The role of DNA polymerase activity in human non-homologous end joining

In mammalian cells, DNA double-strand breaks are repaired mainly by non-hom ologous end joining, which modifies and ligates two DNA ends without requir ing extensive base pairing interactions for alignment. We investigated the role of DNA polymerases in DNA-PK-dependent end joining of restriction-dige sted plasmids in vitro and in vivo. Rejoining of DNA blunt ends as well as those with partially complementary 5' or 3' overhangs was stimulated by 20- 53% in HeLa cell-free extracts when dNTPs were included, indicating that pa rt of the end joining is dependent on DNA synthesis. This DNA synthesis-dep endent end joining was sensitive to aphidicolin, an inhibitor of a-like DNA polymerases. Furthermore, antibodies that neutralize the activity of DNA p olymerase a were found to strongly inhibit end joining in vitro, whereas ne utralizing antibodies directed against DNA polymerases beta and epsilon did not. DNA sequence analysis of end joining products revealed two prominent modes of repair, one of which appeared to be dependent on DNA synthesis. Id entical products of end joining were recovered from HeLa cells after transf ection with one of the model substrates, suggesting that the same end joini ng mechanisms also operate in vivo. Fractionation of cell extracts to separ ate PCNA as well as depletion of cell extracts for PCNA resulted in a moder ate but significant reduction in end joining activity, suggesting a potenti al role in a minor repair pathway.