(A) The work (i.e. ΔE) stored in the ssDNA or SSB-coated ssDNA molecules was determined from the area under the curves from the data in Figure 4C,D, as shown in Figures 5—figure supplements 1, 2, and plotted as a function of the natural logarithm (bottom x-axis) of NaOAc concentration (top x-axis) for ssDNA (black-filled circles, increasing force; black open circles, decreasing force) and for SSB-coated ssDNA (red filled circles). The lines are linear fits where the slope, δkBT/δln[NaOAc], is 1000 (±200) for SSB (red line), 1500 (±200) for ssDNA when decreasing force (black dashed line), and 2200 (±150) for ssDNA when increasing force (black solid line). (B) The force–extension relationship of a single molecule of ssDNA was measured in the absence of salt (black), then again after sequentially adding each of the following: 100 mM NaOAc and 1 mM Mg(OAc)2 (red), 200 nM SSB (blue), 100 nM RecO (purple), and 1 μM RecR (green). The extension in the presence of RecO results in approximately a 10% condensation at each force measured. In the presence of both RecO and RecR, significant hysteresis is observed (compare solid and dashed green lines). (C) The change in energy was determined by integrating the area under the curves in panel B relative to the ssDNA alone curve (black), and are plotted for both increasing (filled bars) and decreasing force (open bars). (D) A cartoon depicting our model for salt-induced intramolecular bridging mediated in cis by oligomers of SSB (either tetramers or octamers). RecOR may mediate bridging either in cis, along the same molecule of SSB-coated ssDNA, or in trans to promote annealing of complementary ssDNA.