Box 1 | The bacterial flagellum
From the following article:
From The Origin of Species to the origin of bacterial flagella
Mark J. Pallen and Nicholas J. Matzke
Nature Reviews Microbiology 4, 784-790 (October 2006)
doi:10.1038/nrmicro1493
The flagellum is the main organelle for motility in bacteria. Despite bearing the same name, bacterial flagella are distinct in form, function and evolution from both archaeal and eukaryotic flagella. The archetypal bacterial flagellum from Salmonella enterica serovar Typhimurium (Salmonella typhimurium) consists of a basal body, embedded in the cell wall, and two axial structures, the hook and filament, which are joined at the hook–filament junction (see figure). The basal body consists of the MS ring, rod, and L- and P-rings. Components of the axial structures are exported from the cell by the flagellar type III secretion system, which consists of several proteins from the MS ring and a peripheral hexameric ATPase FliI that drives the export process. Exported proteins pass through a central pore in the axial structures — therefore, first the flagellar hook and then the filament grow by the incorporation of new subunits at the distal end of the filamentous structure.
Rotation of bacterial flagella is powered by a proton- or sodium-motive force. The flagellar motor converts electrochemical energy into torque through an interaction between two components: the stator and the rotor. The stator consists of multiple copies of two proteins, MotA and MotB, which assemble into a structure that is associated with the inner membrane and anchored to peptidoglycan, so that it remains stationary. The rotor consists of multiple copies of FliG, which together with FliM and FliN form the C ring, mounted on the cytoplasmic face of the MS ring. Torque is transmitted from the C-ring by the MS ring to the rod (a molecular drive shaft) and from there to the hook (a universal joint) and then on to the helical flagellar filament (a molecular propeller). Rotation of this helical filament converts torque into thrust, conferring motility on the cell. The chemotaxis apparatus (not shown) integrates diverse signals to modulate the behaviour of the motor so as to propel the cell towards nutrients. Several soluble factors are involved in coordinating the assembly of the flagellar apparatus, including the flagellar 
factor FliA and the hook-length control protein, FliK.
An excellent movie illustrating flagellar biosynthesis and structure is available, see Further information. L, lipopolysaccharide; MS, membrane/super membrane; P, peptidoglycan.
