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Rhodobacter sphaeroides expresses two different flagellar systems, a subpolar flagellum (fla1) and multiple polar flagella (fla2). These structures are encoded by different sets of flagellar genes. The chemotactic control of the subpolar flagellum (fla1) is mediated by three of the six different CheY proteins (CheY6, CheY4, or CheY3). We show evidence that CheY1, CheY2, and CheY5 control the chemotactic behavior mediated by fla2 flagella and that RSP6099 encodes the fla2 FliM protein.Many bacteria move by using flagella as the locomotive organelle. Flagella alternate between clockwise and counterclockwise rotation or in some cases between rotation and brief stop periods, which allow the bacterial cell to swim in a linear trajectory or to reorient (1,14,15). Bacterial taxis is achieved by modifying the frequency of reorientation events. The bias in the frequency of reorientation is modified by the interaction of the phosphorylated form of the response regulator CheY-P with the switch protein (FliM), which is part of the motor-switch structure located at the base of the flagellum. The concentration of CheY-P is regulated by the chemotactic system (3).A limited number of bacteria possess dual flagellar systems and are able to express two different flagellum types: a polar flagellum for swimming and lateral flagella for swarming; these microorganisms include Vibrio parahaemolyticus (27), Vibrio alginolyticus (8), Aeromonas spp. (26), Azospirillum brasilense (18), Rhodospirillum centenum (17), Helicobacter mustelae (19), and Plesiomonas shigelloides (7). The best-characterized examples are those that belong to the Vibrio genus. The ability to synthesize two different types of flagella allows these microorganisms to populate various niches (16).Rhodobacter sphaeroides is a facultative nonsulfur photosynthetic bacterium. Its genome sequence revealed many intriguing features related to motility and chemotaxis (13), including the presence of two flagellar gene sets. The genes that belong to the first set (fla1) are expressed constitutively and allow the strong swimming previously reported for the WS8 wild-type strain (1,10,21,22). The genes of the second flagellar set are not expressed in the wild-type WS8 strain; however, strains that express it can be isolated. In contrast with the case for other dual-flagellum systems, the fla2 genes of R. sphaeroides produce polar flagella that allow swimming (21).Several chemotactic genes are also reiterated (two cheB genes, three cheR genes, four cheA and cheW genes, and six cheY genes). In spite of this, only some of these gene copies are required when the cell is swimming with the fla1 flagellum. For example, it has been reported that only CheY6 and either CheY3 or CheY4 are required for chemotaxis mediated by the fla1 flagellum (23), despite the fact that the six cheY genes are expressed (6). The presence of a second functional flagellum in R. sphaeroides suggests that some of the chemotactic genes could be involved in its tactic control. To test this hypothesis, we investigated...

Rhodobacter sphaeroides is a free-living alphaproteobacterium that contains two clusters of functional flagellar genes in its genome: one acquired by horizontal gene transfer (fla1) and one that is endogenous (fla2). We have shown that the Fla2 system is normally quiescent and under certain conditions produces polar flagella, while the Fla1 system is always active and produces a single flagellum at a nonpolar position. In this work we purified and characterized the structure and analyzed the composition of the Fla2 flagellum. The number of polar filaments per cell is 4.6 on average. By comparison with the Fla1 flagellum, the prominent features of the ultra structure of the Fla2 HBB are the absence of an H ring, thick and long hooks, and a smoother zone at the hook-filament junction. The Fla2 helical filaments have a pitch of 2.64 m and a diameter of 1.4 m, which are smaller than those of the Fla1 filaments. Fla2 filaments undergo polymorphic transitions in vitro and showed two polymorphs: curly (righthanded) and coiled. However, in vivo in free-swimming cells, we observed only a bundle of filaments, which should probably be left-handed. Together, our results indicate that Fla2 cell produces multiple right-handed polar flagella, which are not conventional but exceptional. ؊ mutant grown phototrophically and in the absence of organic acids. The Fla1 system produces a single lateral or subpolar flagellum, and the Fla2 system produces multiple polar flagella. The two kinds of flagella are never expressed simultaneously, and both are used for swimming in liquid media. The two sets of genes are certainly ready for responding to specific environmental conditions. The characterization of the Fla2 system will help us to understand its role in the physiology of this microorganism. Motility provides microorganisms with a fundamental survival advantage. Flagella are one of the most complex and effective organelles of locomotion, capable of propelling bacteria through liquids (swimming) and through viscous environments or over surfaces (swarming), and are widely used among Bacteria and Archaea. In addition, these organelles play an important role in adhesion to substrates and biofilm formation, and they contribute to the virulence process in pathogenic bacteria (1, 2).The bacterial flagellum is a rotary motor powered by the electrochemical proton or sodium potential. The morphology of the flagellum is similar among different bacterial species. Nevertheless, there are differences in their substructures, which are not yet clearly understood. The improvement in powerful microscopy techniques has revealed variations in the architecture of the bacterial flagellar motors (3, 4). The bacterial flagellum is a supramolecular complex made of about 30 different proteins with copy numbers that range from a few to thousands. The structure has been divided into three parts: filament, hook, and basal body. The basal body spans the bacterial cell envelope and comprises a rod and a series of rings. In the cytoplasm the basal body for...

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