Regression of replication forks stalled by leading-strand template damage: I. Both RecG and RuvAB catalyze regression, but RuvC cleaves the holliday junctions formed by RecG preferentially.

The orderly progression of replication forks formed at the origin of replication in Escherichia coli is challenged by encounters with template damage, slow moving RNA polymerases, and frozen DNA-protein complexes that stall the fork. These stalled forks are foci for genomic instability and must be reactivated. Many models of replication fork reactivation invoke nascent strand regression as an intermediate in the processing of the stalled fork. We have investigated the replication fork regression activity of RecG and RuvAB, two proteins commonly thought to be involved in the process, using a reconstituted DNA replication system where the replisome is stalled by collision with leading-strand template damage. We find that both RecG and RuvAB can regress the stalled fork in the presence of the replisome and SSB; however, RuvAB generates a completely unwound product consisting of the paired nascent leading and lagging strands, whereas RuvC cleaves the Holliday junction generated by RecG-catalyzed fork regression. We also find that RecG stimulates RuvAB-catalyzed regression, presumably because it is more efficient at generating the initial Holliday junction from the stalled fork.