Abstract

Bacterial flagella contain membrane-embedded stators, Mot complexes, that harness the energy of either transmembrane proton or sodium ion gradients to power motility. Use of sodium ion gradients is associated with elevated pH and sodium concentrations. The Mot complexes studied to date contain channels that use either protons or sodium ions, with some bacteria having only one type and others having two distinct Mot types with different ion-coupling. Here, alkaliphilic Bacillus clausii KSM-K16 was shown to be motile in a pH range from 7 to 11 although its genome encodes only one Mot (BCl-MotAB). Assays of swimming as a function of pH, sodium concentration, and ion-selective motility inhibitors showed that BCl-MotAB couples motility to sodium at the high end of its pH range but uses protons at lower pH. This pattern was confirmed in swimming assays of a statorless Bacillus subtilis mutant expressing either BCl-MotAB or one of the two B. subtilis stators, sodium-coupled Bs-MotPS or proton-coupled Bs-MotAB. Pairs of mutations in BCl-MotB were identified that converted the naturally bifunctional BCl-MotAB to stators that preferentially use either protons or sodium ions across the full pH range. We then identified trios of mutations that added a capacity for dual-ion coupling on the distinct B. subtilis Bs-MotAB and Bs-MotPS motors. Determinants that alter the specificity of bifunctional and single-coupled flagellar stators add to insights from studies of other ion-translocating transporters that use both protons and sodium ions.