7,8-dihydro-8-oxoguanine (8-oxoG) is an abundant and mutagenic lesion produced in DNA exposed to free radicals and reactive oxygen species. In Saccharomyces cerevisiae, the OGG1 gene encodes the 8-oxoG DNA N-glycosylase/AP lyase (Ogg1), which is the functional homologue of the bacterial Fpg. Ogg1-deficient strains are spontaneous mutators that accumulate GC to TA transversions due to unrepaired 8-oxoG in DNA. In yeast, DNA mismatch repair (MMR) and translesion synthesis (TLS) by DNA polymerase eta also play a role in the prevention of the mutagenic effect of 8-oxoG. In the present study, we show the RAD18 and RAD6 genes that are required to initiate post-replication repair (PRR) are also involved in the prevention of mutations by 8-oxoG. Consistently, a synergistic increase in spontaneous CanR and Lys+ mutation rates is observed in the absence of Rad6 or Rad18 proteins in ogg1 mutant strains. Spectra of CaR mutations in ogg1 rad18 and ogg1 rad6 double mutants show a strong bias in the favor of GC to TA transversions, which are 137- and 189-fold higher than in the wild-type, respectively. The results also show that Poleta (RAD30 gene product) plays a critical role on the prevention of mutations at 8-oxoG, whereas Polzeta (REV3 gene product) does not. Our current model suggests that the Rad6-Rad18 complex targets Poleta at DNA gaps that result from the MMR-mediated excision of adenine mispaired with 8-oxoG, allowing error-free dCMP incorporation opposite to this lesion.
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