Fission yeast homologs of human XPC and CSB, rhp41 and rhp26, are involved in transcription-coupled repair of methyl methanesulfonate-induced DNA damage

Methyl methanesulfonate (MMS) methylates nitrogen atoms in purines, and predominantly produces 7-methylguanine and 3-methyladenine (3-meA). Previously, we showed that base excision repair (BER) and nucleotide excision repair (NER) synergistically function to repair MMS-induced DNA damage in the fission yeast Schizosaccharomyces pombe. Here, we studied the roles of NER components in repair of 3-meA and BER intermediates such as the AP site and single strand breaks. Mutants of rhp41(XPC homolog) and rhp26 (CSB homolog) exhibited moderate sensitivity to MMS. Transcription of the fbp1 gene, which is induced by glucose starvation, was strongly inhibited by MMS damage in rhp41 Delta and rhp26 Delta strains but not in wild type and 3-meA DNA glycosylase-deficient cells. The results indicate that Rhp41p and Rhp26p are involved in transcription-coupled repair (TCR) of MMS-induced DNA damage. In the BER pathway of S. pombe, AP lyase activity of Nth1p mainly incises the AP site to generate a 3'-blocked end, which is in turn converted to 3'-OH by Apn2p. Deletion of rad16 or rhp26 in the nth1 Delta strain greatly enhanced MMS sensitivity, suggesting that the AP site could also be corrected by TCR. Double mutant apn2 Delta/rad16 Delta exhibited hypersensitivity to MMS, implying that Rad16p provides a backup pathway for removal of the 3'-blocked end. Moreover, an rhp51 Delta strain was extremely sensitive to MMS and double mutants of nth1 Delta/rhp51 Delta and apn2 Delta/rhp51 Delta increased the sensitivity, suggesting that homologous recombination is necessary for repair of three different types of lesions, 3-meA, AP sites and 3'-blocked ends.