Interaction of the recA protein of Escherichia coli with single-stranded DNA

The interaction of recA protein with single-stranded (ss) φX174 DNA has been examined by means of a nuclease protection assay. The stoichiometry of protection was found to be 1 recA monomer/~4 nucleotides of ssDNA both in the absence of a nucleotide cofactor and in the presence of ATP. In contrast, in the presence of adenosine 5'-O-(thiotriphosphate) (ATPγS) the stoichiometry was 1 recA monomer/~ 8 nucleotides. No protection was seen with ADP. In the absence of a nucleotide cofactor, the binding of recA protein to ssDNA was quite stable as judged by equilibration with a challenge DNA (t( 1/2 ) ~ 30 min). Addition of ATP stimulated this transfer (t( 1/2 ) ~ 3 min) as did ATP (t( 1/2 ) ~ 0.2 min). ATPγS greatly reduced the rate of equilibration (t( 1/2 ) > 12 h). Direct visualization of recA·ssDNA complexes at subsaturating recA protein concentrations using electron microscopy revealed individual ssDNA molecules partially covered with recA protein which were converted to highly condensed networks upon addition of ATPγS. These results have led to a general model for the interaction of recA protein with ssDNA.