Energetics at the DNA supercoiling transition.

Twisting a DNA molecule held under constant tension is accompanied by a transition from a linear to a plectonemic DNA configuration, in which part of the applied twist is absorbed in a superhelical structure. Recent experiments revealed the occurrence of an abrupt extension change at the onset of this transition. To elucidate its origin we study this abrupt DNA shortening using magnetic tweezers. We find that it strongly depends on the length of the DNA molecule and the ionic strength of the solution. This behavior can be well understood in the framework of a model in which the energy per writhe for the initial plectonemic loop is larger than for subsequent turns of the superhelix. By quantitative data analysis, relevant plectoneme energies and other parameters were extracted, providing good agreement with a simple theory. As a direct confirmation of the initial-loop model, we find that for a kinked DNA molecule the abrupt extension change occurs at significantly lower twist than the subsequent superhelix formation. This should allow pinning of the plectoneme position within supercoiled DNA if a kinked substrate is used, and enable the detection of enzymes and proteins which, themselves, bend or kink DNA.