A fork-clearing role for UvrD.

The inactivation of a replication protein causes the disassembly of the replication machinery and creates a need for replication reactivation. In several replication mutants, restart occurs after the fork has been isomerized into a four-armed junction, a reaction called replication fork reversal. The repair helicase UvrD is essential for replication fork reversal upon inactivation of the polymerase (DnaE) or the beta-clamp (DnaN) subunits of the Escherichia coli polymerase III, and for the viability of dnaEts and dnaNts mutants at semi-permissive temperature. We show here that the inactivation of recA, recFOR, recJ or recQ recombination genes suppresses the requirement for UvrD for replication fork reversal and suppresses the lethality conferred by uvrD inactivation to Pol IIIts mutants at semi-permissive temperature. We propose that RecA binds inappropriately to blocked replication forks in the dnaEts and dnaNts mutants in a RecQ- RecJ- RecFOR-dependent way and that UvrD acts by removing RecA or a RecA-made structure, allowing replication fork reversal. This work thus reveals the existence of a futile reaction of RecA binding to blocked replication forks, that requires the action of UvrD for fork-clearing and proper replication restart.