Torque-generating units of the bacterial flagellar motor are rotary motors

E. coli swims using helical flagellar filaments driven at their base by a rotary motor. Torque-generating stator units drive the bacterial flagellar motor (BFM) by transmitting mechanical power to a cytoplasmic rotor, the C-ring. Each stator unit is a proton-conducting heteromer. A central dimer of two MotB proteins anchor to the cell wall. A surrounding pentamer of five MotA proteins transmit mechanical power to the C-ring. This asymmetrical 5:2 structure is consistent with rotation as the mechanism of torque generation. Here, we test the hypothesis that the MotA5MotB2 stator units are rotary motors themselves and interact with the rotor like intermeshed gearwheels, where rotation of the C-ring is directly coupled to MotA5 rotation around the MotB2. We used in vivo polarized photo-bleaching microscopy. When a subset of fluorescent domains inside a multimer is rapidly photo-bleached by a strong pulse of polarized light, the induced polarization-dependent fluorescence of unbleached domains becomes a reporter of angular orientation. We applied polarized photo-bleaching microscopy to tethered cells rotating by single flagellar motors. We probed fluorescently-labeled MotA pentamer and MotB dimer calibrated to motor rotation. The MotB dimer rotates at the same angular speed as the cell body, consistent with its anchor to the cell wall. The MotA pentamer rotates [~]6.2x faster than the flagellar motor, revealing the gear ratio between stator and rotor. Significance StatementBacteria swim by rotating rigid helical flagellar filaments. Here, we find that the torque-generating unit that drives flagellar rotation is itself a rotary motor. Each torque-generating unit is a heteromeric macromolecular machine - a pentamer of MotA subunits that surround a dimer of proton-conducting MotB subunits. Torque is generated as the MotA spins around MotB. The MotA pentamer interacts with rotor of the flagellar motor in a manner resembling intermeshing gearwheels. The bacterial flagellar motor is driven by the first set of enmeshed gearwheels that has been described in any living cell.