FliL Differentially Interacts with Two Stator Systems To Regulate Flagellar Motor Output in Pseudomonas aeruginosa.

FliL is present in nearly all flagellated bacterial species and is associated with the flagellar basal body. This protein was found to be important for the function of the flagellar motor, and its absence led to a variety of motility defects in several species. However, the specific function of FliL in Pseudomonas aeruginosa remains elusive. Here, we studied the effects of FliL on motor output in P. aeruginosa using a bead assay, finding that FliL regulates motor output through its differential effects on the two sets of homologous MotAB and MotCD stators. FliL interacts with the MotCD stators to increase the motor torque and the stability of the motor speed, whereas it works with the MotAB stators to maintain a high motor switching rate. These effects of FliL contribute to enhancing P. aeruginosa's motility and chemotaxis. IMPORTANCE FliL emerged as a modulator of flagellar motor function in several bacterial species, but its function in Pseudomonas aeruginosa was unknown. Here, by performing single-motor studies using a bead assay, we elucidated its effects on the flagellar motor in P. aeruginosa. We found that it differentially interacts with two sets of stators (MotAB and MotCD) to regulate different aspects of bacterial motility (motor switching rate and motor rotation speed), thereby enhancing the ability of P. aeruginosa to explore its environment.