Mismatch extension ability of yeast and human DNA polymerase η

DNA polymerase η (Polη) functions in error-free replication of UV-damaged DNA, and in vitro it efficiently bypasses a cis-syn T-T dimer by incorporating two adenines opposite the lesion. Steady state kinetic studies have shown that both yeast and human Polη are low-fidelity enzymes, and they misincorporate nucleotides with a frequency of 10^-2-10^-3 on both undamaged and T-T dimer-containing DNA templates. To better understand the role of Polη in error-free translesion DNA synthesis, here we examine the ability of Polη to extend from base mismatches. We find that both yeast and human Polη extend from mismatched base pairs with a frequency of ∼10^-3 relative to matched base pairs. In the absence of efficient extension of mismatched primer termini, the ensuing dissociation of Polη from DNA may favor the excision of mismatched nucleotides by a proofreading exonuclease. Thus, we expect DNA synthesis by Polη to be more accurate than that predicted from the fidelity of nucleotide incorporation alone.