Interaction of single-strand binding protein and RecA protein at the single-stranded DNA site.

Escherichia coli single-strand binding protein (SSB), which participates in DNA replication, also plays a role in DNA repair and induction of SOS functions. We show that the formation of RecA-dATP-single-stranded DNA complexes is influenced by the presence of SSB. In equilibrium reactions with limiting bacteriophage fd DNA, the mutant SSB113 protein competes more effectively than SSB with RecA protein for sites on the DNA. This result can account for the inability of strain ssb113 to amplify RecA protein synthesis and induce lambda prophage. SSB fails to displace RecA protein completely, even at very high concentrations. Both proteins inhibit the dATPase activity of RecA protein in spite of a large proportion of RecA protein still complexed to single-stranded DNA. Analysis of the multiple RecA protein activities and how they respond to the presence of SSB suggests that they fall into two distinct classes. Those that are enhanced by SSB (proteolysis and strand assimilation) and those inhibited by SSB (NTPase, reannealing of complementary single-stranded DNA). We propose a two-state model of conformational change of RecA protein, affected by the number of available free bases in single-stranded DNA relative to the number of RecA monomers, that would explain the choice of mutually exclusive catalytic activities.