Rotational Fluctuation of the Sodium-driven Flagellar Motor ofVibrio alginolyticusInduced by Binding of Inhibitors

Rotation of the Na⁺-driven flagellar motor ofVibrio alginolyticuswas 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 (ω¯) was about 600 r.p.s. with a standard deviation (σ_ω) of 9% of ω¯. When ω¯ was decreased to about 200 r.p.s. by the presence of 1.5 mM amiloride, σ_ωincreased to 15% of ω¯. On the other hand, when ω¯ was decreased to about 200 r.p.s. by the addition of 0.6 μM phenamil, a large increase in σ_ωup to 50% of ω¯, was observed. Similarly large fluctuations were observed at other concen trations 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.