MMS2, encoding a ubiquitin-conjugating-enzyme-like protein, is a member of the yeast error-free postreplication repair pathway

MPH1, a yeast gene encoding a DEAH protein, plays a role in protection of the genome from spontaneous and chemically induced damage

We have characterized the MPH1 gene from Saccharomyces cerevisiae. mph1 mutants display a spontaneous mutator phenotype. Homologs were found in archaea and in the EST libraries of Drosophila, mouse, and man. Mph1 carries the signature motifs of the DEAH family of helicases. Selected motifs were shown to be necessary for MPH1 function by introducing missense mutations. Possible indirect effects on translation and splicing were excluded by demonstrating nuclear localization of the protein and splicing proficiency of the mutant. A mutation spectrum did not show any conspicuous deviations from wild type except for an underrepresentation of frameshift mutations. The mutator phenotype was dependent on REV3 and RAD6. The mutant was sensitive to MMS, EMS, 4-NQO, and camptothecin, but not to UV light and X rays. Epistasis analyses were carried out with representative mutants from various repair pathways (msh6, mag1, apn1, rad14, rad52, rad6, mms2, and rev3). No epistatic interactions were found, either for the spontaneous mutator phenotype or for MMS, EMS, and 4-NQO sensitivity. mph1 slightly increased the UV sensitivity of mms2, rad6, and rad14 mutants, but no effect on X-ray sensitivity was observed. These data suggest that MPH1 is not part of a hitherto known repair pathway. Possible functions are discussed.

DNA damage-induced mutation: tolerance via translesion synthesis

Translesion synthesis (TLS) appears to be required for most damage-induced mutagenesis in the yeast Saccharomyces cerevisiae, whether the damage arises from endogenous or exogenous sources. Thus, the production of such mutations seems to occur primarily as a consequence of the tolerance of DNA lesions rather than an error-prone repair mechanism. Tolerance via TLS in yeast involves proteins encoded by members of the RAD6 epistasis group for the repair of ultraviolet (UV) photoproducts, in particular two non-essential DNA polymerases that catalyse error-free or error-prone TLS. Homologues of these RAD6 group proteins have recently been discovered in rodent and/or human cells. Furthermore, the operation of error-free TLS in humans has been linked to a reduced risk of UV-induced skin cancer, whereas mutations generated by error-prone TLS may increase the risk of cancer. In this article, we review and link the evidence for translesion synthesis in yeast, and the involvement of nonreplicative DNA polymerases, to recent findings in mammalian cells.

REV3 is required for spontaneous but not methylation damage-induced mutagenesis of Saccharomyces cerevisiae cells lacking O6-methylguanine DNA methyltransferase

O6-methylguanine (O6-MeG) DNA methyltransferase (MTase) removes the methyl group from a DNA lesion and directly restores DNA structure. It has been shown previously that bacterial and yeast cells lacking such MTase activity are not only sensitive to killing and mutagenesis by DNA methylating agents, but also exhibit an increased spontaneous mutation rate. In order to understand molecular mechanisms of endogenous DNA alkylation damage and its effects on mutagenesis, we determined the spontaneous mutational spectra of the SUP4-o gene in various Saccharomyces cerevisiae strains. To our surprise, the mgt1 mutant deficient in DNA repair MTase activity exhibited a significant increase in G:C-->C:G transversions instead of the expected G:C-->A:T transition. Its mutational distribution strongly resembles that of the rad52 mutant defective in DNA recombinational repair. The rad52 mutational spectrum has been shown to be dependent on a mutagenesis pathway mediated by REV3. We demonstrate here that the mgt1 mutational spectrum is also REV3-dependent and that the rev3 deletion offsets the increase of the spontaneous mutation rate seen in the mgt1 strains. These results indicate that the eukaryotic mutagenesis pathway is directly involved in cellular processing of endogenous DNA alkylation damage possibly by the translesion bypass of lesions at the cost of G:C-->C:G transversion mutations. However, the rev3 deletion does not affect methylation damage-induced killing and mutagenesis of the mgt1 mutant, suggesting that endogenous alkyl lesions may be different from O6-MeG.

Methionine reduces spontaneous and alkylation-induced mutagenesis in Saccharomyces cerevisiae cells deficient in O6-methylguanine-DNA methyltransferase

The exposure of DNA to reactive intracellular metabolites is thought to be a major cause of spontaneous mutagenesis. DNA alkylation is implicated in the above process by the fact that bacterial and yeast cells lacking DNA alkylation-specific repair genes exhibit elevated spontaneous mutation rates. The origin of the intracellular alkylating molecules is not clear; however, S-adenosylmethionine (SAM) has been proposed as one source because it has a reactive methyl group known to methylate proteins and DNA. We supplemented yeast cultures with excess methionine and examined the effects of increased endogenous SAM concentration on spontaneous and alkylation-induced mutagenesis in the absence of various DNA repair pathways. Our results show that either the excess methionine, or the increased SAM produced as a result of this treatment, is able to protect yeast cells from mutagenesis, and that this effect is alkylation-damage-specific. The protective effect was observed only in the mgt1 mutant deficient in the O(6)-methylguanine-DNA repair methyltransferase, but not in the wild type or other DNA repair-deficient strains, indicating that the protection is specific for O-methyl lesions. Thus, our results may lend support to the recently reported chemopreventive effect of SAM in rodents and further suggest that the observed tumor prevention by SAM may be, in part, due to its suppression of spontaneous mutagenesis in mammals. Given that a strong correlation has been established between O(6)-methylguanine and carcinogenicity, this study may offer a novel approach to preventing carcinogenesis.

Genetic interactions between error-prone and error-free postreplication repair pathways in Saccharomyces cerevisiae

Evidence obtained from recent studies supports the existence of an error-free postreplication repair (PRR) and a mutagenesis pathway within the Saccharomyces cerevisiae RAD6 DNA repair group. The MMS2 gene is the only known yeast gene involved in error-free PRR that, when mutated, significantly increases the spontaneous mutation rate. In this study, the mutational spectrum of the mms2 mutator was determined and compared to the wild type strain. In addition, mutagenenic effects and genetic interactions of the mms2 mutator and rev3 anti-mutator were examined with respect to forward mutations, frameshift reversions as well as amber and ochre suppressions. It was concluded from these results that the mms2 mutator phenotype is largely dependent on the functional REV3 gene. The synergistic effects of mms2 and rev3 mutations towards killing by a variety of DNA-damaging agents ruled out the possibility that MMS2 simply acts to suppress REV3 activity and favored the hypothesis that MMS2 and REV3 form two alternative subpathways within the RAD6 DNA repair pathway. Taken together, we propose that two pathways represented by MMS2 and REV3 deal with a similar range of endogenous and environmental DNA damage but with different biological consequences, namely, error-free repair and mutagenesis, respectively.

The ubiquitin system in gametogenesis

Ubiquitin is a ubiquitous and highly conserved protein of 76 amino acid residues, that can be covalently attached to cellular acceptor proteins. The attachment of ubiquitin to target proteins is achieved through a multi-step enzymatic pathway, which involves activities of ubiquitin-activating E1 enzymes, ubiquitin-conjugating E2 enzymes, and ligating E3 enzymes. Mono- or poly-ubiquitination of proteins can lead to protein degradation or modification of protein activity. Many components of the complex ubiquitin system show remarkable evolutionary conservation, from yeast to mammalian species. The ubiquitin system is essential to all eukaryotic cells. Among others, several signal transduction cascades show involvement of the ubiquitin system, but there are currently little data supporting a specific role of the ubiquitin system in hormonal control of reproduction. Interestingly, during gametogenesis, many specialized and important aspects of the ubiquitin system become apparent. Components of the ubiquitin system appear to be involved in different steps and processes during gametogenesis, including control of meiosis, and reorganization of chromatin structure.

Noncanonical MMS2-encoded ubiquitin-conjugating enzyme functions in assembly of novel polyubiquitin chains for DNA repair

Ubiquitin-conjugating enzyme variant (UEV) proteins resemble ubiquitin-conjugating enzymes (E2s) but lack the defining E2 active-site residue. The MMS2-encoded UEV protein has been genetically implicated in error-free postreplicative DNA repair in Saccharomyces cerevisiae. We show that Mms2p forms a specific heteromeric complex with the UBC13-encoded E2 and is required for the Ubc13p-dependent assembly of polyubiquitin chains linked through lysine 63. A ubc13 yeast strain is UV sensitive, and single, double, and triple mutants of the UBC13, MMS2, and ubiquitin (ubiK63R) genes display a comparable phenotype. These findings support a model in which an Mms2p/Ubc13p complex assembles novel polyubiquitin chains for signaling in DNA repair, and they suggest that UEV proteins may act to increase diversity and selectivity in ubiquitin conjugation.

Role of Saccharomyces cerevisiae chromatin assembly factor-I in repair of ultraviolet radiation damage in vivo

In vitro, the protein complex Chromatin Assembly Factor-I (CAF-I) from human or yeast cells deposits histones onto DNA templates after replication. In Saccharomyces cerevisiae, the CAC1, CAC2, and CAC3 genes encode the three CAF-I subunits. Deletion of any of the three CAC genes reduces telomeric gene silencing and confers an increase in sensitivity to killing by ultraviolet (UV) radiation. We used double and triple mutants involving cac1Delta and yeast repair gene mutations to show that deletion of the CAC1 gene increases the UV sensitivity of cells mutant in genes from each of the known DNA repair epistasis groups. For example, double mutants involving cac1Delta and excision repair gene deletions rad1Delta or rad14Delta showed increased UV sensitivity, as did double mutants involving cac1Delta and deletions of members of the RAD51 recombinational repair group. cac1Delta also increased the UV sensitivity of strains with defects in either the error-prone (rev3Delta) or error-free (pol30-46) branches of RAD6-mediated postreplicative DNA repair but did not substantially increase the sensitivity of strains carrying null mutations in the RAD6 or RAD18 genes. Deletion of CAC1 also increased the UV sensitivity and rate of UV-induced mutagenesis in rad5Delta mutants, as has been observed for mutants defective in error-free postreplicative repair. Together, these data suggest that CAF-I has a role in error-free postreplicative damage repair and may also have an auxiliary role in other repair mechanisms. Like the CAC genes, RAD6 is also required for gene silencing at telomeres. We find an increased loss of telomeric gene silencing in rad6Delta cac1Delta and rad18Delta cac1Delta double mutants, suggesting that CAF-I and multiple factors in the postreplicative repair pathway influence chromosome structure.

The products of the yeast MMS2 and two human homologs (hMMS2 and CROC-1) define a structurally and functionally conserved Ubc-like protein family

Eukaryotic genes encoding ubiquitin-congugating enzyme (Ubc)-like proteins have been isolated from both human and yeast cells. The CROC-1 gene was isolated by its ability to transactivate c- fos expression in cell culture through a tandem repeat enhancer sequence. The yeast MMS2 gene was cloned by its ability to complement the methyl methanesulfonate sensitivity of the mms2-1 mutant and was later shown to be involved in DNA post-replication repair. We report here the identification of a human MMS2 ( hMMS2 ) cDNA encoding a novel human Ubc-like protein. hMMS2 and CROC-1 share >90% amino acid sequence identity, but their DNA probes hybridize to distinct transcripts. hMMS2 and CROC-1 also share approximately 50% identity and 75% similarity with the entire length of yeast Mms2. Unlike CROC-1 , whose transcript appears to be elevated in all tumor cell lines examined, the hMMS2 transcript is only elevated in some tumor cell lines. Collectively, these results indicate that eukaryotic cells may contain a highly conserved family of Ubc-like proteins that play roles in diverse cellular processes, ranging from DNA repair to signal transduction and cell differentiation. The hMMS2 and CROC-1 genes are able to functionally complement the yeast mms2 defects with regard to sensitivity to DNA damaging agents and spontaneous mutagenesis. Conversely, both MMS2 and hMMS2 were able to transactivate a c- fos - CAT reporter gene in Rat-1 cells in a transient co-transfection assay. We propose that either these proteins function in a common cellular process, such as DNA repair, or they exert their diverse biological roles through a similar biochemical interaction relative to ubiquitination.