ABSENCE OF A BARRIER TO BACKWARDS ROTATION OF THE BACTERIAL FLAGELLARMOTOR DEMONSTRATED WITH OPTICAL TWEEZERS

A cell of the bacterium Escherichia coli was tethered covalently to a glass coverslip by a single flagellum, and its rotation was stopped by using optical tweezers. The tweezers acted directly on the cell body or indirectly, via a trapped polystyrene bead. The torque generated by the flagellar motor was determined by measuring the displacement of t he laser beam on a quadrant photodiode. The coverslip was mounted on a computer-controlled piezo-electric stage that moved the tether point in a circle around the center of the trap so that the speed of rotatio n of the motor could be varied. The motor generated approximate to 450 0 pN nm of torque at all angles, regardless of whether it was stalled, allowed to rotate very slowly forwards, or driven very slowly backwar ds. This argues against models of motor function in which rotation is tightly coupled to proton transit and back-transport of protons is sev erely limited.