Constraints on flagellar rotation.

The motion of tethered cells of Streptococcus was analyzed at low values of protonmotive force (delta p). Cells repeatedly energized and de-energized stopped at discrete angular positions, indicating a rotational symmetry of barriers to rotation of order 5 or 6. At values of delta p smaller than -30 mV, constraints imposed by these barriers were evident when cells were starved and gradually energized, but not when they were energized first and then gradually de-energized. At values of delta p larger than about -30 mV, the cells behaved as if there were no barriers. Cells spinning in this regime also executed rotational Brownian movement. At energy levels above threshold, the motor determines torque; it does not fix the position of the rotor relative to the stator.