Rotational asymmetry of Escherichia coli flagellar motor in the presence of arsenate

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The flagellar motor of Escherichia coli ( E. coli) is driven by a proton-motive force (PMF), hence it was of interest to determine whether the motor is symmetrical in the sense that it can be rotated by any polarity of PMF. For this purpose the cells had to be deenergized first. Conventional deenergization procedures caused irreversible loss of motility, presumably due to ATP-dependent degradative processes. However, E. coli cells deenergized by incubation with arsenate manifested a slow, reversible depletion of PMF. In this procedure there was a sufficiently long time window, during which a considerable proportion of the cells lost their motility and could be made to rotate again by an artificially-imposed PMF. The motors of these cells rotated in response to any PMF polarity, but positive and negative polarities rotated different sub-populations of cells and the direction was almost exclusively counterclockwise. The reason for the unidirectionality of the rotation was not the intervention of the chemotaxis system. A number of potential reasons are suggested. One is the arsenate effect on the motor function found previously [Margolin, Y., Barak, R. and Eisenbach, M. (1994) J. Bacteriol. 176, 5547–5549]. A possible interaction between arsenate and the motor is discussed.