Dynamic exchange of two types of stator units in

Bacteria can migrate towards more suitable environments by rotating flagella that are under the control of sensory signal transduction networks. The bacterial flagellum is composed of the long helical filament functioning as a propeller, the flexible hook as a universal joint and the basal body as a rotary motor powered by ion motive force across the cell membrane. The flagellar motor consists of a rotor and multiple stator units, each of which couples the ion flow through its ion channel with force generation. The flagellar building blocks and motor proteins are highly conserved among bacterial species, but structural and functional diversity of flagella has also been revealed. It has been reported that the structure and function of the flagellar motor of a Gram-positive bacterium, Bacillus subtilis, differ from those of Escherichia coli and Salmonella. The flagellar motor of the B. subtilis BR151MA strain possesses two distinct types of stator complexes, H+-type MotAB and Na+-type MotPS, around the rotor. These two types of stator units dynamically assemble to and disassemble from the rotor in response to environmental changes such as viscosity and external Na+ concentrations. In this mini-review article, we describe our recent understanding of the structure and dynamics of the B. subtilis flagellar motor.