Allosteric effects of E. coli SSB and RecR proteins on RecO protein binding to DNA.

Escherichia coli single stranded (ss) DNA binding protein (SSB) plays essential roles in DNA maintenance. It binds ssDNA with high affinity through its N-terminal DNA binding core and recruits at least 17 different SSB interacting proteins (SIPs) that are involved in DNA replication, recombination, and repair via its nine amino acid acidic tip (SSB-Ct). E. coli RecO, a SIP, is an essential recombination mediator protein in the RecF pathway of DNA repair that binds ssDNA and forms a complex with E. coli RecR protein. Here, we report ssDNA binding studies of RecO and the effects of a 15 amino acid peptide containing the SSB-Ct monitored by light scattering, confocal microscope imaging, and analytical ultracentrifugation (AUC). We find that one RecO monomer can bind the oligodeoxythymidylate, (dT)15, while two RecO monomers can bind (dT)35 in the presence of the SSB-Ct peptide. When RecO is in molar excess over ssDNA, large RecO-ssDNA aggregates occur that form with higher propensity on ssDNA of increasing length. Binding of RecO to the SSB-Ct peptide inhibits RecO-ssDNA aggregation. RecOR complexes can bind ssDNA via RecO, but aggregation is suppressed even in the absence of the SSB-Ct peptide, demonstrating an allosteric effect of RecR on RecO binding to ssDNA. Under conditions where RecO binds ssDNA but does not form aggregates, SSB-Ct binding enhances the affinity of RecO for ssDNA. For RecOR complexes bound to ssDNA, we also observe a shift in RecOR complex equilibrium towards a RecR4O complex upon binding SSB-Ct. These results suggest a mechanism by which SSB recruits RecOR to facilitate loading of RecA onto ssDNA gaps.