Differential correction of lagging-strand replication errors made by DNA polymerases α and δ

Abstract

Mismatch repair (MMR) of replication errors requires DNA ends that can direct repair to the newly synthesized strand containing the error. For all but those organisms that use adeninemethylation to generate nicks, the source of these ends in vivo is unknown. One possibility is thatMMR may have a "special relation to the replication complex" [Wagner R, Jr., Meselson M (1976) Proc Natl Acad Sci USA 73:4135- 4139], perhaps one that allows 5' or 3' DNA ends associated with replication to act as strand discrimination signals. Here we examine this hypothesis, based on the logic that errors made by yeast DNA polymerase a (Pol a), which initiates Okazaki fragments during lagging-strand replication, will always be closer to a 5' end than will bemore internal errors generated byDNApolymerase d (Pol d),which takes over for Pol a to complete lagging-strand replication.Whenwe compared MMR efficiency for errors made by variant forms of these two polymerases,Msh2-dependent repair efficiencies formismatches made by Pol a were consistently higher than for those same mismatcheswhenmade by Pol d. Thus, one special relationship between MMR and replication is thatMMR is more efficient for the least accurate of the major replicative polymerases, exonuclease-deficient Pol a. This observation is consistentwith the close proximity and possible use of 5' ends of Okazaki fragments for strand discrimination, which could increase the probability of Msh2-dependent MMR by 5' excision, by aMsh2-dependent strand displacementmechanism, or both.