On the role of single-stranded DNA binding protein in recA protein-promoted DNA strand exchange.

RecA protein-promoted DNA strand exchange is greatly stimulated by the single-stranded DNA binding protein (SSB) of Escherichia coli. Stimulation is not a consequence of the binding of SSB to excess single-stranded DNA. It results instead from stabilization of recA protein-single-stranded DNA complexes formed in the presence of ATP and SSB. In the presence of SSB, recA protein does not measurably dissociate from these complexes for up to 90 min. However, in its absence, recA protein moves rapidly between two populations of single-stranded DNA, and complete equilibration occurs with t 1/2 of 17 s. Rapid transfer of recA protein to single-stranded DNA occurs during all stages of DNA strand exchange and does not require ATP. The transfer involves an equilibrium between free and bound recA protein rather than a direct redistribution between single-stranded DNA molecules. Thus, SSB prevents dissociation of recA protein from single-stranded DNA, rendering the binding of the recA protein to single-stranded DNA irreversible. Under these conditions, the pairing phase of the strand exchange reaction is accelerated to the point that it is no longer rate-limiting. These results can explain the relative inefficiency of DNA strand exchange in the absence of SSB.