Aphidicolin-resistant and sensitive base excision repair in wild type and DNA polymerase b defective mouse cells

Several DNA polymerases (Pols) can add complementary bases at the gap created during the base excision repair (BER). To characterize the BER resynthesis step, the repair of a single abasic site by wild-type and Pol ²-defective mouse cell extracts was analysed in the presence of aphidicolin, a specific inhibitor of replicative Pols. We show that there is a competition between distributive and processive Pols for the nucleotide addition at the primer terminus. In wild-type cell extracts, the initial nucleotide insertion involves mainly Pol ² but the elongation step is carried out by a replicative Pol. Conversely, in Pol ²-null cell extracts the synthesis step is carried out by a replicative Pol without any switching to an auxiliary polymerase. We present evidence that short-patch repair synthesis occurs even in the absence of both Pol ² and replicative Pols. Exogeneously added purified human Pol » was unable to stimulate this back-up synthesis.