Two binding modes in Escherichia coli single strand binding protein-single stranded DNA complexes. Modulation by NaCl concentration

The binding properties of the Escherichia coli en-coded single strand binding protein (SSB) to a variety of synthetic homopolynucleotides, as well as to single stranded M13 DNA, have been examined as a function of the NaCl concentration (25.0 “C, pH 8.1). Quenching of the intrinsic tryptophan fluorescence of the SSB protein by the nucleic acid is used to monitor binding. We find that the site size (n) for binding of SSB to all single stranded nucleic acids is quite dependent on the NaCl concentration. For SSB-poly(dT), n = 33 2 3 nucleotides/tetramer below 10 mM NaCl and 65 f 5 nucleotides/tetramer above 0.20 M NaCl (up to 5 M)