Different DNA Polymerases Are Involved in the Short- and Long-Patch Base Excision Repair in Mammalian Cells

Abstract

Mammalian cells possess two distinct pathways for completion of base excision repair (BER): the DNA polymerase β (Pol β)-dependent short-patch pathway (replacement of one nucleotide), which is the main route, and the long-patch pathway (resynthesis of 2−6 nucleotides), which is PCNA-dependent. To address the issue of how these two pathways share their role in BER the ability of Pol β-defective mammalian cell extracts to repair a single abasic site constructed in a circular duplex plasmid molecule was tested in a standard in vitro repair reaction. Pol β-deficient extracts were able to perform both BER pathways. However, in the case of the short-patch BER, the repair kinetics was significantly slower than with Pol β-proficient extracts, while the efficiency of the long-patch synthesis was unaffected by the loss of Pol β. The repair synthesis was fully dependent on PCNA for the replacement of long patches. These data give the first evidence that in cell extracts DNA polymerases other than Pol β are specifically involved in the long-patch BER. These DNA polymerases are also able to perform short-patch BER in the absence of PCNA, although less efficiently than Pol β. These findings lead to a novel model whereby the two BER pathways are characterized by different protein requirements, and a functional redundancy at the level of DNA polymerases provides cells with backup systems.