Genetic evidence for a switching and energy-transducing complex in the flagellar motor of Salmonella typhimurium

The flaAII.2, flaQ, and flaN genes of Salmonella typhimurium are important for assembly, rotation, and counterclockwise-clockwise switching of the flagellar motor. Paralyzed and nonchemotactic mutants were subjected to selection pressure for partial acquisition of motility and chemotaxis, and the suppressor mutations of the resulting pseudorevertants were mapped and isolated. Many of the intergenic suppressor mutations were in one of the other two genes. Others were in genes for cytoplasmic components of the chemotaxis system, notably cheY and cheZ; one of the mutations was found in the cheA gene and one in a motility gene, motB. Suppression among the three fla genes was allele specific, and many of the pseudorevertants were either cold sensitive or heat sensitive. We conclude that the FlaAII.2, FlaQ, and FlaN proteins form a complex which determines the rotational sense, either counterclockwise or clockwise, of the motor and also participates in the conversion of proton energy into mechanical work of rotation. This switch complex is probably mounted to the base of the flagellar basal body and, via binding of the CheY and CheZ proteins, receives sensory information and uses it to control flagellar operation.