REGULATION OF SWITCHING FREQUENCY AND BIAS OF THE BACTERIAL FLAGELLARMOTOR BY CHEY AND FUMARATE

The effect of CheY and fumarate on switching frequency and rotational bias of the bacterial flagellar motor was analyzed by computer-aided t racking of tethered Escherichia coli, Plots of cells overexpressing Ch eY in a gutted background showed a bell-shaped correlation curve of sw itching frequency and bias centering at about 50% clockwise rotation, Gutted cells (i.e., with cheA to cheZ deleted) with a low CheY level b ut a high cytoplasmic fumarate concentration displayed the same correl ation of switching frequency and bias as cells overexpressing CheY at the wild-type fumarate level. Hence, a high fumarate level can phenoty pically mimic CheY overexpression by simultaneously changing the switc hing frequency and the bias, A linear correlation of cytoplasmic fumar ate concentration and clockwise rotation bias was found and predicts e xclusively counterclockwise rotation without switching when fumarate i s absent. This suggests that (i) fumarate is essential for clockwise r otation in vivo and (ii) any metabolically induced fluctuation of its cytoplasmic concentration will result in a transient change in bias an d switching probability. A high fumarate level resulted in a dose-resp onse curve linking bias and cytoplasmic CheY concentration that was of fset but with a slope similar to that for a low fumarate level, It is concluded that fumarate and CheY act additively presumably at differen t reaction steps in the conformational transition of the switch comple x from counterclockwise to clockwise motor rotation.