To gain insight into the influence of ATP gamma S on the competitive binding of RecA and single-stranded binding protein (SSB) on single-stranded DNA (ssDNA), A FM imaging was used to examine the three-dimensional structures of the different complexes formed by the binding of the two proteins on ssDNA in the presence and absence of ATP gamma S In the presence of ATP gamma S, RecA attaches to ssDNA, displacing SSB, to form continuous binding regions that caused considerable elongation of the strand When ATP gamma S is absent. RecA could not compete with SSB and only binds at a few sues that correspond to the vacancy in ssDNA left when SSB unbinds These results provide direct evidence that, while SSB binding affinity to DNA is substantially higher than that of RecA, the presence of ATP gamma S is sufficient to alter the events and enable RecA coating of DNA