Suppressor mutants from MotB-D24E and MotS-D30E in the flagellar stator complex of Bacillus subtilis

The bacterial flagellar motor is mainly energized by either a proton (H(+)) or sodium ion (Na(+)) motive force and the motor torque is generated by interaction at the rotor-stator interface. MotA/MotB-type stators use H(+) as the coupling ion, whereas MotP/MotS- and PomA/PomB-type stators use Na(+). Bacillus subtilis employs both H(+)-coupled MotA/MotB and Na(+)-coupled MotP/MotS stators, which contribute to the torque required for flagellar rotation. In Escherichia coli, there is a universally conserved Asp-32 residue of MotB that is critical for motility and is a predicted H(+)-binding site. In B. subtilis, the conserved aspartic acid residue corresponds to Asp-24 of MotB (MotB-D24) and Asp-30 of MotS (MotS-D30). Here we report the isolation of two mutants, MotB-D24E and MotS-D30E, which showed a non-motile and poorly motile phenotype, respectively. Up-motile mutants were spontaneously isolated from each mutant. We identified a suppressor mutation at MotB-T181A and MotP-L172P, respectively. Mutants MotB-T181A and MotP-L172P showed about 50% motility and a poorly motile phenotype compared to each wild type strain. These suppressor sites were suggested to indirectly affect the structure of the ion influx pathway.