Fig. 4

Real-time observation of strand transfer and apical interface stability in STCs. a Scheme depicting signatures of strand transfer. Top: binding and reaction near plectoneme end loops minimally affect σ_z but enables supercoil release through apical interface unbinding (green arrow) and rotational relaxation (black arrow). Bottom: binding near the origin of the plectoneme suppresses σ_z, strand transfer quenches dynamic bridging by intasome-target anchoring. b, c Extension time-traces of supercoiled target DNA reacting with intasome, depicting stepwise extension increments (b) and extension hopping followed by a stable extension level (c). d Extension time-trace of supercoiled target DNA reacting with WT intasome. External magnets introduce supercoils that are released in steps due to transient apical interface unbinding. Extension plateaus quantify dwell times Δt_Apic^STC, step sizes Δz quantify extension increments. e In the plectonemic regime, the number of turns released per unbinding event ΔLk is proportional to Δz. f ΔLk distribution (kernel density estimate; bandwidth 0.2 turns). Inset: Fourier transformation after subtracting an exponential background. g Dependence of step size ΔLk distribution on the sign of ΔLk^TCC in TCCs and on the sign of the torque Γ^STC applied to STCs (red data points are mean <ΔLk> and error bars are 95% CI as obtained from an exponential fit; Supplementary Fig. 7). h Dependence of dwell times Δt_Apic^STC on ΔLk^TCC and Γ^STC (red data points are mean lifetime τ_Apic^STC and error bars are 95% CI as obtained from an exponential fit; Supplementary Fig. 7). Significance calculated using two-sample Kolmogorov–Smirnov test (n^TCC+_STC+ = 227; n^TCC+_STC‒ = 288; n^TCC−_STC+ = 412_; n^TCC‒_STC‒ = 444). Source data are provided as a Source Data file