High-speed Rotation and Speed Stability of the Sodium-driven Flagellar Motor inVibrio alginolyticus

The Na⁺-driven flagellar motor inVibrio alginolyticusrotates very fast. Rotation of a single flagellum on a stuck cell was measured by laser dark-field microscopy with submillisecond temporal resolution. The rotation rate increased with increasing external concentration of NaCl, and reached 1000 r.p.s. at 300 nM NaCl. The Na⁺influx through the motor should determine the rotation period (τ) and affect the speed stability. Fluctuation of the rotation period was analyzed at various rotation rates (from ∼50 r.p.s. to ∼1000 r.p.s.), which were changed by changing the external concentration of NaCl and the addition of a protonophore or a specific inhibitor. At high rotation rates (over 400 r.p.s), the observed rotation was stable, and the standard deviation of τ (σ_τ) ranged from 7% to 16% of the average rotation period (<τ>). At low rotation rates (under 100 r.p.s.), the rotation period tended to fluctuate, and the distributions of τ were non-Gaussian. The value of σ_τranged from 10 to 30% of<τ>. However, the observed minimum value of σ_τat various rotation rates was approximately equal to the calculated standard deviation due to the rotational diffusion of the flagellar filament. These results suggest that the torque was stably generated at various Na⁺influxes through the motor. We observed large fluctuations that cannot be explained by rotational diffusion. We discuss the factors that induce the large fluctuation.^f2 f2 Professor Yasou Imae died suddenly of a cerebral haemorrhage on July 2nd 1993. This article is dedicated to him. Abbreviations used: LDM, laser dark-field microscopy; CCCP, carbonylcyanidem-chlorophenylhydrazone; r.p.s., revolutions per second.