The in vivo characterization of translesion synthesis across UV-induced lesions in Saccharomyces cerevisiae: Insights into Polζ- and Polη-dependent frameshift mutagenesis

UV irradiation, a known carcinogen, induces the formation of dipyrimidine dimers with the predominant lesions being cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone adducts (6-4PPs). The relative roles of the yeast translesion synthesis DNA polymerases Polζ and Poη in UV survival and mutagenesis were examined using strains deficient in one or both polymerases. In addition, photoreactivation was used to specifically remove CPDs, thus allowing an estimate to be made of the relative contributions of CPDs vs. 6-4PPs to overall survival and mutagenesis. In terms of UV-induced mutagenesis, we focused on the +1 frameshift mutations detected by reversion of the lys2ΔA746 allele, as Polζ produces a distinct mutational signature in this assay. Results suggest that CPDs are responsible for most of the UV-associated toxicity as well as for the majority of UV-induced frameshift mutations in yeast. Although the presence of Polη generally suppresses UV-induced mutagenesis, our data suggest a role for this polymerase in generating some classes of +1 frameshifts. Finally, the examination of frameshift reversion spectra indicates a hierarchy between Polη and Polζ with respect to the bypass of UV-induced lesions.