Fig. 4.

Topo IV unbraiding of single DNA braids. (A) Preferential relaxation of left-handed braids. Topo IV activity was assayed by monitoring the extension of the braids held under constant tension (1 pN). Raw data are plotted in green (left-handed braid) and red (right-handed braid), and a 1-sec moving average is shown as a solid black line. (Inset) Histogrammed rates of unbraiding were calculated as ΔZ/Δt for successive 5-sec windows in 10 independent runs each on right- and left-handed braids (|σ_br| = 0.05). To avoid effects of limiting substrate, only those portions of the traces where |σ_br| > 0.02 were included. The mean unbraiding rate was 9 ± 1.4 nm/sec for left-handed braids (green) and 0.3 ± 0.3 nm/sec for right-handed braids (red). (B) Topo IV relaxation of high-density DNA braids for |σ_br| > 0.15 and 2 pN of tension. The first-order DNA braid folds into a plectonemic second-order superhelix, with DNA crossings possessing the opposite handedness of the first-order superhelix. Symbols are as in A. The dashed black line is the extension for |σ_br^crit| at 2 pN.