The interaction dynamics of a negative feedback loop regulates flagellar number in Salmonella enterica serovar Typhimurium

Each Salmonella enterica serovar Typhimurium cell produces a discrete number of complete flagella. Flagellar assembly responds to changes in growth rates through FlhD₄C₂ activity. FlhD₄C₂ activity is negatively regulated by the type 3 secretion chaperone FliT. FliT is known to interact with the flagellar filament cap protein FliD as well as components of the flagellar type 3 secretion apparatus. FliD is proposed to act as an anti-regulator, in a manner similar to FlgM inhibition of σ²⁸ activity. We have found that efficient growth-dependent regulation of FlhD₄C₂ requires FliT regulation. In turn, FliD regulation of FliT modulates the response. We also show that, unlike other flagellar-specific regulatory circuits, deletion of fliT or fliD did not lead to an all-or-nothing response in FlhD₄C₂ activity. To investigate why, we characterized the biochemical interactions in the FliT FliD FlhD₄C₂ circuit. When FlhD₄C₂ was not bound to DNA, FliT disrupted the FlhD₄C₂ complex. Interestingly, when FlhD₄C₂ was bound to DNA it was insensitive to FliT regulation. This suggests that the FliT circuit regulates FlhD₄C₂ activity by preventing the formation of the FlhD₄C₂:DNA complex. Our data would suggest that this level of endogenous regulation of FlhD₄C₂ activity allows the flagellar system to efficiently respond to external signals.