Influence of neighbouring base sequences on the mutagenesis induced by 7,8-dihydro-8-oxoguanine in yeast

We have analysed the influence of neighbouring base sequences on the mutagenesis induced by 7,8-dihydro-8-oxoguanine (8-oxoG or G^o), a typical oxidative lesion of DNA, using the yeast oligonucleotide transformation technique. Two oligonucleotides, oligo-CCG^o and oligo-CGG ^o, each possessing a single 8-oxoG residue and represented by the sequences 5′-CCG^o-3′ and 5′-CGG^o- 3′, respectively, were introduced into a chromosome of Saccharomyces cerevisiae and their mutagenic potentials were compared. In a wild-type strain, 8-oxoG showed very weak mutagenic potential in both cases. However, the lesion in 5′-CCG^o-3′ can cause efficient G-to-T transversion in a strain lacking the rad30 gene which encodes yeast DNA polymerase η (Ypolη). To explore the properties associated with this translesion synthesis (TLS), the same two oligonucleotides possessing an 8-oxoG were used as templates for a standing-start primer extension assay, and the nucleotide incorporation opposite 8-oxoG was investigated. We found that dATP incorporation opposite 8-oxoG with Ypolη was low for both sequences. In particular, very low dATP incorporation was observed for the 5′-CCG^o-3′ sequence. These results account for the efficient inhibition of mutagenesis by Ypolη. TLS plays an important role in one DNA sequence in terms of avoiding mutagenesis induced by 8-oxoG in yeast. In contrast, human yeast DNA polymerase η showed higher dATP incorporation rates even with the 5′-CCG ^o-3′ sequence.