High hydrostatic pressure induces counterclockwise to clockwise reversals of the Escherichia coli flagellar motor

The bacterial flagellar motor is a reversible rotary machine that rotates a left-handed helical filament, allowing bacteria to swim toward a more favorable environment. The direction of rotation reverses from counterclockwise (CCW) to clockwise (CW), and vice versa, in response to input from the chemotaxis signaling circuit.CWrotation is normally caused by binding of the phosphorylated response regulator CheY (CheY-P), and strains lacking CheY are typically locked in CCW rotation. The detailed mechanism of switching remains unresolved because it is technically difficult to regulate the level of CheY-P within the concentration range that produces flagellar reversals. Here, we demonstrate that high hydrostatic pressure can induceCWrotation even in the absence of CheY-P. The rotation of single flagellar motors in Escherichia coli cells with the cheY gene deleted was monitored at various pressures and temperatures. Application of>120 MPa pressure induced a reversal from CCW toCWat 20°C, although at that temperature, no motor rotatedCWat ambient pressure (0.1 MPa). At lower temperatures, pressure-induced changes in direction were observed at pressures of<120 MPa.CWrotation increased with pressure in a sigmoidal fashion, as it does in response to increasing concentrations of CheY-P. Application of pressure generally promotes the formation of clusters of ordered water molecules on the surfaces of proteins. It is possible that hydration of the switch complex at high pressure induces structural changes similar to those caused by the binding of CheY-P.