The binding-site sizes of Escherichia coli single-stranded-DNA-binding protein and mammalian replication protein A are 65 and >/= 54 nucleotides respectively.

The electrophoretic mobilities of complexes formed with single-stranded (ss) DNA and tetrameric Escherichia coli ssDNA-binding protein (EcoSSB) or mammalian replication protein A (RPA) were analysed. The electrophoretic mobilities of the complexes in a native polyacrylamide gel increased as the lengths of the DNA increased from 28 to 70 nt, thus revealing paradoxical 'descending-staircase' patterns. Increases in the electrophoretic mobilities of EcoSSB.ssDNA complexes were observed when the lengths of the bound DNA were increased by 1 nt. Quantitative analyses of the complexes suggested that the binding-sites sizes of EcoSSB and RPA were 65 and >=54 nt respectively. The binding-site size for RPA is at least 24 nt larger than previously reported.