Evaluation of the duty ratio of bacterial flagellar motor by a dynamic load control

TL;DR

This study introduces a dynamic load control method to measure the duty ratio of bacterial flagellar motors, revealing a small duty ratio of approximately 0.14, which clarifies previous conflicting observations.

Contribution

The paper develops an electrorotation technique to dynamically control load and accurately determine the duty ratio of bacterial flagellar motors.

Findings

Duty ratio of approximately 0.14 was measured.

The method allows precise control of load and stator unit number.

Results support a small duty ratio in bacterial flagellar motors.

Abstract

Bacterial flagellar motor is one of the most complex and sophisticated nano machineries in nature. A duty ratio $D$ is a fraction of time that the stator and the rotor interact and is a fundamental property to characterize the motor but remains to be determined. It is known that the stator units of the motor bind to and dissociate from the motor dynamically to control the motor torque depending on the load on the motor. At low load where the kinetics such as a proton translocation speed limits the rotation rate, the dependency of the rotation rate on the number of stator units $N$ infers $D$; the dependency becomes larger for smaller $D$. Contradicting observations supporting both the small and large $D$ have been reported. A dilemma is that it is difficult to explore a broad range of $N$ at low load because the stator units easily dissociate, and $N$ is limited to one or two at vanishing load. Here, we develop an electrorotation method to dynamically control the load on the flagellar motor of {\it Salmonella} with a calibrated magnitude of the torque. By instantly reducing the load for keeping $N$ high, we observed that the speed at low load depends on $N$, implying a small duty ratio. We recovered the torque-speed curves of individual motors and evaluated the duty ratio to be $0.14 \pm 0.04$ from the correlation between the torque at high load and the rotation rate at low load.