An ATP-dependent partner switch links flagellar C-ring assembly with gene expression

Bacterial flagella differ in their number and spatial arrangement. In many species, the MinD-type ATPase FIhG (also YIxH/FIeN) is central to the numerical control of bacterial flagella, and its deletion in polarly flagellated bacteria typically leads to hyperflagellation. The molecular mechanism underlying this numerical control, however, remains enigmatic. Using the model species Shewanella putrefaciens, we show that FIhG links assembly of the flagellar C ring with the action of the master transcriptional regulator FIrA (named FIeQ in other species). While FIrA and the flagellar C-ring protein FIiM have an overlapping binding site on FIhG, their binding depends on the ATP-dependent dimerization state of FIhG. FIiM interacts with FIhG independent of nucleotide binding, while FIrA exclusively interacts with the ATP-dependent FIhG dimer and stimulates FIhG ATPase activity. Our in vivo analysis of FIhG partner switching between FIiM and FIrA reveals its mechanism in the numerical restriction of flagella, in which the transcriptional activity of FIrA is down-regulated through a negative feedback loop. Our study demonstrates another level of regulatory complexity underlying the spationumerical regulation of flagellar biogenesis and implies that flagellar assembly transcriptionally regulates the production of more initial building blocks.