Spontaneous mutation, oxidative DNA damage, and the roles of base and nucleotide excision repair in the yeast Saccharomyces cerevisiae

The OGG1 gene of Saccharomyces cerevisiae encodes a DNA glycosylase that ex cises 7,8-dihydro-8-oxoguanine (8-OxoG). When compared to wild-type, ogg1 m utants show an increase in the frequency of GC to TA transversions, indicat ing a role for Ogg1 in the repair of 8-OxoG. Here we report an increased fr equency of forward mutation to canavanine resistance in mutants defective i n the nucleotide excision repair (NER) gene RAD14. This was not increased f urther in strains additionally defective in OGG1. However, when compared to strains solely defective in OGG1, ogg1rad14 mutants displayed an increase in spontaneous GC to TA transversions. Intriguingly, reversion of the lys1 -1 ochre allele was not increased in rad14 mutants, suggesting that oxidati ve base damage may only represent a substrate for NER in certain regions of the genome. We also examined repair of oxidative DNA damage by transformin g mutant strains with plasmid DNA treated with methylene blue plus visible light. Mutants defective in OGG1 showed no significant reduction in transfo rmation efficiency compared with wild-type strains. In contrast, disruption of RAD14 reduced the efficiency of transformation, yet there was no furthe r decrease in an ogg1 rad14 mutant. This strongly supports a role for NER i n the repair of oxidative base damage in yeast, and differs from similar ex periments carried out in E. coli, where transformation efficiency is only r educed in mutants defective in both fpg and uvrA. Finally, the repair of Fp g-sensitive sites was examined at the MAT alpha and HML alpha mating type l oci, and NER was found to play a role in their removal. Copyright (C) 1999 John Wiley & Sons, Ltd.