Synergism between yeast nucleotide and base excision repair pathways in the protection against DNA methylation damage.

The treatment of cells with simple DNA methylating agents such as methyl methanesulfonate (MMS) results in genotoxic lesions, including 3-methyladenine which blocks DNA replication. All the organisms studied to date contain an alkylation-specific base excision repair pathway. In the yeast Saccharomyces cerevisiae, the base excision repair pathway is initiated by a Mag1 3-methyladenine DNA glycosylase that removes the damaged base, followed by the Apn1 apurinic/apyrimidinic endonuclease which cleaves the DNA strand at the abasic site for subsequent repair and synthesis. Several nucleotide excision repair pathway mutants display only slightly increased sensitivity to killing by MMS, indicating that nucleotide excision repair per se does not play a major role in the repair of DNA methylation damage. However, mag1 and apn1 mutants that are also defective in nucleotide excision repair are extremely sensitive to MMS-induced killing and the effects are synergistic. These observations suggest that nucleotide excision repair and alkylation-specific base excision repair provide alternative pathways for the repair of DNA methylation damage. In addition to their role in nucleotide excision repair, Rad1 and Rad10 form a complex that is involved in recombination repair. It was found that the apn1 rad1 and apn1 rad10 double mutants have a growth defect and are significantly more sensitive to MMS killing than apn1 rad2 and apn1 rad4 double mutants in a gradient plate assay. Furthermore, the apn1 rad1 double mutant increased both the spontaneous and MMS-induced mutation frequency. Thus, the recombination repair defects of rad1 and rad10 may confer an additional synergistic effect when combined with the apn1 mutation.