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

Significance Flagella, bacterial organelles of locomotion, appear in a defined number and localization at the bacterial cell surface. The MinD-type ATPase FlhG numerically regulates flagellation patterns through a molecular mechanism only poorly understood. Depending on its ATP-dependent oligomerization state, FlhG interacts either with the C-ring protein FliM during flagellar assembly or with flagellar master regulator FlrA. This partner switch between FliM and FlrA establishes a regulatory network critical for the numerical regulation of flagella, in which the physical assembly of the flagellum transcriptionally feeds back to prevent the production of more building blocks.