Mutagenicity of N3-methyladenine: A multi-translesion polymerase affair

We recently demonstrated that Polζ and Rev1 contribute to alleviate the lethal effects of Me-lex, which selectively generates 3-methyladenine, by error prone lesion bypass. In order to determine the role of Polη in the biological fate of Me-lex induced lesions, the RAD30 (Polη) gene was deleted in the yIG397 parental strain and in its rev3 (Polζ) derivative, and the strains transformed with plasmid DNA damaged in vitro by Me-lex. While deletion of RAD30 increased the toxicity of Me-lex, the impact on mutagenicity varied depending on the concentration of Me-lex induced DNA damage and the overall TLS capacity of the cells. For the first time the Me-lex induced mutation spectrum in rad30 strain was determined and compared with the spectrum previously determined in WT strain. Overall, the two mutation spectra were not significantly different. The effect on mutation frequency and the features of the Me-lex induced mutation spectra were suggestive of error prone (significant decrease of mutation frequency and significant decrease of AT > TA at a mutation hotspot in rad30 vs RAD30) but also error free (significant increase of AT > GC in rad30 vs RAD30) Polη dependent bypass of lesions. In summary, our previous results with Polζ and Rev1 mutants, the present results with Polη, and the known physical and functional interactions among TLS proteins, lead us to propose that the bypass of Me-lex induced lesions is a multi-DNA polymerases process that is mostly effective when all three yeast TLS polymerases are present.