STABLE COMPLEXES OF recA PROTEIN AND SINGLE-STRANDED DNA FORMED IN THE PRESENCE OF ATP AND SINGLE-STRANDED DNA BINDING PROTEIN*

The recA protein of Escherichia coli promotes the complete exchange of strands between full length linear duplex and single-stranded circular DNA rnolecules. An early step in this reaction consists of the binding of recA protein to single-stranded DNA. In the presence of ATP and the single-stranded DNA binding protein, recA protein and single-stranded DNA interact to form a complex whose stability depends upon the single-stranded DNA binding protein. Duplex DNA is not required for complex formation. Subsequent steps occur within this complex which contains up to 1 recA protein monomer per 2 nucleotides of single-stranded DNA. Although several hundred ATPs are hydrolyzed per recA protein monomer, recA protein is ot released from the complex at any step during or subsequent to strand exchange. The complex is kinetically competent with respect to both its rate of formation and its rate of reaction with homologous duplex DNA. It is therefore a significant intermediate in the overall reaction pathway. These results have served as the basis for an expanded model for recA protein-promoted strand exchange.