DIFFERENT DNA-POLYMERASES ARE INVOLVED IN THE SHORT-PATCH AND LONG-PATCH BASE EXCISION-REPAIR IN MAMMALIAN-CELLS

Mammalian cells possess two distinct pathways for completion of base e xcision repair (BER): the DNA polymerase beta (Pol beta)-dependent sho rt-patch pathway (replacement of one nucleotide), which is the main ro ute, and the long-patch pathway (resynthesis of 2-6 nucleotides), whic h is PCNA-dependent. To address the issue of how these two pathways sh are their role in BER the ability of Pol beta-defective mammalian cell extracts to repair a single abasic site constructed in a circular dup lex plasmid molecule was tested in a standard in vitro repair reaction . Pol beta-deficient extracts were able to perform both BER pathways. However, in the case of the short-patch BER, the repair kinetics was s ignificantly slower than with Pol beta-proficient extracts, while the efficiency of the long-patch synthesis was unaffected by the loss of P ol beta. The repair synthesis was fully dependent on PCNA for the repl acement of long patches. These data give the first evidence that in ce ll extracts DNA polymerases other than Pol beta are specifically invol ved in the long-patch BER. These DNA polymerases are also able to perf orm short patch BER in the absence of PCNA, although less efficiently than Pol beta. These findings lead to a novel model whereby the two BE R pathways are characterized by different protein requirements, and a functional redundancy at the level of DNA polymerases provides cells w ith backup systems.