleases results in cell death. However, ET-743 damage can be tolerated in BER and/or NER mutants by TLS (error-prone) or in combination with HR (error-free).

Expression of a human cytochrome p450 in yeast permits analysis of pathways for response to and repair of aflatoxin-induced DNA damage

Aflatoxin B1 (AFB1) is a human hepatotoxin and hepatocarcinogen produced by the mold Aspergillus flavus. In humans, AFB1 is primarily bioactivated by cytochrome P450 1A2 (CYP1A2) and 3A4 to a genotoxic epoxide that forms N7-guanine DNA adducts. A series of yeast haploid mutants defective in DNA repair and cell cycle checkpoints were transformed with human CYP1A2 to investigate how these DNA adducts are repaired. Cell survival and mutagenesis following aflatoxin B1 treatment was assayed in strains defective in nucleotide excision repair (NER) (rad14), postreplication repair (PRR) (rad6, rad18, mms2, and rad5), homologous recombinational repair (HRR) (rad51 and rad54), base excision repair (BER) (apn1 apn2), nonhomologous end-joining (NHEJ) (yku70), mismatch repair (MMR) (pms1), translesion synthesis (TLS) (rev3), and checkpoints (mec1-1, mec1-1 rad53, rad9, and rad17). Together our data suggest the involvement of homologous recombination and nucleotide excision repair, postreplication repair, and checkpoints in the repair and/or tolerance of AFB1-induced DNA damage in the yeast model. Rev3 appears to mediate AFB1-induced mutagenesis when error-free pathways are compromised. The results further suggest unique roles for Rad5 and abasic endonuclease-dependent DNA intermediates in regulating AFB1-induced mutagenicity.

Saccharomyces cerevisiae MPH1 gene, required for homologous recombination-mediated mutation avoidance, encodes a 3' to 5' DNA helicase

The MPH1 (mutator pHenotype 1) gene of Saccharomyces cerevisiae was identified on the basis of elevated spontaneous mutation rates of haploid cells deleted for this gene. Further studies showed that MPH1 functions to channel DNA lesions into an error-free DNA repair pathway. The Mph1 protein contains the seven conserved motifs of the superfamily 2 (SF2) family of nucleic acid unwinding enzymes. Genetic analyses have found epistasis of the mph1 deletion with mutations in the RAD52 gene group that mediates homologous recombination and DNA repair by homologous recombination. To begin dissecting the biochemical functions of the MPH1-encoded product, we have expressed it in yeast cells and purified it to near homogeneity. We show that Mph1 has a robust ATPase function that requires single-stranded DNA for activation. Consistent with its homology to members of the SF2 helicase family, we find a DNA helicase activity in Mph1. We present data to demonstrate that the Mph1 DNA helicase activity is fueled by ATP hydrolysis and has a 3' to 5' polarity with respect to the DNA strand on which this protein translocates. The DNA helicase activity of Mph1 is enhanced by the heterotrimeric single-stranded DNA binding protein replication protein A. These results, thus, establish Mph1 as an ATP-dependent DNA helicase, and the availability of purified Mph1 should facilitate efforts at deciphering the role of this protein in homologous recombination and mutation avoidance.