RecG regresses stalled DNA replication forks

RecG and RuvAB are proposed to act at stalled DNA replication forks to facilitate replication restart. To clarify the roles of these proteins in fork regression, we used a coupled spectrophotometric ATPase assay to determine how these helicases act on two groups of model fork substrates: the first group mimics nascent stalled forks while the second mimics regressed fork structures. The results show that RecG is active on the substrates in group one whereas these are poor substrates for RuvAB. In addition, in the presence of group one forks, the single stranded DNA binding protein (SSB) enhances the activity of RecG and enables it to compete with excess RuvA. In contrast, SSB inhibits the activity of RuvAB on these substrates. Results also show that the preferred regressed fork substrate for RuvAB is a Holliday Junction not a forked DNA. The active form of the enzyme on the Holliday Junction contains a single RuvA tetramer. In contrast, and although the enzyme is active on a regressed fork structure, RuvB loading by a single RuvA tetramer is impaired and full activity requires the cooperative binding of two, forked DNA substrate molecules. Collectively, the data support a model that RecG is responsible for stalled DNA replication fork regression, not RuvAB. SSB ensures that if the nascent fork has single stranded DNA character, RuvAB is inhibited while the activity of RecG is preferentially enhanced. Only once the fork has been regressed and the DNA is relaxed, can RuvAB bind to a RecG-extruded, Holliday