  We deduced the torque applied on a single stretched and twisted DNA by
integrating with respect to force the change in the molecule's extension as it
is coiled. While consistent with previous direct measurements of the torque at
high forces (F>1 pN) this method, which is simple and does not require a
sophisticated set-up, allows for lower force estimates. We used this approach
to deduce the effective torsional modulus of DNA, which decreases with force
and to estimate the buckling torque of DNA as a function of force in various
salt conditions.
