Magnetic torque tweezers: measuring torsional stiffness in DNA and RecA-DNA filaments

Abstract
Magnetic torque tweezers are used to directly measure the torsional stiffness of single molecules of bare DNA and RecA filaments. We introduce magnetic torque tweezers, which enable direct single-molecule measurements of torque. Our measurements of the effective torsional stiffness C of dsDNA indicated a substantial force dependence, with C = ∼40 nm at low forces up to C = ∼100 nm at high forces. The initial torsional stiffness of RecA filaments was nearly twofold larger than that for dsDNA, yet at moderate torques further build-up of torsional strain was prevented.