Regulatory Role of a Hydrophobic Core in the FliG C-Terminal Domain in the Rotary Direction of a Flagellar Motor

A flagellar motor can rotate either counterclockwise (CCW) or clockwise (CW), and rotational switching is triggered by conformational changes in FliG, although the molecular mechanism is still unknown. Here, we found that <i>cheY</i> deletion, which locks motor rotation in the CCW direction, restored the motility abolished by the <i>fliG</i> L259Q mutation. We found that the CCW-biased <i>fliG</i> G214S mutation also restored the swimming of the L259Q mutant, but the CW-biased <i>fliG</i> G215A mutation did not. Since the L259 residue participates in forming the FliG hydrophobic core at its C-terminal domain, mutations were introduced into residues structurally closer to L259, and their motility was examined. Two mutants, D251R and L329Q, exhibited CW-biased rotation. Our results suggest that mutations in the hydrophobic core of FliG_C collapse its conformational switching and/or stator interaction; however, the CCW state of the rotor enables rotation even with this disruption.