| An electrochemical proton gradient or protonmotive force (pmf) across the bacterium's inner membrane powers the flagellar rotary motor of Escherichia coli. We show that the motor's rotation rate is proportional to its pmf though most of its dynamic range (0--250 Hz). This extends previous work (Fung, 1994; Fung & Berg, 1995), which found a similar relation but was restricted to heavily loaded, slowly rotating motors (<10 Hz). Two motors on the same bacterium were monitored as the cell was slowly de-energized. One motor rotated slowly due to the heavy viscous load of the entire cell body and measured the cell's pmf, given the known speed-pmf relation at these speeds. The second motor rotated quickly, due to the light viscous load of a small latex bead, and was compared to the first to give the speed-pmf relation at high speeds. Experiments were done at 24.0, 16.2 and 13.2°C, at speeds well within the high-speed, low-torque regime of the motor.; These results are consistent with a tightly coupled motor, whose rotation speed under light viscous loads is limited by proton translocation rates. Such a motor translates a fixed number of protons per revolution and, based on our results, has an ohmic response to pmf. This agrees with other high-speed motor measurements suggesting that in its high-speed, low-torque regime the motor mimics a voltage-gated proton channel in its open conformation (Chen & Berg 2000b; Decoursey & Cherny 2000). Proton translation in these channels is attributed to a hydrogen-bonded chain mechanism, which moves proton across the membrane and dissipates energy through a series of small hops. We find that motor models based on such a mechanism produce linear speed-pmf relations under light viscous loads, but are incapable of reproducing the flagellar motor's characteristic torque-speed curve. In contrast, current motor models, which reproduce the motors torque-speed curve (Berry & Berg, 1998), generally do not predict linear speed-pmf relations under light loads. Thus our findings suggest that part of the proton translocation process through the motor is similar to that of a simple proton channel, but the specifics of how this translocation is coupled to torque generation in the motor remains a mystery. |
