Control of speed modulation (chemokinesis) in the unidirectional rotary motor of Sinorhizobium meliloti

Abstract

941 943 3163. Dedicated to the memory of Robert M. Macnab, who was a pioneer in studying bacterial chemotaxis, motility and flagellar motor structure, and who was a cherished friend and colleague of ours until his sudden death on 7 September 2003. Control of speed modulation (chemokinesis) in SummarySwimming cells of Sinorhizobium meliloti are driven by flagella that rotate only clockwise. They can modulate rotary speed (achieve chemokinesis) and reorient the swimming path by slowing flagellar rotation. The flagellar motor is energized by proton motive force, and torque is generated by electrostatic interactions at the rotor/stator (FliG/MotA-MotB) interface. Like the Escherichia coli flagellar motor that switches between counterclockwise and clockwise rotation, the S. meliloti rotary motor depends on electrostatic interactions between conserved charged residues, namely, Arg294 and Glu302 (FliG) and Arg90, Glu98 and Glu150 (MotA). Unlike in E. coli , however, Glu150 is essential for torque generation, whereas residues Arg90 and Glu98 are crucial for the chemotaxis-controlled variation of rotary speed. Substitutions of either Arg90 or Glu98 by charge-neutralizing residues or even by their smaller, charge-maintaining isologues, lysine and aspartate, resulted in top-speed flagellar rotation and decreased potential to slow down in response to tactic signalling (chemokinesisdefective mutants). The data infer a novel mechanism of flagellar speed control by electrostatic forces acting at the rotor/stator interface. These features have been integrated into a working model of the speedmodulating rotary motor.

Functional Analysis of Nine Putative Chemoreceptor Proteins inSinorhizobium meliloti

“…Of the 20 L-amino acids tested, the formation of distinct swarm rings was observed only in the presence of glutamate, glutamine, histidine, lysine, or proline. We also As a response to attractant stimuli, S. meliloti increases its swimming speed, a phenomenon called chemokinesis (8,64). To assess the potency of organic acids, amino acids, and sugars as attractants to induce chemokinesis, we used computerized motion analysis to monitor and average the free-swimming speeds of cell populations.…”

“…Some of these receptor proteins have no periplasmic domains, owing to the lack of obvious transmembrane regions (28,44,50). The motility system of the alphaproteobacterium S. meliloti deviates from the enterobacterial paradigm in its mode of flagellar rotation and in the ability to increase its swimming speed as a reaction toward attractants (8,58,64).…”

“…The rigid complex flagellar filaments consist of four related flagellin subunits, and interflagellin bonds lock the filaments into righthandedness (21,29,60). Hence, S. meliloti cells are propelled by flagella that rotate exclusively clockwise, and swimming cells respond to tactic stimuli by modulating their rotary speed (8,58). In E. coli, tactic signals are processed by a single response regulator, CheY, and a phosphatase, CheZ.…”

Roles of Charged Residues of Rotor and Stator in Flagellar Rotation: Comparative Study using H + -Driven and Na + -Driven Motors in Escherichia coli

“…In the native E. coli motor, rotation is prevented by mutations that neutralize both Arg90 and Glu98 of MotA or that reverse the charge of either (28). (The conserved charged residue Glu150 was shown to make a secondary contribution to rotation in E. coli and is important for rotation in Sinorhizobium meliloti [3], but Glu150 mutations were not tested here.) Arg90 and Glu98 also proved essential for rotation in the motor using FliG EV .…”

“…A recent analysis of charged residues in the S. meliloti motor casts additional light on the variation that can occur at the rotor-stator interface (3). Unlike E. coli, which steers by means of reversals in motor direction, S. meliloti directs its movements by modulating the speed of exclusively clockwise-rotating motors, a behavior termed chemokinesis (23).…”