FliW and FliS function independently to control cytoplasmic flagellin levels in Bacillus subtilis

The cytoplasmic level of flagellin (called Hag) is homeostatically regulated in the Gram-positive bacterium Bacillus subtilis by a partner-switching mechanism between the protein FliW and either the Hag structural protein or CsrA, an RNA binding protein that represses hag translation. Here we show that FliW and the putative secretion chaperone FliS bind to Hag simultaneously but control Hag translation by different mechanisms. While FliW directly inhibits CsrA activity, FliS antagonizes CsrA indirectly by binding to Hag, enhancing Hag secretion, and depleting Hag in the cytoplasm to trigger the FliW partner switch. Consistent with a role for FliS in potentiating Hag secretion, the mutation of fliS crippled both motility and flagellar filament assembly, and both phenotypes could be partially rescued by artificially increasing the concentration of the Hag substrate through the absence of CsrA. Furthermore, the absence of FliS resulted in an approximately 30-fold reduction in extracellular Hag accumulation in cells mutated for CsrA (to relieve homeostatic control) and the filament cap protein FliD (to secrete flagellin into the supernatant). Thus, we mechanistically discriminate between the FliW regulator and the FliS chaperone to show that secretion disrupts flagellin homeostasis and promotes high-level flagellin synthesis during the period of filament assembly in B. subtilis.