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DNA base excision repair (BER) is initiated by DNA glycosylases that recognize and remove damaged bases. The phosphate backbone adjacent to the resulting apurinic/apyrimidinic (AP) site is then cleaved by an AP endonuclease or glycosylase-associated AP lyase to invoke subsequent BER steps. We have used a genetic approach in Saccharomyces cerevisiae to determine whether or not AP sites are blocks to DNA replication and the biological consequences if AP sites persist in the genome. We previously reported that yeast cells deficient in the two AP endonucleases (apn1 apn2 double mutant) are extremely sensitive to killing by a model DNA alkylating agent methyl methanesulfonate (MMS) and that this sensitivity can be reduced by deleting the MAG1 3-methyladenine DNA glycosylase gene. Here we report that in the absence of the AP endonucleases, deletion of two Escherichia coli endonuclease III homologs, NTG1 and NTG2, partially suppresses MMS-induced killing, which indicates that the AP lyase products are deleterious unless they are further processed by an AP endonuclease. The severe MMS sensitivity seen in AP endonuclease deficient strains can also be rescued by treatment of cells with the AP lyase inhibitor methoxyamine, which suggests that the product of AP lyase action on an AP site is indeed an extremely toxic lesion. In addition to the AP endonuclease interactions, deletion of NTG1 and NTG2 enhances the mag1 mutant sensitivity to MMS, whereas overexpression of MAG1 in either the ntg1 or ntg2 mutant severely affects cell growth. These results help to delineate alkylation base lesion flow within the BER pathway.

DNA base excision repair (BER) is initiated by DNA glycosylases that recognize and remove damaged bases. The phosphate backbone adjacent to the resulting apurinic/apyrimidinic (AP) site is then cleaved by an AP endonuclease or glycosylase-associated AP lyase to invoke subsequent BER steps. We have used a genetic approach in Saccharomyces cerevisiae to address whether AP sites are blocks to DNA replication and the biological consequences if AP sites persist in the genome. We found that yeast cells deficient in the two AP endonucleases (apn1 apn2 double mutant) are extremely sensitive to killing by methyl methanesulfonate (MMS), a model DNA alkylating agent. Interestingly, this sensitivity can be reduced up to 2500-fold by deleting the MAG1 3-methyladenine DNA glycosylase gene, suggesting that Mag1 not only removes lethal base lesions, but also benign lesions and possibly normal bases, and that the resulting AP sites are highly toxic to the cells. This rescuing effect appears to be specific for DNA alkylation damage, since the mag1 mutation reduces killing effects of two other DNA alkylating agents, but does not alter the sensitivity of apn cells to killing by UV, gamma-ray or H(2)O(2). Our mutagenesis assays indicate that nearly half of spontaneous and almost all MMS-induced mutations in the AP endonuclease-deficient cells are due to Mag1 DNA glycosylase activity. Although the DNA replication apparatus appears to be incapable of replicating past AP sites, Polzeta-mediated translesion synthesis is able to bypass AP sites, and accounts for all spontaneous and MMS-induced mutagenesis in the AP endonuclease-deficient cells. These results allow us to delineate base lesion flow within the BER pathway and link AP sites to other DNA damage repair and tolerance pathways.

The RAD6 postreplication repair and mutagenesis pathway is the only major radiation repair pathway yet to be extensively characterized. It has been previously speculated that the RAD6 pathway consists of two parallel subpathways, one error free and another error prone (mutagenic). Here we show that the RAD6 group genes can be exclusively divided into three rather than two independent subpathways represented by the RAD5, POL30, and REV3 genes; the REV3 pathway is largely mutagenic, whereas the RAD5 and the POL30 pathways are deemed error free. Mutants carrying characteristic mutations in each of the three subpathways are phenotypically indistinguishable from a single mutant such as rad18, which is defective in the entire RAD6 postreplication repair/tolerance pathway. Furthermore, the rad18 mutation is epistatic to all single or combined mutations in any of the above three subpathways. Our data also suggest that MMS2 and UBC13 play a key role in coordinating the response of the error-free subpathways; Mms2 and Ubc13 form a complex required for a novel polyubiquitin chain assembly, which probably serves as a signal transducer to promote both RAD5 and POL30 error-free postreplication repair pathways. The model established by this study will facilitate further research into the molecular mechanisms of postreplication repair and translesion DNA synthesis. In view of the high degree of sequence conservation of the RAD6 pathway genes among all eukaryotes, the model presented in this study may also apply to mammalian cells and predicts links to human diseases.

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.

Among the three Saccharomyces cerevisiae DNA repair epistasis groups, the RAD6 group is the most complicated and least characterized, primarily because it consists of two separate repair pathways: an error-free postreplication repair pathway, and a mutagenesis pathway. The rad6 and rad18 mutants are defective in both pathways, and the rev3 mutant affects only the mutagenesis pathway, but a yeast gene that is involved only in error-free postreplication repair has not been reported. We cloned the MMS2 gene from a yeast genomic library by functional complementation of the mms2-1 mutant [Prakash, L. & Prakash, S. (1977) Genetics 86, 33-55]. MMS2 encodes a 137-amino acid, 15.2-kDa protein with significant sequence homology to a conserved family of ubiquitin-conjugating (Ubc) proteins. However, Mms2 does not appear to possess Ubc activity. Genetic analyses indicate that the mms2 mutation is hypostatic to rad6 and rad18 but is synergistic with the rev3 mutation, and the mms2 mutant is proficient in UV-induced mutagenesis. These phenotypes are reminiscent of a pol30-46 mutant known to be impaired in postreplication repair. The mms2 mutant also displayed a REV3-dependent mutator phenotype, strongly suggesting that the MMS2 gene functions in the error-free postreplication repair pathway, parallel to the REV3 mutagenesis pathway. Furthermore, with respect to UV sensitivity, mms2 was found to be hypostatic to the rad6Delta1-9 mutation, which results in the absence of the first nine amino acids of Rad6. On the basis of these collective results, we propose that the mms2 null mutation and two other allele-specific mutations, rad6Delta1-9 and pol30-46, define the error-free mode of DNA postreplication repair, and that these mutations may enhance both spontaneous and DNA damage-induced mutagenesis.

The Saccharomyces cerevisiae REV3 gene encodes the catalytic subunit of a non-essential DNA polymerase zeta, which is required for mutagenesis. The rev3 mutants significantly reduce both spontaneous and DNA damage-induced mutation rates. We have identified human cDNA clones from two different libraries whose deduced amino acid sequences bear remarkable homology to the yeast Rev3, and named this gene hREV3. The hREV3 gene was mapped to chromosome 1p32-33 by fluorescence in situ hybridization. The hREV3 encodes an mRNA of >10 kb, and its expression varies in different tissues and appears to be elevated in some but not all of the tumor cell lines we have examined. In light of recent reports of a putative mouse REV3, these results indicate that mammalian cells may also contain a mutagenic pathway which aids in cell survival at the cost of increased mutation.

mms4-1 is one of several Saccharomyces cerevisiae mutants that exhibit an increased sensitivity to methyl methanesulfonate (MMS), but not to UV or X-rays. We have isolated the MMS4 gene by functional complementation of the MMS-sensitive phenotype in the mms4-1 strain. The MMS4 gene encodes a 691-amino acid, 78.7-kDa protein. The deduced Mms4 protein does not show significant homology to any of the known proteins in the database. However, several putative functional domains suggest that it may be a nuclear protein capable of interacting with other proteins. Examination of the mms4delta mutant phenotype indicates that the mutation not only sensitizes DNA to methylating and ethylating agents, but also to other DNA damage that blocks DNA replication. However, the mms4delta mutant appears to be more sensitive to chronic treatment than to acute treatment by DNA-damaging agents. Furthermore, the spontaneous mutation rate increases significantly in the mms4delta mutant. Mms4 alone, when fused to a Gal4 DNA-binding domain, is able to activate P(GAL1)-lacZ and P(GAL1)-HIS3 reporter genes in a two-hybrid system; the Mms4 transactivation domain maps to the highly acidic N-terminal region. These results collectively suggest that Mms4 may function as a transcriptional (co)activator and play an important role in DNA repair and/or synthesis.

The major genotoxicity of methyl methanesulfonate (MMS) is due to the production of a lethal 3-methyladenine (3MeA) lesion. An alkylation-specific base-excision repair pathway in yeast is initiated by a Mag1 3MeA DNA glycosylase that removes the damaged base, followed by an Apn1 apurinic/ apyrimidinic endonuclease that cleaves the DNA strand at the abasic site for subsequent repair. MMS is also regarded as a radiomimetic agent, since a number of DNA radiation-repair mutants are also sensitive to MMS. To understand how these radiation-repair genes are involved in DNA methylation repair, we performed an epistatic analysis by combining yeast mag1 and apn1 mutations with mutations involved in each of the RAD3, RAD6 and RAD52 groups. We found that cells carrying rad6, rad18, rad50 and rad52 single mutations are far more sensitive to killing by MMS than the mag1 mutant, that double mutants were much more sensitive than either of the corresponding single mutants, and that the effects of the double mutants were either additive or synergistic, suggesting that post-replication and recombination-repair pathways recognize either the same lesions as MAG1 and APN1, or else some differ- ent lesions produced by MMS treatment. Lesions handled by recombination and post replication repair are not simply 3MeA, since over-expression of the MAG1 gene does not offset the loss of these pathways. Based on the above analyses, we discuss possible mechanisms for the repair of methylation damage by various pathways.