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The predatory bacterium Bdellovibrio bacteriovorus uses flagellar motility to locate regions rich in Gramnegative prey bacteria, colliding and attaching to prey and then ceasing flagellar motility. Prey are then invaded to form a "bdelloplast" in a type IV pilus-dependent process, and prey contents are digested, allowing Bdellovibrio growth and septation. After septation, Bdellovibrio flagellar motility resumes inside the prey bdelloplast prior to its lysis and escape of Bdellovibrio progeny. Bdellovibrio can also grow slowly outside prey as long flagellate host-independent (

Flagellate bacteria such as Escherichia coli and Salmonella enterica serovar Typhimurium typically express 5 to 12 flagellar filaments over their cell surface that rotate in clockwise (CW) and counterclockwise directions. These bacteria modulate their swimming direction towards favorable environments by biasing the direction of flagellar rotation in response to various stimuli. In contrast, Rhodobacter sphaeroides expresses a single subpolar flagellum that rotates only CW and responds tactically by a series of biased stops and starts. Rotor protein FliG transiently links the MotAB stators to the rotor, to power rotation and also has an essential function in flagellar export. In this study, we sought to determine whether the FliG protein confers directionality on flagellar motors by testing the functional properties of R. sphaeroides FliG and a chimeric FliG protein, EcRsFliG (N-terminal and central domains of E. coli FliG fused to an R. sphaeroides FliG C terminus), in an E. coli FliG null background. The EcRsFliG chimera supported flagellar synthesis and bidirectional rotation; bacteria swam and tumbled in a manner qualitatively similar to that of the wild type and showed chemotaxis to amino acids. Thus, the FliG C terminus alone does not confer the unidirectional stop-start character of the R. sphaeroides flagellar motor, and its conformation continues to support tactic, switch-protein interactions in a bidirectional motor, despite its evolutionary history in a bacterium with a unidirectional motor.Bacteria swim and respond chemotactically to various stimuli by biasing the rotational behavior of their flagella when an internal phosphorelay network signals a change in chemotactic receptor occupancy on the cell surface. In Escherichia coli, flagellar rotation alternates between clockwise (CW) and counter-clockwise (CCW) directions, where CW rotation leads to cell tumbling and reorientation, and CCW rotation produces smooth swimming and thus forward movement (21). In contrast, Rhodobacter sphaeroides has a unidirectional flagellum that alternates between CW rotation and brief stops, where the bacterium is reoriented by Brownian motion and changes in flagellar filament morphology (2).The molecular mechanisms underlying flagellar torque generation are not fully understood, due mainly to a lack of structural data. For recent reviews, see references 4, 5, 22, and 26. The nonrotating stator component is composed of MotA and MotB proteins, which together form proton-conducting ion channels. MotAB channels encircle the MS ring and C ring at the base of the flagellum. A ring of FliG subunits, attached at their N termini to the MS ring, project into the cytoplasm from the C ring. As protons flow through MotAB, they are thought to transiently bind to and dissociate from a critical aspartate residue (38) located on MotB, inducing conformational changes (17) in the cytoplasmic domain of MotA that are believed to apply force to rotor protein subunits of FliG, thus turning the motor (17, 26). These stator-rotor interactions ar...