Rotational fluctuation of the sodium-driven flagellar motor of Vibrio alginolyticus induced by binding of inhibitors

Rotation of the Na⁺-driven flagellar motor of Vibrio alginolyticus was investigated under the influence of inhibitors specific to the motor, amiloride and phenamil. The rotation rate of a single flagellum on a cell stuck to a glass slide was examined using laser dark-field microscopy In the presence of 50 mM NaCl, the average rotation rate (ω̄) was about 600 r.p.s. with a standard deviation (σ(ω)) of 9% of ω̄. When ω̄ was decreased to about 200 r.p.s. by the presence of 1.5 mM amiloride, σ(ω) increased to 15% of ω̄. On the other hand, when ω̄ was decreased to about 200 r.p.s. by the addition of 0.6 μM phenamil, a large increase in σ(ω) up to 50% of ω̄ was observed. Similarly large fluctuations were observed at other concentrations of phenamil. These observations suggest that dissociation of phenamil from the motor was much slower than that of amiloride. A very low concentration of phenamil caused a transient but substantial reduction in rotation rate. This might suggest that binding of only a single molecule of phenamil strongly inhibits the torque generation in the flagellar motor.