Properties of motility in Bacillus subtilis powered by the H ⁺-coupled MotAB flagellar stator, Na⁺-coupled MotPS or hybrid stators MotAS or MotPB

Bacillus subtilis has a single set of flagellar rotor proteins that interact with two distinct stator-force generators, the H⁺-coupled MotAB complex and the Na⁺-coupled MotPS complex, that energize rotation. Here, motility on soft agar plates and in liquid was assayed in wild-type B. subtilis and strains expressing only one stator, either MotAB, MotPS or hybrid MotAS or MotPB. The strains expressing MotAB or MotAS had an average of 11 flagella/cell while those expressing MotPS or MotPB had an average of seven flagella/cell, and a Mot-less double mutant had three to four flagella/cell. MotAB had a more dominant role in motility than MotPS under most conditions, but MotPS supported comparable motility to MotAB on malate-containing soft agar plating media at elevated pH and Na⁺. MotAB supported much faster swimming speeds in liquid than MotPS, MotAS or MotPB under all conditions, but a contribution of MotPS to wild-type swimming was discernible from differences in swimming speeds of wild-type and MotAB at elevated viscosity, pH and Na⁺. Swimming supported by MotPS and MotAS was stimulated by Na⁺ and elevated pH whereas the converse was true of MotAB and MotPB. This suggests that MotAS is Na⁺-coupled and MotPB is H⁺-coupled and that MotB and MotS are major determinants of ion-coupling. However, the swimming speed supported by MotPB, as well as MotPS and MotAS, was inhibited severely at Na⁺ concentrations above 300 mM whereas MotAB-dependent swimming was not. The presence of either the MotP or MotS component in the stator also conferred sensitivity to inhibition by an amiloride analogue. These observations suggest that MotP contributes to Na ⁺-coupling and inhibition by Na⁺ channel inhibitors. Similarly, a role for MotA in H⁺-dependent stator properties is indicated by the larger effects of pH on the Na⁺-response of MotAS versus MotPS. Finally, optimal function at elevated viscosity was found only in MotPS and MotPB and is therefore conferred by MotP.