Stopped-flow studies of the kinetics of single-stranded DNA binding and wrapping around the Escherichia coli SSB tetramer

We have examined the kinetic mechanism for binding of the homotetrameric Escherichia coli SSB protein to single-stranded oligodeoxynucleotides [(dT)₇₀ and (dT)₃₅] under conditions that favor the formation of a fully wrapped ssDNA complex in which all four subunits interact with DNA. Under these conditions, a so-called (SSB)65 complex is formed in which either one molecule of (dT)₇₀ or two molecules of (dT)₃₅ bind per tetramer. Stopped-flow studies monitoring quenching of the intrinsic SSB Trp fluorescence were used to examine the initial binding step. To examine the kinetics of ssDNA wrapping, we used a single-stranded oligodeoxythymidylate, (dT)₆₆, that was labeled on its 3′-end with a fluorescent donor (Cy3) and on its 5′-end with a fluorescent acceptor (Cy5). Formation of the fully wrapped structure was accompanied by extensive fluorescence resonance energy transfer (FRET) from Cy3 to Cy5 since the two ends of (dT)₆₆ are in close proximity in the fully wrapped complex. Our results indicate that initial ssDNA binding to the tetramer is very rapid, with a bimolecular rate constant, κ_l.app, of nearly 10⁹ M^-1 s^-1 in the limit of low salt concentration (<0.2 M NaCl, pH 8.1, 25.0 °C), whereas the rate of dissociation is very low at all salt concentrations that were examined (20 mM to 2 M NaCl or NaBr). However, the rate of initial binding and the rate of formation of the fully wrapped complex are identical, indicating that the rate of wrapping of the ssDNA around the SSB tetramer is very rapid, with a lower limit rate of 700 s^-1. The implications of this rapid binding and wrapping reaction are discussed.