Fig. 1.

Participation of DNA polymerases at replication fork in E. coli. Shown is a scheme delineating events that might occur upon Pol III HE creating a 3' terminal mismatch (M·m) by misinsertion in lagging or leading strand. The HE is depicted in a simplified form as two Pol III cores connected by the dimeric τ subunit. The most likely outcome of a misinsertion is removal of the misinserted nucleotide by the Pol III proofreading activity. However, occasionally proofreading might not occur, and the resulting stalling of HE may provide an opportunity for other polymerases to participate. Here, we have shown a dissociation of the Pol III core from the mismatch in the lagging strand, which may occur in a manner similar to the dissociation normally occurring at the end of an Okazaki fragment. This provides an opportunity for Pol II to act as a back-up proofreader (Banach-Orlowska et al., 2005) or for Pol IV or Pol V to extend the mismatch (mutator activity). Polymerase exchange is also possible in the leading strand, but possibly through a different mechanism that strongly favors action by Pol II. The function of Pol I appears limited to fixing the Okazaki fragment gap, generally in an error-free way. See Text for more details.