The Energy for Chemotaxis
Application of genetic and biochemical methods has allowed determination of the energy source required for chemotaxis. On one hand it seems logical that ATP would be the direct source of the mechanical energy required for swimming since most energy transductions in higher cells appear to use ATP. On the other hand, the flagella originate in the cell membrane, and a substantial proton gradient exists across the inner membrane under most growth conditions. Therefore the direct source of energy could also derive from the membrane potential.
ATP and the proton motive force across the inner membrane are normally interconvertible by means of the membrane-bound ATPase (Fig. 22.9). The membrane potential generates ATP and, conversely, ATP can be used to generate a membrane potential. Therefore, blocking formation of either ATP or the membrane potential could have an effect on the other. Arsenate permits decoupling to determine what actually
Figure 22.9 A schematic of the inner membrane of a cell showing that electronflow down the electron transport chain leads to export of protons and ultimate transfer of electrons to oxygen. Reentry of protons to the cell through the ATPase generates ATP. Conversely, ATP can be hydrolyzed by ATPase to pump protons out of the cell.
drives the motor of the flagella. It blocks ATP formation both by glycolysis and by ATPase, but does not directly affect the membrane potential. Cells treated with arsenate are found to be motile, but they do not swim up gradients of attractants. This shows that ATP is not required for motility, but probably is involved with the process of modulating run duration.
Next, to investigate the role of the membrane potential, cells can be grown anaerobically. This blocks the usual means of generation of the membrane potential because the electron transfer chain becomes inac-tive owing to the lack of a terminal electron acceptor. Then, to prevent energy from ATP from being used to create a membrane potential, an ATPase-negative mutant can be used. Anaerobically grown ATPase mutants are found not to be motile, leading to the conclusion that the motors that drive the flagella are run by the cell’s membrane potential. Other experiments show that cells swim when transmembrane pH gradient or membrane potentials are generated artificially.