Figure 1.
Dan causes DNA stiffening through cooperative DNA-binding. (A) An illustration of the transverse magnetic tweezers setup for the single-DNA stretching experiment. (B) Force-extension curves of single-DNA stretching experiments were performed using increasing concentrations of Dan. DNA stiffening is saturated by 60 nM Dan concentration. Solid curves are fitting curves according to the WLC model. The apparent persistence length and contour length of Dan nucleoprotein filament is 488 ± 290 and 16 002 ± 320 nm. (C) The occupancy fraction of Dan–DNA binding was calculated from the measured apparent persistence length (inserted equation) and plotted with respect to the Dan concentration. Fitting with Hill equation (solid line) suggests that Dan has DNA-binding Kd of 13.20 ± 4.67 nM and Hill coefficient of 2.90 ± 1.59, indicating strong cooperative DNA binding by Dan.