Switching of bacterial flagellar motors is triggered by mutant FliG

Binding of the chemotaxis response regulator CheY-P promotes switching between rotational states in flagellar motors of the bacterium Escherichia coli. Here, we induced switching in the absence of CheY-P by introducing copies of a mutant FliG locked in the clockwise (CW) conformation (FliG^CW). The composition of the mixed FliG ring was estimated via fluorescence imaging, and the probability of CW rotation (CW_bias) was determined from the rotation of tethered cells. The results were interpreted in the framework of a 1D Ising model. The data could be fit by assuming that mutant subunits are more stable in the CW conformation than in the counterclockwise conformation. We found that CW_bias varies depending on the spatial arrangement of the assembled subunits in the FliG ring. This offers a possible explanation for a previous observation of hysteresis in the switch function in analogous mixed FliM motors - in motors containing identical fractions of mutant FliM^CW in otherwise wild-type motors, the CW_bias differed depending on whether mutant subunits were expressed in strains with native motors or native subunits were expressed in strains with mutant motors.