Abstract
The flagellar motor of Bacillus subtilis possesses two distinct H+-type MotAB and Na+-type MotPS stators. In contrast to the MotAB motor, the MotPS motor functions efficiently at elevated viscosity in the presence of 200 mM NaCl. Here, we analyzed the torque-speed relationship of the Bacillus MotAB and MotPS motors over a wide range of external loads. The stall torque of the MotAB and MotPS motors at high load was about 2,200 pN nm and 220 pN nm, respectively. The number of active stators in the MotAB and MotPS motors was estimated to be about ten and one, respectively. However, the number of functional stators in the MotPS motor was increased up to ten with an increase in the concentration of a polysaccharide, Ficoll 400, as well as in the load. The maximum speeds of the MotAB and MotPS motors at low load were about 200 Hz and 50 Hz, respectively, indicating that the rate of the torque-generation cycle of the MotPS motor is 4-fold slower than that of the MotAB motor. Domain exchange experiments showed that the C-terminal periplasmic domain of MotS directly controls the assembly and disassembly dynamics of the MotPS stator in a load- and polysaccharide-dependent manner.
Highlights
Many motile bacteria can swim in liquid media and move on solid surface by rotating flagella
We show that the maximum rotation rate of the MotPS motor at low load is about 4-fold slower than that of the MotAB motor but the torque produced by the MotPS stator is nearly the same as that produced by the MotAB stator
Since high-speed rotation of the flagellar motor at low load is limited by the rate of torque-generation reaction cycle of the motor, we suggest that the maximum activity of the MotPS stator is about 4 times lower than that of the MotAB stator
Summary
Many motile bacteria can swim in liquid media and move on solid surface by rotating flagella. A similar observation has been reported in E. coli cells that express both the H+-type MotAB and Na+-type PomA/PotB stators[24] These suggest that the Na+-type stator acts as a Na+ sensor to regulate the number of active stators around the rotor in response to external Na+ concentrations. The fully-induced MotPS motor support motility in soft agar at a level comparable to the wild-type and MotAB motors but not in liquid media[25,26] This raises the possibility that the MotPS stator could act as a viscosity or load sensor as well as a Na+ sensor. We will discuss the load- and polysaccharide-dependent activation mechanism of the MotPS motor
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