RuvABC-dependent double-strand breaks in dnaBts mutants require RecA

Replication fork arrest can cause DNA double-strand breaks (DSBs). These DS Bs are caused by the action of the Holliday junction resolvase RuvABC, indi cating that they are made by resolution of Holliday junctions formed at blo cked forks. In this work, we study the homologous recombination functions r equired for RuvABC-mediated breakage in cells deficient for the accessory r eplicative helicase Rep or deficient for the main Escherichia coli replicat ive helicase DnaB. We show that, in the rep mutant, RuvABC-mediated breakag e occurs in the absence of the homologous recombination protein RecA. In co ntrast, in dnaBts mutants, most of the RuvABC-mediated breakage depends on the presence of RecA, which suggests that RecA participates in the formatio n of Holliday junctions at forks blocked by the inactivation of DnaB. This action of RecA does not involve the induction of the SOS response and does not require any of the recombination proteins essential for the presynaptic step of homologous recombination, RecBCD, RecF or RecO. Consequently, our observations suggest a new function for RecA at blocked replication forks, and we propose that RecA acts by promoting homologous recombination without the assistance of known presynaptic proteins.