The MotB protein of Escherichia coli is an essential component in each of eight torque generators in the flageliar rotary motor. Based on its membrane topology, it has been suggested that MotB might be a linker that fastens the torque-generating machinery to the cell wall. Here, we report the isolation and characterization of a number of motB mutants. As found previously for motA, many alleles of motB were dominant, as expected if MotB is a component of the motor. In other respects, however, the motB mutants differed from the motA mutants. Most of the mutations mapped to a hydrophilic, periplasmic domain of the protein, and nothing comparable to the slow-swimming alleles of motA, which show normal torque when tethered, was found. Some motB mutants retained partial function, but when tethered they produced subnormal torque, indicating that their motors contained only one or two functional torque generators. These results support the hypothesis that MotB is a linker.Cells of Escherichia coli and many other motile bacteria are propelled by helical filaments driven at their base by a reversible rotary motor (1,20); the motor-filament organelle is called a flagellum (for recent reviews, see references 3 and 14). The energy for rotation comes from the movement of protons down an electrochemical gradient across the cytoplasmic membrane, rather than from the hydrolysis of ATP (12, 16). The detailed mechanism underlying this energy conversion is not understood.The motor apparatus contains about 25 different kinds of proteins (14). Mutations in most of the genes encoding the motor parts causes the flagellum to be assembled incompletely or not at all. Accordingly, these proteins are believed to be important for flagellar structure but probably not for torque generation per se. Five proteins, called MotA, MotB, FliG, FliM, and FliN, can be mutated to give a paralyzed phenotype, in which a normal-looking basal body is built but does not rotate. These proteins are likely to be more directly involved in torque generation. Certain alleles of fliG, fliM, andfliN are nonflagellate, whereas other alleles are defective in switching the direction of motor rotation, so these proteins also are important for flagellar assembly and for chemotaxis (28,29). Mutations in motA and motB can impair motility, but none has been found that affects assembly or switching.Earlier work showed that MotA and MotB are components in each of eight independent torque generators in the motor (4, 6). The MotA and MotB proteins are associated with the cytoplasmic membrane (7,8,21,26,27 linker that fastens MotA, or other components of the torquegenerating machinery, to the cell wall (8). This linkage is necessary to ensure that the motor components that apply force at the base of the filament remain stationary with respect to the cell as a whole.We have used hydroxylamine to mutagenize plasmids carrying motB, examined the phenotypes of motB-deficient and wild-type cells transformed with the mutant plasmids, and determined the corresponding nucleotide base cha...
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