The dynamic response of the bacterial flagellar motor to its direct intracellular input signal

Abstract

The bacterial flagellar motor drives bacterial swimming and chemotaxis by rotating helical flagellar filaments. When Escherichia coli navigates chemical gradients, the motor switches from counterclockwise (CCW) during forward swimming to clockwise (CW) during direction-changing tumbles. The motor responds indirectly to extracellular chemosensory input to membrane-bound chemoreceptors using an intervening intracellular signaling pathway. How the motor responds to its direct input signal – the diffusible messenger CheY-P – remains poorly understood. Steady-state motor measurements have been modeled as an allosteric switch between CCW/CW states that depend on mean CheY-P levels. Allosteric models have suggested that as many as 20 CheY-P molecules can be bound to the motor when it switches rotational direction. But steady-state models cannot predict the sensitivity of the motor to dynamic changes in CheY-P that essentially modulate chemotactic behavior. We present an optogenetic reagent that precisely controls the direct dynamical input signal to the motor. We designed a “caged” molecule, Opto-CheY, that is transiently activated by photon absorption. We find that activation and binding of 1-3 CheY-P molecules is sufficient to switch the motor from the CCW to CW state. The sensitivity of the motor to small changes in CheY-P occupancy helps resolve a long-standing paradox about the high sensitivity of the chemotactic response to external sensory input. Optogenetic biochemistry by light-activated uncaging of signal molecules is a new strategy to dissect information-processing in the living cell. Significance Statement: Motile bacteria swim to better environments by modulating the rotation of the bacterial flagellar motor. How this motor responds to intracellular signaling activity is poorly understood. The physiologically-relevant response of the motor is to transient activation of intracellular signaling molecules on the sub-second time scale of bacterial decision-making. Here, we report the first optogenetic probe that targets in vivo the output module of the the chemotactic network. We demonstrate that the motor has high dynamical sensitivity to the binding of single intracellular signaling molecules. This solves a long-standing problem of high sensitivity and signal amplification in the bacterial chemotactic response.