Accurate and Efficient Bypass of 8,5'-Cyclopurine-2'-Deoxynucleosides by Human and Yeast DNA Polymerase η

Reactive oxygen species (ROS), which can be produced during normal aerobic metabolism, can induce the formation of tandem DNA lesions, including 8,5'-cyclo-2'-deoxyadenosine (cyclo-dA) and 8,5'-cyclo-2'-deoxyguanosine (cyclo-dG). Previous studies have shown that cyclo-dA and cyclo-dG accumulate in cells and can block mammalian RNA polymerase II and replicative DNA polymerases. Here, we used primer extension and steady-state kinetic assays to examine the efficiency and fidelity for polymerase η to insert nucleotides opposite, and extend primer past, these cyclopurine lesions. We found that Saccharomyces cerevisiae and human polymerase η inserted 2'-deoxynucleotides opposite cyclo-dA, cyclo-dG, and their adjacent 5' nucleosides at fidelities and efficiencies that were similar as their respective undamaged nucleosides. Moreover, the yeast enzyme exhibited similar processivity in DNA synthesis on templates housing a cyclo-dA or cyclo-dG as those carrying an unmodified dA or dG; the human polymerase, however, dissociated from the primer-template complex after inserting one or two additional nucleotides after the lesion. Pol η's accurate and efficient bypass of cyclo-dA and cyclo-dG indicate that this polymerase is likely responsible for error-free bypass of these lesions, whereas mutagenic bypass of these lesions may involve other translesion synthesis DNA polymerases. Together, our results suggested that pol η may have an additional function in cells, i.e., to alleviate the cellular burden of endogenously induced DNA lesions, including cyclo-dA and cyclo-dG.