Figure 5

Negatively-supercoiled DNA held at low force undergoes length fluctuations. Large amplitude length fluctuations occurred when 4 kbp, oriD-containing, DNA templates were negatively supercoiled (by −8 to −16 turns) and held at 0.67 to 0.91 pN load. (A) Example traces showing DNA length fluctuations at −12 turns of supercoiling and a range forces. Grey points are raw data and red lines (overlaid) are the running average of 25 points. (B) Data point values show the fraction of time the DNA molecule spent at the extended length at different forces and supercoiling levels. Pseudo-color coded background (from black to blue) is an interpolated heat-map to illustrate the fraction of time in the alternative state at each load and supercoiling level. The data points were obtained by histogramming z-displacement data (from 20–25 molecules, recorded for >300 seconds for each condition) so that the proportion of time spent in the extended state could then be determined by fitting to a dual Gaussian function (see Fig. S3 for details). The boxed values are from the same conditions as the raw data records in (A). The SEM of each point is around ±0.05. Experiments were performed at 23 °C, K100 + buffer.