Flagella

The bacterial flagellum is a motility apparatus commonly found among motile bacterial species. The flagellum is a supramolecular structure composed of about 30 protein components and divided into three substructures: the filament, the hook, and the basal body. The filament is a helical assembly, which takes on several distinct forms under various physicochemical conditions. Helical forms of peritrichous, polar, and lateral flagella are independent of each other and belong to different flagellar families. The basal body contains a rotary motor, which is powered by ion motive force across the cytoplasmic membrane. The C ring is a cup-shaped structure attached to the cytoplasmic side of the basal body MS ring and not only works as a rotor of the motor but also provides docking sites for the cytoplasmic ATPase complex of a type III protein export apparatus. About 40 genes required for flagellar assembly are ordered in a hierarchical manner at the transcriptional level. The type III protein export apparatus coordinates flagellar gene expression to assembly. The flagellar basal body shares common features with the secretion apparatus for virulence factors, indicating that these two systems were derived from a common ancestor.