Role of DNA polymerase η in the bypass of a (6-4) TT photoproduct

UV light-induced DNA lesions block the normal replication machinery. Eukaryotic cells possess DNA polymerase eta (Pol eta), which has the ability to replicate past a cis-syn thymine thymine (TT) dimer efficiently and accurately, and mutations in human Pol eta result in the cancer-prone syndrome, the variant form of xeroderma pigmentosum. Here, we test Pol eta for its ability to bypass a (6-4) TT lesion which distorts the DNA helix to a much greater extent than a cis-syn TT dimer, Opposite the 3' T of a (6-4) TT photoproduct, both yeast and human Pol eta preferentially insert a G residue, but they are unable to extend from the inserted nucleotide. DNA Pol eta, essential for UV induced mutagenesis, efficiently extends from the G residue inserted opposite the 3' T of the (6-4) TT lesion by Pol eta, and Pol zeta inserts the correct nucleotide A opposite the 5' T of the lesion. Thus, the efficient bypass of the (6-4) TT photoproduct is achieved by the combined action of Pol eta and Pol zeta, wherein Pol eta inserts a nucleotide opposite the 3' T of the lesion and Pol zeta extends from it. These biochemical observations are in concert with genetic studies in yeast indicating that mutations occur predominantly at the 3' T of the (6-4) TT photoproduct and that these mutations frequently exhibit a 3' T -->C change that would result from the insertion of a G opposite the 3' T of the (6-4) TT lesion.