DIRECT DETERMINATION OF DNA TWIST-STRETCH COUPLING

The symmetries of the DNA double helix require a new term in its linea r response to stress: the coupling between twist and stretch. Recent e xperiments with torsionally constrained single molecules give the firs t direct measurement of this important material parameter. We extract its value from a recent experiment of Strick et al. (Science, 271 (199 6) 1835) and find agreement with an independent experimental estimate recently given by Marko. We also present a very simple microscopic the ory predicting a value comparable to the one observed.