Abstract
The introduction of a replication-inhibiting lesion into the DNA of Escherichia coli produces a marked elevation in mutation rate. The mutation pathway is a component of the induced, multigene SOS response. SOS mutagenesis is a tightly regulated process dependent on two RecA-mediated proteolytic events: cleavage of the LexA repressor to induce the UmuC and UmuD mutagenesis proteins, and cleavage of UmuD to UmuD' to activate the mutation pathway. To investigate the protein-protein interactions responsible for SOS mutagenesis, we have studied the interaction of UmuC, UmuD, and UmuD'. To probe intracellular interaction, we have used immunoprecipitation techniques with antibodies against UmuC or UmuD and UmuD'. We have found that antibody to UmuC precipitates UmuD' from cell extracts, and antibody to UmuD and UmuD' precipitates UmuC. Thus we conclude that UmuC probably associates tightly with UmuD' in cells. For biochemical studies, we have purified the UmuC and UmuD' proteins to use with the previously purified UmuD. UmuC associates strongly with an affinity column of UmuD and UmuD', eluting only under strongly dissociating conditions (2 M urea or 1.5 M KSCN). UmuC also associates efficiently with UmuD or UmuD' in solution, as judged by velocity sedimentation in a glycerol gradient. The likely stoichiometry is one UmuC with a dimeric UmuD or UmuD'. From these experiments and previous work, we infer that SOS mutagenesis depends on the action of the UmuC-UmuD' complex and probably RecA to rescue a stalled DNA polymerase III holoenzyme at the DNA lesion.