This report discovers a role of Escherichia coli RecA, the cellular recombinase, in directing the action of several DNA polymerases at the replication fork. Bulk chromosome replication is performed by DNA polymerase (Pol) III. However, E. coli contains translesion synthesis (TLS) Pols II, IV, and V that also function with the helicase, primase, and sliding clamp in the replisome. Surprisingly, we find that RecA specifically activates replisomes that contain TLS Pols. In sharp contrast, RecA severely inhibits the Pol III replisome. Given the opposite effects of RecA on Pol III and TLS replisomes, we propose that RecA acts as a switch to regulate the occupancy of polymerases within a moving replisome. Previous studies have shown that all three Pols function at replication forks with DnaB helicase, primase, the β sliding clamp, and the clamp loader (2). Bulk chromosome replication is performed by Pol III, and the mechanism that regulates the access of TLS Pols to the replisome is largely unexplored.Heavy DNA damage in E. coli induces the SOS response, which slows down chromosomal replication (3) and expresses over 40 proteins that aid cell survival and enable genome replication to continue (4, 5). Induction of the SOS response requires formation of ssDNA by replication forks to which the cellular recombinase, RecA, binds. The binding of RecA to ssDNA facilitates RecAmediated self-cleavage of the LexA repressor, thereby activating the SOS response genes (6). Among these are TLS Pol II (polB) and TLS Pol IV (dinB), which are rapidly (<5 min) up-regulated during the SOS response (7-10). TLS Pol V (UmuD′ 2 C) is induced late (≥45 min) and requires RecA bound to ssDNA (RecA*) for synthetic activity (8,11,12). Pol IV and Pol V belong to the Yfamily Pols (1), whereas Pol II, even though involved in TLS and mutagenesis, is a B-family polymerase (13).During normal chromosome duplication, the intracellular levels of Pols II and IV are not sufficient to take over Pol III-based replisomes (2). Upon DNA damage, the induced levels of TLS Pols take over the cellular replicase (2, 14, 15). Our previous studies have shown that Pols II and IV replace Pol III within the replisome without causing fork collapse, as they preserve the DNAbound helicase and sliding clamp to form TLS Pol replisomes (2). Pol II and Pol IV replisomes progress at significantly slower rates than the intrinsic rate of DnaB helicase, and thus regulate the speed of helicase unwinding. Slower fork progression may give the cell additional time to repair DNA lesions using normal repair processes (e.g., nucleotide excision repair), thereby preventing direct encounters of the replication fork with DNA lesions.It has been suggested that polymerase selection at the replication fork is mainly guided by mass action dictated by the concentration of each polymerase and its affinity for the clamp (1,14,16,17). However, the findings of this report demonstrate that RecA adds an additional level of control, acting as a switch that regulates DNA polymerase access to the replicat...