Figure 3.

A scale drawing of the bacterial flagellar motor. Structural components are named after their genes, which fall in classes flg, flh, fli (originally called fla). Null mutants in the fla class are nonflagellate. Null mutants in the mot class are flagellate but nonmotile. The switch controls the direction of rotation. It interacts with certain cytoplasmic proteins, part of the cell’s sensory system (not shown). The transport apparatus (as yet poorly defined) controls the excretion of axial motor components. The motor is assembled from the inside-out, with new components added at its distal end (1).