Scheme 2.

Model for mtSSB binding to ssDNA dT₆₀ (D₆₀) and for DNA exchange. The minimal model is shown for the mtSSB tetramer (SSB) binding to D₆₀ to form SSB–D₆₀. Isomerization of SSB–D₆₀ to the fully wrapped form, SSB–D_60,wrapped, leads to tighter binding. A second DNA (D₆₀) can bind to the SSB–D₆₀ to form a ternary intermediate SSB–D₆₀–D₆₀ to achieve the direct transfer of mtSSB between two ssDNA molecules. The model was used to globally fit kinetic and equilibrium binding data shown in Figs. 5 and 6. The rate constants describe the reaction in buffer A with 100 mm NaCl and 10 mm Mg²⁺, with confidence intervals summarized in Table 4. In fitting the data, the rate constants in brackets were held fixed at the values shown, as described under “Results.”