Unexpected Discontinuous Supercoiling of Torsionally Buckled DNA: Evidence for a Solenoid?

Overtwisted elastic rods are known to buckle into a coiled loop (a plectoneme) to relieve torsional stress. This buckling event is marked by a discontinuous drop in the extension of a single supercoiled DNA molecule, as observed in experiments. Contrary to expectations and current models, we observe a subsequent cascade of highly regular discontinuous extension changes during plectoneme extrusion, indicating a series of kinetic barriers. We reconcile these data within the context of elastic rod theory and present a self-consistent model in which the extended DNA adopts a solenoid structure. In this scenario, kinetic barriers to plectoneme extension arise from boundary matching conditions between the solenoid and the plectoneme. Although evidence for a solenoid has not been previously reported for DNA, extended solenoids have been imaged in actin filaments which bend and form superstructures on a much larger characteristic length scale. Our data and model provide a framework for further measurements and theories that capture the structures and mechanics of supercoiled biopolymers.