Stabilization of recA Protein-ssDNA Complexes by the Single-Stranded DNA Binding Protein of Escherichia coli?

ABSTRACT: In vitro recombination reactions promoted by the recA protein of Escherichia coli are enhanced by the single-stranded DNA binding protein (SSB). SSB affects the assembly of the filamentous complexes between recA protein and ssDNA that are the active form of the recA protein. Here, we present evidence that SSB plays a complex role in maintaining the stability and activity of recA-ssDNA filaments. Results of ATPase, nuclease protection, and D N A strand exchange assays suggest that the continuous presence of SSB is required to maintain the stability of recAssDNA complexes under reaction conditions that support their recombination activity. We also report data that indicate that there is a functional distinction between the species of SSB present a t 10 mM magnesium chloride, which enhances recA-ssDNA binding, and a species present a t 1 mM magnesium chloride, which displaces recA protein from ssDNA. These results are discussed in the context of current models of SSB conformation and of SSB action in recombination activities of the recA protein. x e recA protein of Escherichia coli is required for ho-mologous recombination and DNA repair functions in vivo. In vitro, the recA protein promotes DNA strand exchange reactions between homologous molecules of single-strande