Figure 4.

(a) Kinetic scheme of protein-induced DNA looping. (b) Time traces of the RMS motion of a single DNA tether (left panel, black line) calculated over a 2 s time-window, and the corresponding histograms (right panel), with increasing optical pushing force. The histograms show two distinct states corresponding to the unlooped (large RMS) and looped state (small RMS). From the histograms a threshold value is obtained, which is used to determine the dwell times (red lines, left panels). (c) Average loop lifetime plotted as a function of optical pushing force. (Black circles) Experimental data points (error bars, mean ± SE); (red line) fit using Arrhenius’ law; (inset) looped fraction as a function of the optical pushing force (error bars, mean ± SE). (d) Energy landscape obtained from the force spectroscopy measurements. The corrected free energy G_cor takes the volume exclusion force into account due to the close proximity of the surface.