EVIDENCE THAT MUTY AND MUTM COMBINE TO PREVENT MUTATIONS BY AN OXIDATIVELY DAMAGED FORM OF GUANINE IN DNA

It has been previously shown both in vivo and in vitro that DNA synthesis past an oxidatively damaged form of guanine, 7,8-dihydro-8-oxoguanine (8-oxoG), can result in the misincorporation of adenine (A) opposite the 8-oxodG. In this study we show that MutY glycosylase is active on a site-specific, oxidatively damaged A/8-oxoG mispair and that it removes the undamaged adenine from this mispair. Strains that lack active MutY protein have elevated rates of G.C --> T.A transversions. We find that the mutator phenotype of a mutY strain can be fully complemented by overexpressing MutM protein (Fpg protein) from a plasmid clone. The MutM protein removes 8-oxoG lesions from DNA. In addition, we have isolated a strain with a chromosomal mutation that suppresses the mutY phenotype and found that this suppressor also overexpresses MutM. Finally, a mutY mutM double mutant has a 25- to 75-fold higher mutation rate than either mutator alone. The data strongly suggest that MutY is part of an intricate repair system directed against 8-oxoG lesions in nucleic acids and that the primary function of MutY in vivo is the removal of adenines that are misincorporated opposite 8-oxoG lesions during DNA synthesis.