Non-replicative helicases at the replication fork

Reactivation of stalled or collapsed replication forks is an essential process in bacteria. Restart systems operate to restore the 5′ → 3′ replicative helicase, DnaB, to the lagging-strand template. However, other non-replicative 3′ → 5′ helicases play an important role in the restart process as well. Here we examine the DNA-binding specificity of three of the latter group, PriA, Rep, and UvrD. Only PriA and Rep display structure-specific fork binding. Interestingly, their specificity is opposite: PriA binds a leading-strand fork, presumably reflecting its restart activity in directing loading of DnaB to the lagging-strand template. Rep binds a lagging-strand fork, presumably reflecting its role in partially displacing Okazaki fragments that originate near the fork junction. This activity is necessary for generating a single-stranded landing pad for DnaB. While UvrD shows little structure-specificity, there is a slight preference for lagging-strand forks, suggesting that there might be some redundancy between Rep and UvrD and possibly explaining the observed synthetic lethality that occurs when mutations in the genes encoding these two proteins are combined.