We report a new kind of DNA nanomachine that, fueled by Hg2+ binding and sequestration, couples mechanical motion to the multiply reversible switching of through DNA charge transport This rnechano-electronic DNA switch consists of a three-way helical junction, one arm of which is a T-T mismatch containing Hg2+-binding domain. We demonstrate, using chemical footprinting and by monitoring charge-flow-dependent guanine oxidation, that the formation of T-Hg2+-T base pairs in the Hg2+-binding domain sharply increases electron-hole transport between the other two Watson-Crick-paired stems, across the three-way junction. FRET measurements are then used to demonstrate that Hg2+ binding/dissociation, and the concomitant increase/decrease of hole transport efficiency, are strongly linked to specific mechanical movements of the two conductive helical stems. The increase in hole transport efficiency upon Hg2+ binding is tightly coupled to the movement of the conductive stems from a bent arrangement toward a more linear one, in which coaxial stacking is facilitated This switch offers a paradigm wherein the performance of purely mechanical work by a nanodevice can be conveniently monitored by electronic measurement.