FIG. 1: (a) Tethered DNA is trapped in the linear region of an optical potential (dashed line indicates laser focus). The figure also represents a kinetic model of DNA looping. (b) Raw experi- mental recording of LacI-mediated DNA looping. The looped/unlooped threshold is chosen at the minimum between the two state distributions displayed in a histogram of the binned data. unaffected by forces as large as 183 fN. Third, our measurements strongly suggest that the anti-parallel conformation is the dominant topology of a generic LacI-mediated DNA loop [8]. We mechanically attenuate the thermal fluctuations that drive loop formation and breakdown, and measure the associated changes to the looping and unlooping rates, by em- ploying a variety of optical tweezers that differs from the more conventional tweezers setup. Constant-force axial optical tweezers stretch the molecule away from the surface and trap the attached microsphere slightly below the laser focus in an approximately linear region of the optical potential. This provides effectively a constant force in the axial direction that does not change when the protein binds to or dissociates from the DNA (see Fig. 1a). Details of this set-up are described in Ref.[9]. Because of this novel optical tweezers setup, we have been able to measure the formation and breakdown rates of LacI-mediated loops as 3