Mechanosensitive dose response of the bacterial flagellar motor.

The bacterial flagellar motor is both chemo- and mechanosensitive. It is sensitive to the intracellular concentration of the chemotaxis response regulator CheY-P and to external load conditions. The motor's dose-response curve, which represents the probability of the motor rotating clockwise (CW bias) as a function of CheY-P concentration, characterizes its chemical sensitivity. However, it remains unclear how this dose-response curve depends on the load conditions. Here, we measured the dose-response curves of the motor under various load conditions. Surprisingly, we found that the dose-response curve exhibited minimal changes with load at low CW biases, but shifted leftward with higher sensitivity to CheY-P concentration at high CW biases when the load increased. This observation contradicts previous model predictions that incorporated the effect of stator-rotor interaction on motor switching. Through the development of an Ising-type model for the coupled chemo- and mechanosensitivity of the flagellar switch, we revealed that the mechanism underlying the mechanosensitive dose response is the synergistic interplay between the adaptive remodeling of the motor switch complex and the nonequilibrium effect of the stator-rotor interaction.