Evidence for a Solenoid Phase of Supercoiled DNA

Abstract

In mechanical manipulation experiments, a single DNA molecule overwound at constant force undergoes a discontinuous drop in extension as it buckles and forms a superhelical loop (a plectoneme). Further overwinding the DNA, we observe an unanticipated cascade of highly regular discontinuous extension changes associated with stepwise plectoneme lengthening. This phenomenon is consistent with a model in which the force-extended DNA forms barriers to plectoneme lengthening caused by topological writhe. Furthermore, accounting for writhe in a fluctuating solenoid gives an improved description of the measured force-dependent effective torsional modulus of DNA, providing a reliable formula to estimate DNA torque. Our data and model thus provide context for further measurements and theories that capture the structures and mechanics of supercoiled biopolymers.Comment: See https://www.biorxiv.org/content/biorxiv/suppl/2018/04/16/302661.DC1/302661-1.pdf for supporting information and https://drive.google.com/open?id=1sd6j4P-lYrZpgPbPAA1N8z2yWG2kLeQT for cod