Escherichia coli single-strand binding protein organizes single-stranded DNA in nucleosome-like units

Electron microscopy shows that complexes of the single-strand DNA binding protein (SSB) of Escherichia coli and phage fd DNA appear as beaded fiber loops containing an average of 38 beads, 1 per 170 bases of DNA. Extensive digestion of native unfixed SSB-fd DNA complexes with micrococcal nuclease reveals a protected DNA fragment of 145 bases, while shorter digestion periods result in a sequence of fragments in multiples of 160 +/- 25 bases. Digestion of these complexes with DNase I produces a repeating pattern of bands, multiples of approximately 15 bases with strong bands at 60, 105, 118, 130, 145, 150, and 210 bases. Isopycnic banding in CsCl solution yields densities of 1.272 and 1.700 g/ml, respectively, for SSB alone and for fd DNA and, after fixation, of 1.388 g/ml for fd DNA-SSB beaded fibers and 1.373 g/ml for the individual protein-DNA beads. Based on these data and the molecular weights of SSB and fd DNA, we suggest that the nucleoprotein chain consists of eight molecules of SSB bound to 145 bases of DNA, with these units linked by roughly 30 bases of protein-free DNA. The excellent concord between results obtained by enzyme digestion of unfixed native samples and, after fixation, by electron microscopy and density banding supports the conclusion that SSB organizes single-stranded DNA in a manner similar to the organization of duplex DNA by histones.