Localization of an accessory helicase at the replisome is critical in sustaining efficient genome duplication

Genome duplication requires accessory helicases to displace proteins ahead of advancing replication forks. E. coli contains three helicases, Rep, UvrD and DinG, that might promote replication of protein-bound DNA. One of these helicases, Rep, also interacts with the replicative helicase DnaB. We demonstrate that Rep is the only putative accessory helicase whose absence results in an increased chromosome duplication time. We show also that the interaction between Rep and DnaB is required for Rep to maintain rapid genome duplication. Furthermore, this Rep-DnaB interaction is critical in minimising the need for both recombinational processing of blocked replication forks and replisome reassembly, indicating that colocalisation of Rep and DnaB minimises stalling and subsequent inactivation of replication forks. These data indicate that E. coli contains only one helicase that acts as an accessory motor at the fork in wild type cells, that such an activity is critical for the maintenance of rapid genome duplication and that colocalisation with the replisome is crucial for this function. Given that the only other characterised accessory motor, S. cerevisiae Rrm3p, physically associates with the replisome, our demonstration of the functional importance of such an association indicates that colocalisation may be a conserved feature of accessory replicative motors.