Direct imaging of intracellular signaling components that regulate bacterial chemotaxis.

The bacterial chemotaxis system regulates the rotational direction of flagellar motors through an intracellular signaling molecule, the phosphorylated form of CheY (CheY-P). The binding of CheY-P to a motor is believed to switch the motor's rotational direction from counterclockwise to clockwise. We demonstrated that the rotational switch of a motor was directly regulated by the binding and dissociation of CheY-P by simultaneously visualizing CheY tagged with green fluorescent protein and the rotational switching of a motor in live cells. The binding of 13 ± 7 CheY-P molecules was sufficient to induce clockwise rotation, and CheY-P molecules bound to and dissociated from a motor within ~100 ms during switching. Thus, we have directly measured the regulation of the output from a signal transduction pathway by intracellular signaling proteins.