Role of human DNA polymerase as an extender in translesion synthesis

Human DNA polymerase (Pol) is a member of the Y family of DNA polymerases. Unlike Pol , another member of this family, which carries out efficient translesion synthesis through various DNA lesions, the role of Pol in lesion bypass has remained unclear. Recent studies, however, have indicated that Pol is a proficient extender of mispaired primer termini on undamaged DNAs and also on cis-syn thymine-thymine (T-T) dimer-containing DNA. Here we determine whether Pol can promote the efficient bypass of DNA lesions by extending from the nucleotides inserted opposite the lesion site by another DNA polymerase. From steady-state kinetic analyses, we find that Pol is highly inefficient at incorporating nucleotides opposite an O6-methyl guanine (m6G) lesion, but it efficiently extends from the T or C nucleotide incorporated opposite this lesion by Pol . Opposite an 8-oxoguanine (8-oxoG) lesion, Pol efficiently inserts an A and then proficiently extends from it. Importantly, for both these DNA lesions, however, the most efficient bypass occurs when Pol is combined with Pol ; in this reaction, Pol performs the extension step after the incorporation of nucleotides opposite these lesion sites by Pol . These studies reveal a role for Pol in the extension phase of lesion bypass.