Fig. 1.
Experimental set-up. (a) A single DNA molecule tethering a magnetic bead to a surface can be twisted and stretched by using small magnets placed above the sample. DNA loop formation by GalR and HU reduces the bead-to-surface distance by an amount Δz at the expense of the work, FΔz, performed against the stretching force F. The tension on the DNA may be used to tune the transition rates, (τunlooped)–1 and (τlooped)–1, between the unlooped and looped state. (b) A typical telegraph-like signal. (c) A diagram illustrating the variation of ΔG for the reaction involving the DNA conformational change associated with loop opening. The activation energy for loop opening, Eb – δF, is slightly decreased on pulling, whereas the activation energy for loop formation, Ef + FΔz, is increased on pulling.