Calibration of the model. (A and B) Fourier transformed linear response function. (A) Magnitude of the response function of the rotary motor measured by Block et al.  (circles) and Segall et al.  (plus symbols). The fit of our model is shown by the solid line. The dashed line represents a 3rd-order filter for comparison. (B) Magnitude (left) and phase (right) of the response function at the level of the response regulator CheY measured by Shimizu et al. . The symbols are measurements at 22°C (circles) and 32°C (plus symbols). Grey lines represent the fit of our model to the magnitude of the response. (C) Switching rates of the motor from CCW to CW rotation k+ (squares) and from CW to CCW rotation k- (circles) as a function of the concentration of signalling-active unphosphorylated double-mutant Y**. The cellular concentration of Y** varied as a result of induced expression in cells in which wild-type CheY and other cytoplasmic chemotaxis proteins had been deleted. A fit using the model of Turner et al.  is shown as well (solid and dashed lines; cf. Methods).