Homeostatic regulation of flagellar number and assembly in Bacillus subtilis

Many bacteria are motile by rotating flagella. Each flagellum is built inside-out from over 30 different proteins and assembled in three architectural domains: basal body, hook and filament. Flagellar assembly is an energy-expensive process, primarily because the filament is polymerized from over 20,000 subunits of flagellin protein. The high demand of flagellin during filament assembly is met by transcription and translation of flagellin from strong promoter and near consensus ribosome-binding site respectively. However, the energy-expense is magnified in peritrichously-flagellated bacteria like Bacillus subtilis that synthesize numerous flagella. The work presented here establishes a novel post-transcriptional paradigm where the structural protein flagellin participates in a negative auto-feedback loop that involves partner switching with the regulatory protein pair FliW-CsrA to homeostatically restrict intracellular flagellin levels. The flagellin-FliW-CsrA partner switching limits flagellin synthesis to periods of active filament assembly when the intracellular pool of flagellin is depleted by FliS-aided secretion of flagellin and polymerization into the growing filament. Further, this work identifies a second way to control the metabolic investment in flagellar assembly: regulation of the number of flagella per cell in response to the environmental conditions. I found that B. subtilis cells have about 18 flagella per cell for swimming in liquid but over 30 flagella per cell during swarming over semi-solid surfaces. Flagellar number is regulated by SwrA protein, the master regulator of flagellar biosynthesis, that activates the transcription of most genes required for flagellar assembly. I showed that the AAA+ protease LonA degrades SwrA and restricts flagellar number in swimmer cells but under conditions favorable to swarming LonA is somehow inhibited and SwrA levels accumulate to allow increase in flagellar number and swarming motility.