Rusty, jammed, and well-oiled hinges:: Mutations affecting the interdomain region of FliG, a rotor element of the Escherichia coli flagellar motor

The FliG protein is a central component of the bacterial flagellar motor. It is one of the first proteins added during assembly of the flagellar basal body, and there are 26 copies per motor. FliG interacts directly with the Mot protein complex of the stator to generate torque, and it is a crucial player in switching the direction of flagellar rotation from clockwise (CW) to counterclockwise and vice versa. A primarily helical linker joins the N-terminal assembly domain of FliG, which is firmly attached to the FliF protein of the MS ring of the basal body, to the motility domain that interacts with MotA/MotB. We report here the results of a mutagenic analysis focused on what has been called the hinge region of the linker. Residue substitutions in this region generate a diversity of phenotypes, including motors that are strongly CW biased, infrequent switchers, rapid switchers, and transiently or permanently paused. Isolation of these mutants was facilitated by a sensitizing mutation (E232G) outside of the hinge region that was accidentally introduced during cloning of the chromosomal fliG gene into our vector plasmid. This mutation partially interferes with flagellar assembly and accentuates the defects associated with mutations that by themselves have little phenotypic consequence. The effects of these mutations are analyzed in the context of a conformational-coupling model for motor switching and with respect to the structure of the C-terminal 70% of FliG from Thermotoga maritima.