Mutations in the fliK gene of Salmonella typhimurium commonly cause failure to terminate hook assembly and initiate filament assembly (polyhook phenotype). Polyhook mutants give rise to pseudorevertants which are still defective in hook termination but have recovered the ability to assemble filament (polyhook-filament phenotype). The polyhook mutations have been found to be either frameshift or nonsense, resulting in truncation of the C terminus of FliK. Intragenic suppressors of frameshift mutations were found to be ones that restored the original frame (and therefore the C-terminal sequence), but in most cases with substantial loss of natural sequence and sometimes the introduction of artificial sequence; in no cases did intragenic suppression occur when significant disruption remained within

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FliG, FIiM, and FliN are three proteins of Salnonella typhimurium that affect the rotation and switching of direction of the flagellar motor. An analysis of mutant alleles of FliM has been described recently (H. Sockett, S. Yamaguchi, M. Kihara, V. M. Irikura, and R. M. Macnab, J. Bacteriol. 174:793-806, 1992). We have now analyzed a large number of mutations in thefliG andfliN genes that are responsible for four different types of defects: failure to assembly flagella (nonflagellate phenotype), failure to rotate flagella (paralyzed phenotype), and failure to display normal chemotaxis as a result of an abnormally high bias to clockwise (CW) or counterclockwise (CCW) rotation (CW-bias and CCW-bias phenotypes, respectively). The null phenotype for fliG, caused by nonsense or frameshift mutations, was nonflagellate. However, a considerable part of the FliG amino acid sequence was not needed for flagellation, with several substantial in-frame deletions preventing motor rotation but not flagellar assembly. Missense mutations infliG causing paralysis or abnormal switching occurred at a number of positions, almost all within the middle one-third of the gene. CW-bias and CCW-bias mutations tended to segregate into separate subclusters. The null phenotype of fliN is uncertain, since frameshift and nonsense mutations gave in some cases the nonflagellate phenotype and in other cases the paralyzed phenotype; in none of these cases was the phenotype a consequence of polar effects on downstream flagellar genes. Few positions in FliN were found to affect switching: only one gave rise to the CW mutant bias and only four gave rise to the CCW mutant bias. The different properties of the FliM, FliG, and FliN proteins with respect to the processes of assembly, rotation, and switching are discussed.The direction of rotation of the bacterial flagellum of Salmonella typhimurium is under the control of the flagellar switch, a structure that on genetic grounds is thought to consist of three proteins, FliG, FliM, and FliN, all of which are needed to permit rotation to occur and to control its direction (27). These proteins also affect the structural integrity of the flagellum, so that some mutations, especially severe ones (19), result in failure to assemble flagella at all.The default rotational state of the switch is counterclockwise (CCW), with the clockwise (CW) state being stabilized by a component of the sensory transduction system, CheY, following its phosphorylation by another component, CheA; the simplest hypothesis is that phosphorylated CheY acts by binding to the switch. Still another chemotaxis protein, CheZ, catalyzes the dephosphorylation of CheY, rendering it incapable of stabilizing the CW state. Thus, CheZ has the effect of favoring the CCW state of the switch (for a review, see reference 3).We have recently described an extensive analysis of mutations infliM of S. typhimurium (19

Previous studies have led to the conclusion that in Salmonella typhimurium (and also in Escherichia coli) there are three flagellar proteins, FliG, FliM, and FliN, which function together in enabling motor rotation and in controlling its direction of rotation (2,4,10,35,41). The part of the flagellum constructed from these proteins is called the flagellar switch. The state of the switch, counterclockwise (CCW) or clockwise (CW), is influenced by the chemotaxis sensory system, such that CCW rotation is enhanced by favorable stimuli and CW rotation is enhanced by unfavorable stimuli (14). On the basis of intergenic suppression analysis in E. coli (24) and the effect of CheY on cell envelopes lacking any of the other chemotaxis proteins (25), CheY is thought to bind to the switch and bias it to the CW state. CheZ has the opposite effect on switch bias (13), and intergenic suppression analysis (23,24) suggests that it, too, binds to the switch; however, in this case no biochemical evidence is available to support the hypothesis. CheY is activated by CheA (by phosphorylation of aspartate 57) and deactivated by CheZ (by dephosphorylation) (1,6,27,38).