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BP: Fachverband Biologische Physik

BP 7: Poster I

BP 7.54: Poster

Monday, March 23, 2009, 17:45–20:00, P3

Buckling Transition during DNA Supercoiling studied by Magnetic Tweezers — •Hergen Brutzer, Daniel Klaue, and Ralf Seidel — DNA motors group, BIOTECnology Center, University of Technology Dresden, D-01062 Dresden

In contrast to its well-characterized stretching and bending behavior, the response of DNA upon twisting is less understood. Initially, under the action of an external force, the molecule extension remains almost constant upon twisting. Once a critical buckling torque is reached a linear decrease in extension with added twist is observed, due to the formation of a superhelical structure. Recent experiments, however, revealed the existence of an abrupt extension change at the buckling transition, i.e. upon superhelix formation. Here we studied this abrupt buckling using magnetic tweezers, in order to elucidate its origin. We recorded the population of the pre and post-buckling states as function of the applied twist with high resolution. Depending on the applied force, the superhelix in the post buckling state comprises considerably more than one turn. Applying a two-state model in which the energy for the first turn of superhelix formation is larger than for the subsequent turns, the observed buckling transition can be explained nearly quantitatively. The model suggests a plectonemic structure with one initial loop of high curvature and a subsequent superhelix with lower DNA curvature. With decreasing salt concentration the appearance of the buckling transition is less pronounced, which is also supported by the model.