DNA damage alters DNA polymerase to a form that exhibits increased discrimination against modified template bases and mismatched primers

Human DNA polymerase (Pol 4), a key enzyme in chromosomal replication, is a heterotetramer composed of the p125, p50, p68 and p12 subunits. Genotoxic agents such as UV and alkylating chemicals trigger a DNA damage response in which Pol 4 is converted to a trimer (Pol 3) by degradation of p12. We show that Pol 3 has altered enzymatic properties: it is less able to perform translesion synthesis on templates containing base lesions (O-6-MeG, 8-oxoG, an abasic site or a thymine-thymine dimer); a greater proofreading activity; an increased exonuclease/polymerase activity ratio; a decreased tendency for the insertion of wrong nucleotides, and for the extension of mismatched primers. Overall, our findings indicate that Pol 3 exhibits an enhanced ability for the detection of errors in both primers and templates over its parent enzyme. These alterations in Pol 3 show that p12 plays a major role in Pol 4 catalytic functions, and provides significant insights into the rationale for the conversion of Pol 4 to Pol 3 in the cellular response to DNA damage.