The systematic substitutions around the conserved charged residues of the cytoplasmic loop of Na⁺-driven flagellar motor component PomA

PomA, a homolog of MotA in the H⁺-driven flagellar motor, is an essential component for torque generation in the Na⁺-driven flagellar motor. Previous studies suggested that two charged residues, R90 and E98, which are in the single cytoplasmic loop of MotA, are directly involved in this process. These residues are conserved in PomA of Vibrio alginolyticus as R88 and E96, respectively. To explore the role of these charged residues in the Na⁺-driven motor, we replaced them with other amino acids. However, unlike in the H⁺-driven motor, both of the single and the double PomA mutants were functional. Several other positively and negatively charged residues near R88 and E96, namely K89, E97 and E99, were neutralized. Motility was retained in a strain producing the R88A/K89A/E96Q/E97Q/E99Q (AAQQQ) PomA protein. The swimming speed of the AAQQQ strain was as fast as that of the wild-type PomA strain, but the direction of motor rotation was abnormally counterclockwise-biased. We could, however, isolate non-motile or poorly motile mutants when certain charged residues in PomA were reversed or neutralized. The charged residues at positions 88-99 of PomA may not be essential for torque generation in the Na⁺-driven motor and might play a role in motor function different from that of the equivalent residues of the H⁺-driven motor.