Continuous association of Escherichia coli single-stranded DNA binding protein with stable complexes of recA protein and single-stranded DNA.

The single-stranded DNA binding protein of Escherichia coli (SSB) stimulates recA protein promoted DNA strand exchange reactions by promoting and stabilizing the interaction between recA protein and single-stranded DNA (ssDNA). Utilizing the intrinsic tryptophan fluorescence of SSB, an ATP-dependent interaction has been detected between SSB and recA-ssDNA complexes. This interaction is continuous for periods exceeding 1 h under conditions that are optimal for DNA strand exchange. Our data suggest that this interaction does not involve significant displacement of recA protein in the complex by SSB when ATP is present. The properties of this interaction are consistent with the properties of SSB-stabilized recA-ssDNA complexes determined by other methods. The data are incompatible with models in which SSB is displaced after functioning transiently in the formation of recA-ssDNA complexes. A continuous association of SSB with recA-ssDNA complexes may therefore be an important feature of the mechanism by which SSB stimulates recA protein promoted reactions.