摘要：
Flagellated bacteria swim by rotating thin helical filaments, each driven at
its base by a reversible rotary motor embedded in the cell wall and powered by
an ion flux. The motor contains about 20 different kinds of proteins,
comprising a rotor driven by torque-generating units, a drive shaft, a
bushing, a universal joint (the hook), and a variable-pitch propeller (the
flagellar filament). We developed a new technique that allows systematic
study of the motor near zero load. Sixty-nanometer-diameter gold spheres were
attached to motors lacking flagellar filaments, and a novel laser darkfield
setup was used to monitor their rotation. Paralyzed motors without torque-
generating units were resurrected by adding torque generating units to the
motor one at a time. In contrast to the incremental increase in rotation rate
observed at high load, the rotation rate jumped to the maximum value upon
addition of the first torque-generating unit. Switching properties of
the flagellar motor near zero load also were investigated, and the switching
rates showed a linear dependence on motor torque. Rotation in either direction
(clockwise: CW or counterclockwise: CCW) has been thought to be symmetric,
exhibiting the same torques and speeds. We measured the torque-speed
relationship across all load regimes for CW rotation, and found that the
torque decreases linearly with speed, a result remarkably different from that
for CCW rotation. This work provides further insights into the torque-
generating and switching mechanisms, and places tighter constraints on
possible motor models.