The bacterial rotary motor

The bacterial flagellar motor is a molecular machine that converts chemical energy stored in a transmembrane proton gradient into the mechanical work of rotation. Knowledge of the structure, genetics and dynamics of this motor has increased steadily. Recent progress is reviewed, with an emphasis on the identification of parts most likely to be involved in torque generation and on the assignment of specific mechanistic roles to these parts. An explicit hypothesis for torque generation is described, and some tests of this hypothesis are proposed.