Mutations in the stator protein PomA affect switching of rotational direction in bacterial flagellar motor.

The flagellar motor rotates bi-directionally in counter-clockwise (CCW) and clockwise (CW) directions. The motor consists of a stator and a rotor. Recent structural studies have revealed that the stator is composed of a pentameric ring of A subunits and a dimer axis of B subunits. Highly conserved charged and neighboring residues of the A subunit interacts with the rotor, generating torque through a gear-like mechanism. The rotational direction is controlled by chemotaxis signaling transmitted to the rotor, with less evidence for the stator being involved. In this study, we report novel mutations that affect the switching of the rotational direction at the putative interaction site of the stator to generate rotational force. Our results highlight an aspect of flagellar motor function that appropriate switching of the interaction states between the stator and rotor is critical for controlling the rotational direction.

ZomB is essential for chemotaxis of Vibrio alginolyticus by the rotational direction control of the polar flagellar motor.

Bacteria exhibit chemotaxis by controlling flagellar rotation to move toward preferred places or away from nonpreferred places. The change in rotation is triggered by the binding of the chemotaxis signaling protein CheY-phosphate (CheY-P) to the C-ring in the flagellar motor. Some specific bacteria, including Vibrio spp. and Shewanella spp., have a single transmembrane protein called ZomB. ZomB is essential for controlling the flagellar rotational direction in Shewanella putrefaciens and Vibrio parahaemolyticus. In this study, we confirmed that the zomB deletion results only in the counterclockwise (CCW) rotation of the motor in Vibrio alginolyticus as previously reported in other bacteria. We found that ZomB is not required for a clockwise-locked phenotype caused by mutations in fliG and fliM, and that ZomB is essential for CW rotation induced by overproduction of CheY-P. Purified ZomB proteins form multimers, suggesting that ZomB may function as a homo-oligomer. These observations imply that ZomB interacts with protein(s) involved in either flagellar motor rotation, chemotaxis, or both. We provide the evidence that ZomB is a new player in chemotaxis and is required for the rotational control in addition to CheY in Vibrio alginolyticus.

A slight bending of an α-helix in FliM creates a counterclockwise-locked structure of the flagellar motor in Vibrio.

Many bacteria swim by rotating flagella. The chemotaxis system controls the direction of flagellar rotation. Vibrio alginolyticus, which has a single polar flagellum, swims smoothly by rotating the flagellar motor counterclockwise (CCW) in response to attractants. In response to repellents, the motor frequently switches its rotational direction between CCW and clockwise (CW). We isolated a mutant strain that swims with a CW-locked rotation of the flagellum, which pulls rather than pushes the cell. This CW phenotype arises from a R49P substitution in FliM, which is the component in the C-ring of the motor that binds the chemotaxis signalling protein, phosphorylated CheY. However, this phenotype is independent of CheY, indicating that the mutation produces a CW conformation of the C-ring in the absence of CheY. The crystal structure of FliM with the R49P substitution showed a conformational change in the N-terminal α-helix of the middle domain of FliM (FliMM). This helix should mediates FliM-FliM interaction. The structural models of wild type and mutant C-ring showed that the relatively small conformational change in FliMM induces a drastic rearrangement of the conformation of the FliMM domain that generates a CW conformation of the C-ring.