Fig. 6.

Relaxation of braids by single Topo IV molecules. (A) Representative single-enzyme traces are shown for the indicated tensions and σ_br = –0.05. (B) Processivity of unbraiding by single Topo IV molecules. The burst sizes (b) from 31 experiments are plotted as a cumulative histogram. Fitting of this histogram with a single exponential (neglecting undersampled short burst sizes) yields a decay constant (β) of 81 crossings. (C) Processive relaxation of a 44-kb (+) supercoiled DNA. A representative single Topo IV molecule supercoil relaxation experiment is shown for a DNA molecule (σ = +0.03) held at 0.7 pN. Seventy-five crossings were relaxed in a single burst. (D) The effect of force on the rate of Topo IV unbraiding. The rates of relaxation were determined over each burst of activity and were fit to v(F) = v_o/[1 + (v_o/v₁)e^{(0.5FΔx)/(kT)}] (black line), where v(F) is the reaction velocity at force F, k is Boltzmann's constant, T is temperature, and v_o is the rate of the force-independent kinetic step. For the force-dependent step, v₁ and Δx are, respectively, the unperturbed rate and the distance along the pulling axis to the transition state. (v_o = 2.5 catalytic cycles per sec, v₁ = 360 catalytic cycles per sec, and Δx = 4.3 nm.)