Compartmentalization of the replication fork by single-stranded DNA binding protein regulates translesion synthesis

Abstract DNA replication is mediated by the coordinated actions of multiple enzymes within replisomes. Processivity clamps tether many of these enzymes to DNA, allowing access to the primer/template junction. Many clamp-interacting proteins (CLIPs) are involved in genome maintenance pathways including translesion synthesis (TLS). Despite their abundance, DNA replication in bacteria is not perturbed by these CLIPs. Here we show that while the TLS polymerase Pol IV is largely excluded from moving replisomes, the remodeling of ssDNA binding protein (SSB) upon replisome stalling enriches Pol IV at replication forks. This enrichment is indispensable for Pol IV-mediated TLS on both the leading and lagging strands as it enables Pol IV-processivity clamp binding by overcoming the gatekeeping role of the Pol III epsilon subunit. As we have demonstrated for the Pol IV-SSB interaction, we propose that the binding of CLIPs to the processivity clamp must be preceded by interactions with factors that serve as localization markers for their site of action.