Accuracy of lesion bypass by yeast and human DNA polymerase eta

DNA polymerase eta (Pol eta) functions in the error-free bypass of UV-induc ed DNA lesions, and a defect in Pol eta in humans causes the cancer-prone s yndrome, the variant form of xeroderma pigmentosum. Both yeast and human Po l eta replicate through a cis-syn thymine-thymine dimer (TT dimer) by inser ting two As opposite the two Ts of the dimer. Pol eta, however, is a low-fi delity enzyme, and it misinserts nucleotides with a frequency of approximat e to 10(-2) to 10(-3) opposite the two Ts of the TT dimer as well as opposi te the undamaged template bases. This low fidelity of nucleotide insertion seems to conflict with the role of Poll, in the error-free bypass of UV les ions. To resolve this issue, we have examined the ability of human and yeas t Pol eta to extend from paired and mispaired primer termini opposite a TT dimer by using steady-state kinetic assays. We find that Pol eta extends fr om mispaired primer termini on damaged and undamaged DNAs with a frequency of approximate to 10(-2) to 10(-3) relative to paired primer termini. Thus, after the incorporation of an incorrect nucleotide, Pol eta would dissocia te from the DNA rather than extend from the mispair. The resulting primer-t erminal mispair then could be subject to proofreading by a 3 ' -->5 ' exonu clease. Replication through a TT dimer by Pol eta then would be more accura te than that predicted from the fidelity of nucleotide incorporation alone.