Using Optical Tweezers and the Optical Torque Wrench to Probe Force and Torque Generation by Single Bio-Molecules Arthur LaPorta Department of Physics Stanford University   Although scientists and engineers have recently come to appreciate the possibility of making a nano-scale machines, nature contains a great variety of single molecule machines that use chemical energy to do work or process information. A great deal has been learned about such machines using optical tweezers, as well as other technologies. I will describe experiments to study the kinetics of the enzyme RNA polymerase, a molecular motor which moves along a double-stranded DNA molecule and synthesizes an RNA strand with matching base sequence. By comparing with wild-type behavior, we use this technique to study the effect of microcin, a protein which kills bacteria by interfering with RNA polymerase. Our experiments support the view that microcin works by binding to the secondary channel and prevents NTPs from reaching the catalytic site. By studying pausing statistics we are able to measure the microcin binding and dissociation rate. But the conventional optical tweezer is missing part of the picture, since molecular motors can generate torque and rotation in as well as force and displacement. I will describe the Optical Torque Wrench, a new kind of optical tweezers that can trap particles at a specific angular orientation and measure the torque acting on the particle. Using this technique we are able to rotate particles composed of anisotropic materials by varying the polarization state of the trapping beam, and we are able to determine the angular deviation and torque acting on the particles by detecting amount of angular momentum transferred to the trapping beam. The detection scheme is able to measure torques of a few pN-nm. Finally, I will discuss our preliminary results using the system to study DNA elasticity under rotational constraint.