Adaptation at the output of the chemotaxis signalling pathway

Abstract

In the bacterial chemotaxis network, receptor clusters process input, and flagellar motors generate output. Receptor and motor complexes are coupled by the diffusible protein CheY-P. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell. However, the motor is ultrasensitive, with a narrow operating range of CheY-P concentrations. How the match between receptor output and motor input might be optimized is unclear. Here we show that the motor can shift its operating range by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. This shift in sensitivity explains the slow partial adaptation observed in mutants that lack the receptor methyltransferase and methylesterase and why motors show signal-dependent FliM turnover. Adaptive remodelling is likely to be a common feature in the operation of many molecular machines.

Fig. 1

Motor responses to stepwise addition of chemical attractants monitored by the bead assay. The attractants were applied at the times indicated by the arrows. Error bounds for standard errors of the mean are shown as dotted lines. (a) Averaged responses of 7 cheR cheB cells (JY35 carrying pKAF131) to 1 mM MeAsp (weak attractant). (b) Averaged responses of 4 cheR cheB cheZ cells (JY32 carrying pVS7 and pKAF131) to 2 mM MeAsp + 0.5 mM L-serine (strong attractant).

Fig. 2

FRET responses (Y/C ratio) of cheR cheB cells to stepwise addition of chemical attractants. The attractants were added at the times indicated by the arrows. (a) CheY-YFP/CheZ-CFP FRET responses to 1 mM MeAsp (weak attractant). (b) CheY-YFP/CFP-FliM FRET responses to 2 mM MeAsp + 0.5 mM L-serine (strong attractant).

Fig. 3

CW bias as a function of CheY-P concentration. The red curve is for pre-stimulus wild-type motors, as measured by Cluzel et al. and fit with an MWC model with 34 FliM units. The blue dots are data for motors that have partially adapted to stepwise addition of attractant, with a two-parameter fit to the MWC model with N_av = 36 FliM units shown by the green curve; see text.

Fig. 4

Changes in single-motor FliM-YFP fluorescence intensities in cheR cheB cells tethered by hooks and stimulated by addition of attractant (at time t₀, arrow). The intensity for a given motor was normalized by its intensity at time 0, intensities for different motors were averaged, and the fit to the control of panel c was subtracted. Fits with the model are shown in magenta and the parameters for these fits are given. Error bars are standard errors of the mean. (a) Responses of 20 motors to addition of 2 mM MeAsp + 0.5 mM L-serine. (b) Responses of 22 motors to addition of 1 mM MeAsp. (c) Responses of 15 motors without addition of attractant, fit with an exponential decay function plus a constant.