FliL is essential for swarming: motor rotation in absence of FliL fractures the flagellar rod in swarmer cells of Salmonella enterica.

fliL is the first gene in a flagellar operon that specifies members of the switch complex and type III export system in Salmonella enterica and Escherichia coli, but no function has been ascribed to this gene thus far. Here we report that a fliL mutant is slightly impaired for swimming but completely defective in swarming in both organisms, and have studied this phenotype further in S. enterica. We have found that on swarm agar, mutant cells release or 'eject' their flagellar filaments. The released filaments are attached to the hook and part of the rod structure; we have identified the distal rod protein FlgG but not the proximal rod protein FlgF in these filaments. Rod fracture was not observed if flagellar rotation was prevented by removal of proteins that supply proton flow through the motor. Based on these and other results, we propose that motors experience a higher torque on swarm agar owing to an increased proton motive force, and that FliL allows the rod to withstand the increased torsional stress. The flagella-release phenotype of the S. enterica fliL mutant has a bearing on FliL-dependent flagellar ejection during the swimmer- to stalk-cell transition in the developmental cycle of Caulobacter crescentus.

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

Swimming 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 (chemokinesis-defective 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 speed-modulating rotary motor.