TORQUE GENERATED BY THE FLAGELLAR MOTOR OF ESCHERICHIA-COLI

Cells of the bacterium Escherichia coli were tethered and spun in a hi gh-frequency rotating electric field at a series of discrete field str engths. This was done first at low field strengths, then at field stre ngths generating speeds high enough to disrupt motor function, and fin ally at low field strengths. Comparison of the initial and final speed versus applied-torque plots yielded relative motor torque. For backwa rd rotation, motor torque rose steeply at speeds close to zero, peakin g, on average, at about 2.2 times the stall torque. For forward rotati on, motor torque remained approximately constant up to speeds of about 60% of the zero-torque speed. Then the torque dropped linearly with s peed, crossed zero, and reached a minimum, on average, at about -1.7 t imes the stall torque. The zero-torque speed increased with temperatur e (about 90 Hz at 11-degrees-C, 140 Hz at 16-degrees-C, and 290 Hz at 23-degrees-C), while other parameters remained approximately constant. Sometimes the motor slipped at either extreme (delivered constant tor que over a range of speeds), but eventually it broke. Similar results were obtained whether motors broke catastrophically (suddenly and comp letely) or progressively or were de-energized by brief treatment with an uncoupler. These results are consistent with a tightly coupled ratc het mechanism, provided that elastic deformation of force-generating e lements is limited by a stop and that mechanical components yield at h igh applied torques.