TORQUE GENERATION IN THE FLAGELLAR MOTOR OF ESCHERICHIA-COLI - EVIDENCE OF A DIRECT ROLE FOR FLIG BUT NOT FOR FLIM OR FLIN

Among the many proteins needed for assembly arid function of bacterial flagella, FliG, FliM, and FliN have attracted special attention becau se mutant phenotypes suggest that they are needed not only for flagell ar assembly but also for torque generation and for controlling the dir ection of motor rotation. A role for these proteins in torque generati on is suggested by the existence of mutations in each of them that pro duce the Mot(-) (or paralyzed) phenotype, in which flagella are assemb led and appear normal but do not rotate. The presumption is that Mot(- ) defects cause paralysis by specifically disrupting functions essenti al for torque generation, while preserving the features of a protein n eeded for flagellar assembly. Here, we present evidence that the repor ted mot mutations fliM and fliN do not disrupt torque-generating funct ions specifically but, instead, affect the incorporation of proteins i nto the flagellum. The fliM and fliN mutants are immotile at normal ex pression levels but become motile when the mutant proteins and/or othe r, evidently interacting flagellar proteins are overexpressed. In cont rast, many of the reported fliG mot mutations abolish motility at all expression levels, while permitting flagellar assembly, and thus appea r to disrupt torque generation specifically. These mutations are clust ered in a segment of about 100 residues at the carboxyl terminus of Fl iG. A slightly larger carboxyl-terminal segment of 126 residues accumu lates in the cells when expressed alone and thus probably constitutes a stable, independently folded domain. We suggest that the carboxyl-te rminal domain of FliG functions specifically in torque generation, for ming the rotor portion of the site of energy transduction in the flage llar motor.