TEMPERATURE-INDUCED SWITCHING OF THE BACTERIAL FLAGELLAR MOTOR

Chemotaxis signaling proteins normally control the direction of rotati on of the flagellar motor of Escherichia coli. In their absence, a wil d-type motor spins exclusively counterclockwise. Although the signalin g pathway is well defined, relatively little is known about switching, the mechanism that enables the motor to change direction, We found th at switching occurs in the absence of signaling proteins when cells ar e cooled to temperatures below about 10 degrees C. The forward rate co nstant (for counterclockwise to clockwise, CCW to CW, switching) incre ases and the reverse rate constant (for CW to CCW switching) decreases as the temperature is lowered. At about -2 degrees C, most motors spi n exclusively CW. At temperatures for which reversals are frequent eno ugh to generate a sizable data set, both CCW and CW interval distribut ions appear to be exponential, From the rate constants we computed equ ilibrium constants and standard free energy changes, and from the temp erature dependence of the standard free energy changes we determined s tandard enthalpy and entropy changes. Using transition-slate theory, w e also calculated the activation free energy, enthalpy, and entropy. W e conclude that the CW state is preferred at very low temperatures and that it is relatively more highly bonded and restricted than the CCW state.