In Escherichia coli and Salmonella typhimurium, ATP is required for chemotaxis and for a normal probability of clockwise rotation of the flagellar motors, in addition to the requirement for S-adenosylmethionine (J. Shioi, R. J. Galloway, M. Niwano, R. E. Chinnock, and B. L. Taylor, J. Biol. Chem. 257:7969-7975, 1982). The site of the ATP requirement was investigated. The times required for S. typhimurium ST23 (hisF) to adapt to a step increase in serine, phenol, or benzoate were similar in cells depleted of ATP and in cells with normal levels of ATP. This established that ATP was not required for the chemotactic signal to cross the inner membrane or for adaptation to the transmembrane signal to occur. Depletion of ATP did not affect the probability of clockwise rotation in E. coli cheYZ scy strains that were defective in the cheY and cheZ genes and had a partially compensating mutation in the motor switch. Strain HCB326 (cheAWRBYZ tar tap tsr trg::Tn10), which was deficient in all chemotaxis components except the switch and motor, was transformed with the pCK63 plasmid (ptac-cheY+). Induction of cheY in the transformant increased the frequency of clockwise rotation, but except at the highest levels of CheY overproduction, clockwise rotation was abolished by depleting ATP. It is proposed that the CheY protein is normally in an inactive form and that ATP is required for formation of an active CheY* protein that binds to the switch on the flagellar motors and initiates clockwise rotation. Depletion of ATP partially inhibits feedback regulation of the cheB product, protein methylesterase, but this may reflect a second site of ATP action in chemotaxis
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