Kinetics of transfer of Escherichia coli single strand DNA binding protein between single-stranded DNA molecules

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Abstract
The binding of homogeneous E. coli K12 single strand DNA binding protein to a 3H-labeled 375-nucleotide single-stranded DNA of known sequence was detected with a filter-binding assay. The binding of protein is sufficiently cooperative that an all-or-nothing mechanism governs and DNA is either free of or saturated with protein. The stoichiometry of filter binding agrees with this model. Also, the 1st-order kinetics of transfer of protein from donor DNA-protein complexes to recipient DNA of the same size is the same whether 3H-labeled donor or recipient DNA is used. The rate of transfer of protein from donor to recipient is weakly temperature dependent, independent of salt concentration over a defined range, and inversely proportional to solvent viscosity. These results are consistent with a diffusion-controlled reaction mechanism. When much larger recipient DNA is used, the rate of transfer of protein is greatly reduced. The magnitude of and the length dependence of the rate constants for protein transfer are incompatible with a mechanism involving uptake of free protein from solution. A model is proposed involving direct transfer of cooperative units of protein from donor to recipient strands. EM consistent with this model are presented.