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

UV light-induced DNA lesions block the normal replication machinery. Eukary otic cells possess DNA polymerase eta (Pol eta), which has the ability to r eplicate past a cis-syn thymine thymine (TT) dimer efficiently and accurate ly, 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 abili ty to bypass a (6-4) TT lesion which distorts the DNA helix to a much great er extent than a cis-syn TT dimer, Opposite the 3' T of a (6-4) TT photopro duct, both yeast and human Pol eta preferentially insert a G residue, but t hey are unable to extend from the inserted nucleotide. DNA Pol eta, essenti al for UV induced mutagenesis, efficiently extends from the G residue inser ted opposite the 3' T of the (6-4) TT lesion by Pol eta, and Pol zeta inser ts the correct nucleotide A opposite the 5' T of the lesion. Thus, the effi cient bypass of the (6-4) TT photoproduct is achieved by the combined actio n of Pol eta and Pol zeta, wherein Pol eta inserts a nucleotide opposite th e 3' T of the lesion and Pol zeta extends from it. These biochemical observ ations are in concert with genetic studies in yeast indicating that mutatio ns occur predominantly at the 3' T of the (6-4) TT photoproduct and that th ese 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.