High-speed rotation and speed stability of the sodium-driven flagellar motor in Vibrio alginolyticus

Regulation of the Single Polar Flagellar Biogenesis

Some bacterial species, such as the marine bacterium Vibrio alginolyticus, have a single polar flagellum that allows it to swim in liquid environments. Two regulators, FlhF and FlhG, function antagonistically to generate only one flagellum at the cell pole. FlhF, a signal recognition particle (SRP)-type guanosine triphosphate (GTP)ase, works as a positive regulator for flagellar biogenesis and determines the location of flagellar assembly at the pole, whereas FlhG, a MinD-type ATPase, works as a negative regulator that inhibits flagellar formation. FlhF intrinsically localizes at the cell pole, and guanosine triphosphate (GTP) binding to FlhF is critical for its polar localization and flagellation. FlhG also localizes at the cell pole via the polar landmark protein HubP to directly inhibit FlhF function at the cell pole, and this localization depends on ATP binding to FlhG. However, the detailed regulatory mechanisms involved, played by FlhF and FlhG as the major factors, remain largely unknown. This article reviews recent studies that highlight the post-translational regulation mechanism that allows the synthesis of only a single flagellum at the cell pole.

Microbial Morphology and Motility as Biosignatures for Outer Planet Missions

Meaningful motion is an unambiguous biosignature, but because life in the Solar System is most likely to be microbial, the question is whether such motion may be detected effectively on the micrometer scale. Recent results on microbial motility in various Earth environments have provided insight into the physics and biology that determine whether and how microorganisms as small as bacteria and archaea swim, under which conditions, and at which speeds. These discoveries have not yet been reviewed in an astrobiological context. This paper discusses these findings in the context of Earth analog environments and environments expected to be encountered in the outer Solar System, particularly the jovian and saturnian moons. We also review the imaging technologies capable of recording motility of submicrometer-sized organisms and discuss how an instrument would interface with several types of sample-collection strategies. Key Words: In situ measurement-Biosignatures-Microbiology-Europa-Ice. Astrobiology 16, 755-774.

Effect of the MotB(D33N) mutation on stator assembly and rotation of the proton-driven bacterial flagellar motor

The bacterial flagellar motor generates torque by converting the energy of proton translocation through the transmembrane proton channel of the stator complex formed by MotA and MotB. The MotA/B complex is thought to be anchored to the peptidoglycan (PG) layer through the PG-binding domain of MotB to act as the stator. The stator units dynamically associate with and dissociate from the motor during flagellar motor rotation, and an electrostatic interaction between MotA and a rotor protein FliG is required for efficient stator assembly. However, the association and dissociation mechanism of the stator units still remains unclear. In this study, we analyzed the speed fluctuation of the flagellar motor of Salmonella enterica wild-type cells carrying a plasmid encoding a nonfunctional stator complex, MotA/B(D33N), which lost the proton conductivity. The wild-type motor rotated stably but the motor speed fluctuated considerably when the expression level of MotA/B(D33N) was relatively high compared to MotA/B. Rapid accelerations and decelerations were frequently observed. A quantitative analysis of the speed fluctuation and a model simulation suggested that the MotA/B(D33N) stator retains the ability to associate with the motor at a low affinity but dissociates more rapidly than the MotA/B stator. We propose that the stator dissociation process depends on proton translocation through the proton channel.

Load-sensitive coupling of proton translocation and torque generation in the bacterial flagellar motor

The Salmonella flagellar motor consists of a rotor and about a dozen stator elements. Each stator element, consisting of MotA and MotB, acts as a proton channel to couple proton flow with torque generation. A highly conserved Asp33 residue of MotB is directly involved in the energy coupling mechanism, but it remains unknown how it carries out this function. Here, we show that the MotB(D33E) mutation dramatically alters motor performance in response to changes in external load. Rotation speeds of the MotA/B(D33E) and MotA(V35F)/B(D33E) motors were markedly slower than the wild-type motor and fluctuated considerably at low load but not at high load, whereas the rotation rate of the wild-type motor was stable at any load. At low load, pausing events were frequently observed in both mutant motors. The proton conductivities of these mutant stator channels in their 'unplugged' forms were only half of the conductivity of the wild-type channel. These results suggest that the D33E mutation induces a load-dependent inactivation of the MotA/B complex. We propose that the stator element is a load-sensitive proton channel that efficiently couples proton translocation with torque generation and that Asp33 of MotB is critical for this co-ordinated proton translocation.

Ecology and physics of bacterial chemotaxis in the ocean

Intuitively, it may seem that from the perspective of an individual bacterium the ocean is a vast, dilute, and largely homogeneous environment. Microbial oceanographers have typically considered the ocean from this point of view. In reality, marine bacteria inhabit a chemical seascape that is highly heterogeneous down to the microscale, owing to ubiquitous nutrient patches, plumes, and gradients. Exudation and excretion of dissolved matter by larger organisms, lysis events, particles, animal surfaces, and fluxes from the sediment-water interface all contribute to create strong and pervasive heterogeneity, where chemotaxis may provide a significant fitness advantage to bacteria. The dynamic nature of the ocean imposes strong selective pressures on bacterial foraging strategies, and many marine bacteria indeed display adaptations that characterize their chemotactic motility as "high performance" compared to that of enteric model organisms. Fast swimming speeds, strongly directional responses, and effective turning and steering strategies ensure that marine bacteria can successfully use chemotaxis to very rapidly respond to chemical gradients in the ocean. These fast responses are advantageous in a broad range of ecological processes, including attaching to particles, exploiting particle plumes, retaining position close to phytoplankton cells, colonizing host animals, and hovering at a preferred height above the sediment-water interface. At larger scales, these responses can impact ocean biogeochemistry by increasing the rates of chemical transformation, influencing the flux of sinking material, and potentially altering the balance of biomass incorporation versus respiration. This review highlights the physical and ecological processes underpinning bacterial motility and chemotaxis in the ocean, describes the current state of knowledge of chemotaxis in marine bacteria, and summarizes our understanding of how these microscale dynamics scale up to affect ecosystem-scale processes in the sea.

Mutation in the a-subunit of F(1)F(O)-ATPase causes an increased motility phenotype through the sodium-driven flagella of Vibrio

Bacterial flagellar motors exploit the electrochemical potential gradient of a coupling ion as energy source and are composed of stator and rotor proteins. Vibrio alginolyticus has a Na(+)-driven motor and its stator is composed of PomA and PomB. Recently, we isolated increased motility strains (sp1-sp4) from the PomA-N194D/PomB-D24N mutant whose motility was quite weak. To detect the responsible mutation, we have used a next-generation sequencer and determined the entire genome sequences of the sp1 and sp2 strains. Candidate mutations were identified in the gene encoding the a-subunit of F1Fo-ATPase (uncB). To confirm this, we constructed a deletion strain, which gave the increased motility phenotype. The amount of membrane-bound ATPase was reduced in the sp2 and ΔuncB mutants. From these results, we conclude that a mutation in the uncB gene causes the increased motility phenotype in V. alginolyticus. They confer faster motility in low concentrations of sodium than in the parental strain and this phenotype is suppressed in the presence of KCN. Those results may suggest that the proton gradient generated by the respiratory chain is increased by the uncB mutation, consequently the sodium motive force is increased and causes the increased motility phenotype.

Control and benefits of CP4-57 prophage excision in Escherichia coli biofilms

Earlier, we discovered that the global regulator, Hha, is related to cell death in biofilms and regulates cryptic prophage genes. Here, we show that Hha induces excision of prophages, CP4-57 and DLP12, by inducing excision genes and by reducing SsrA synthesis. SsrA is a tmRNA that is important for rescuing stalled ribosomes, contains an attachment site for CP4-57 and is shown here to be required for CP4-57 excision. These prophages impact biofilm development, as the deletion of 35 genes individually of prophages, CP4-57 and DLP12, increase biofilm formation up to 17-fold, and five genes decrease biofilm formation up to sixfold. In addition, CP4-57 excises during early biofilm development but not in planktonic cells, whereas DLP12 excision was detected at all the developmental stages for both biofilm and planktonic cells. CP4-57 excision leads to a chromosome region devoid of prophage and to the formation of a phage circle (which is lost). These results were corroborated by a whole-transcriptome analysis that showed that complete loss of CP4-57 activated the expression of the flg, flh and fli motility operons and repressed expression of key enzymes in the tricarboxylic acid cycle and of enzymes for lactate utilization. Prophage excision also results in the expression of cell lysis genes that reduce cell viability (for example, alpA, intA and intD). Hence, defective prophages are involved in host physiology through Hha and in biofilm formation by generating a diversified population with specialized functions in terms of motility and nutrient metabolism.

MEMS technology for nanobio research

Micro- and nanotechnology have gathered 20 years of increasing research efforts. This research activity began and developed with the design and the fabrication of micro- and nanomechanisms, sensors and actuators, which range from 10nm to 100mum. More recent trends focus on the transfer of this technology know-how towards nanobiological topics and very wide range applications can be addressed. Among them, this review proposes various examples that include MEMS tweezers for molecular direct handling and characterization, single molecular characterization in femto-L chambers and dynamic microarray for cell positioning. The micromachined devices are described with bio-oriented experiences that are relevant to foresee their future contribution to drug discovery.

Bacterial flagellar motor

The bacterial flagellar motor is a reversible rotary nano-machine, about 45 nm in diameter, embedded in the bacterial cell envelope. It is powered by the flux of H+or Na+ions across the cytoplasmic membrane driven by an electrochemical gradient, the proton-motive force or the sodium-motive force. Each motor rotates a helical filament at several hundreds of revolutions per second (hertz). In many species, the motor switches direction stochastically, with the switching rates controlled by a network of sensory and signalling proteins. The bacterial flagellar motor was confirmed as a rotary motor in the early 1970s, the first direct observation of the function of a single molecular motor. However, because of the large size and complexity of the motor, much remains to be discovered, in particular, the structural details of the torque-generating mechanism. This review outlines what has been learned about the structure and function of the motor using a combination of genetics, single-molecule and biophysical techniques, with a focus on recent results and single-molecule techniques.

Towards single biomolecule handling and characterization by MEMS

Applications of microelectromechanical systems (MEMS) technology are widespread in both industrial and research fields providing miniaturized smart tools. In this review, we focus on MEMS applications aiming at manipulations and characterization of biomaterials at the single molecule level. Four topics are discussed in detail to show the advantages and impact of MEMS tools for biomolecular manipulations. They include the microthermodevice for rapid temperature alternation in real-time microscopic observation, a microchannel with microelectrodes for isolating and immobilizing a DNA molecule, and microtweezers to manipulate a bundle of DNA molecules directly for analyzing its conductivity. The feasibilities of each device have been shown by conducting specific biological experiments. Therefore, the development of MEMS devices for single molecule analysis holds promise to overcome the disadvantages of the conventional technique for biological experiments and acts as a powerful strategy in molecular biology.

Figure

Properties of motility in Bacillus subtilis powered by the H+-coupled MotAB flagellar stator, Na+-coupled MotPS or hybrid stators MotAS or MotPB

Bacillus subtilis has a single set of flagellar rotor proteins that interact with two distinct stator-force generators, the H+-coupled MotAB complex and the Na+-coupled MotPS complex, that energize rotation. Here, motility on soft agar plates and in liquid was assayed in wild-type B.subtilis and strains expressing only one stator, either MotAB, MotPS or hybrid MotAS or MotPB. The strains expressing MotAB or MotAS had an average of 11 flagella/cell while those expressing MotPS or MotPB had an average of seven flagella/cell, and a Mot-less double mutant had three to four flagella/cell. MotAB had a more dominant role in motility than MotPS under most conditions, but MotPS supported comparable motility to MotAB on malate-containing soft agar plating media at elevated pH and Na+. MotAB supported much faster swimming speeds in liquid than MotPS, MotAS or MotPB under all conditions, but a contribution of MotPS to wild-type swimming was discernible from differences in swimming speeds of wild-type and MotAB at elevated viscosity, pH and Na+. Swimming supported by MotPS and MotAS was stimulated by Na+ and elevated pH whereas the converse was true of MotAB and MotPB. This suggests that MotAS is Na+-coupled and MotPB is H+-coupled and that MotB and MotS are major determinants of ion-coupling. However, the swimming speed supported by MotPB, as well as MotPS and MotAS, was inhibited severely at Na+ concentrations above 300 mM whereas MotAB-dependent swimming was not. The presence of either the MotP or MotS component in the stator also conferred sensitivity to inhibition by an amiloride analogue. These observations suggest that MotP contributes to Na+-coupling and inhibition by Na+ channel inhibitors. Similarly, a role for MotA in H+-dependent stator properties is indicated by the larger effects of pH on the Na+-response of MotAS versus MotPS. Finally, optimal function at elevated viscosity was found only in MotPS and MotPB and is therefore conferred by MotP.

The rotary motor of bacterial flagella

Flagellated bacteria, such as Escherichia coli, swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. A motor is about 45 nm in diameter and is assembled from about 20 different kinds of parts. It develops maximum torque at stall but can spin several hundred Hz. Its direction of rotation is controlled by a sensory system that enables cells to accumulate in regions deemed more favorable. We know a great deal about motor structure, genetics, assembly, and function, but we do not really understand how it works. We need more crystal structures. All of this is reviewed, but the emphasis is on function.

Helix rotation model of the flagellar rotary motor

A new model of the flagellar motor is proposed that is based on established dynamics of the KcsA potassium ion channel and on known genetic, biochemical, and biophysical facts, which accounts for the mechanics of torque generation, force transmission, and reversals of motor rotation. It predicts that proton (or in some species sodium ion) flow generates short, reversible helix rotations of the MotA-MotB channel complex (the stator) that are transmitted by Coulomb forces to the FliG segments at the rotor surface. Channels are arranged as symmetric pairs, S and T, that swing back and forth in synchrony. S and T alternate in attaching to the rotor, so that force transmission proceeds in steps. The sense of motor rotation can be readily reversed by conformationally switching the position of charged groups on the rotor so that they interact with the stator during the reverse rather than forward strokes. An elastic device accounts for the observed smoothness of rotation and a prolonged attachment of the torque generators to the rotor, i.e., a high duty ratio of each torque-generating unit.

Torque-speed relationship of the Na+-driven flagellar motor of Vibrio alginolyticus

The torque-speed relationship of the Na(+)-driven flagellar motor of Vibrio alginolyticus was investigated. The rotation rate of the motor was measured by following the position of a bead, attached to a flagellar filament, using optical nanometry. In the presence of 50mM NaCl, the generated torque was relatively constant ( approximately 3800pNnm) at lower speeds (speeds up to approximately 300Hz) and then decreased steeply, similar to the H(+)-driven flagellar motor of Escherichia coli. When the external NaCl concentration was varied, the generated torque of the flagellar motor was changed over a wide range of speeds. This result could be reproduced using a simple kinetic model, which takes into consideration the association and dissociation of Na(+) onto the motor. These results imply that for a complete understanding of the mechanism of flagellar rotation it is essential to consider both the electrochemical gradient and the absolute concentration of the coupling ion.

Effect of intracellular pH on rotational speed of bacterial flagellar motors

Weak acids such as acetate and benzoate, which partially collapse the transmembrane proton gradient, not only mediate pH taxis but also impair the motility of Escherichia coli and Salmonella at an external pH of 5.5. In this study, we examined in more detail the effect of weak acids on motility at various external pH values. A change of external pH over the range 5.0 to 7.8 hardly affected the swimming speed of E. coli cells in the absence of 34 mM potassium acetate. In contrast, the cells decreased their swimming speed significantly as external pH was shifted from pH 7.0 to 5.0 in the presence of 34 mM acetate. The total proton motive force of E. coli cells was not changed greatly by the presence of acetate. We measured the rotational rate of tethered E. coli cells as a function of external pH. Rotational speed decreased rapidly as the external pH was decreased, and at pH 5.0, the motor stopped completely. When the external pH was returned to 7.0, the motor restarted rotating at almost its original level, indicating that high intracellular proton (H+) concentration does not irreversibly abolish flagellar motor function. Both the swimming speeds and rotation rates of tethered cells of Salmonella also decreased considerably when the external pH was shifted from pH 7.0 to 5.5 in the presence of 20 mM benzoate. We propose that the increase in the intracellular proton concentration interferes with the release of protons from the torque-generating units, resulting in slowing or stopping of the motors.

MotX and MotY, specific components of the sodium-driven flagellar motor, colocalize to the outer membrane in Vibrio alginolyticus

Rotation of the sodium-driven polar flagella of Vibrio alginolyticus requires four motor proteins: PomA, PomB, MotX and MotY. MotX and MotY, which are unique components of the sodium-driven motor of Vibrio, have been believed to be localized in the inner (cytoplasmic) membrane via their N-terminal hydrophobic segments. Here we show that MotX and MotY colocalize to the outer membrane. Both proteins, when expressed together, were detected in the outer membrane fraction separated by sucrose density gradient centrifugation. As mature MotX and MotY proteins do not have N-terminal hydrophobic segments, the N-termini of the primary translation products must have signal sequences that are removed upon translocation across the inner membrane. Moreover, MotX and MotY require each other for efficient localization to the outer membrane. Based on these lines of evidence, we propose that MotX and MotY form a complex in the outer membrane. This is the first case that describes motor proteins function in the outer membrane for flagellar rotation.

Na(+)-driven flagellar motor of Vibrio

Bacterial flagellar motors are molecular machines powered by the electrochemical potential gradient of specific ions across the membrane. Bacteria move using rotating helical flagellar filaments. The flagellar motor is located at the base of the filament and is buried in the cytoplasmic membrane. Flagellar motors are classified into two types according to the coupling ion: namely the H(+)-driven motor and the Na(+)-driven motor. Analysis of the flagellar motor at the molecular level is far more advanced in the H(+)-driven motor than in the Na(+)-driven motor. Recently, the genes of the Na(+)-driven motor have been cloned from a marine bacterium of Vibrio sp. and some of the motor proteins have been purified and characterized. In this review, we summarize recent studies of the Na(+)-driven flagellar motor.

Grabbing the cat by the tail: manipulating molecules one by one

Methods for manipulating single molecules are yielding new information about both the forces that hold biomolecules together and the mechanics of molecular motors. We describe here the physical principles behind these methods, and discuss their capabilities and current limitations.

Multimeric structure of PomA, a component of the Na+-driven polar flagellar motor of vibrio alginolyticus

Four integral membrane proteins, PomA, PomB, MotX, and MotY, are thought to be directly involved in torque generation of the Na(+)-driven polar flagellar motor of Vibrio alginolyticus. Our previous study showed that PomA and PomB form a complex, which catalyzes sodium influx in response to a potassium diffusion potential. PomA forms a stable dimer when expressed in a PomB null mutant. To explore the possible functional dependence of PomA domains in adjacent subunits, we prepared a series of PomA dimer fusions containing different combinations of wild-type or mutant subunits. Introduction of the mutation P199L, which completely inactivates flagellar rotation, into either the first or the second half of the dimer abolished motility. The P199L mutation in monomeric PomA also altered the PomA-PomB interaction. PomA dimer with the P199L mutation even in one subunit also had no ability to interact with PomB, indicating that the both subunits in the dimer are required for the functional interaction between PomA and PomB. Flagellar rotation by wild-type PomA dimer was completely inactivated by phenamil, a sodium channel blocker. However, activity was retained in the presence of phenamil when either half of the dimer was replaced with a phenamil-resistant subunit, indicating that both subunits must bind phenamil for motility to be fully inhibited. These observations demonstrate that both halves of the PomA dimer function together to generate the torque for flagellar rotation.

A slow-motility phenotype caused by substitutions at residue Asp31 in the PomA channel component of a sodium-driven flagellar motor

PomA is thought to be a component of the ion channel in the sodium-driven polar-flagellar motor of Vibrio alginolyticus. We have found that some cysteine substitutions in the periplasmic region of PomA result in a slow-motility phenotype, in which swarming and swimming speeds are reduced even in the presence of high concentrations of NaCl. Most of the mutants showed a sodium ion dependence similar to that of the wild type but with significantly reduced motility at all sodium ion concentrations. By contrast, motility of the D31C mutant showed a sharp dependence on NaCl concentration, with a threshold at 38 mM. The motor of the D31C mutant rotates stably, as monitored by laser dark-field microscopy, suggesting that the mutant PomA protein is assembled normally into the motor complex. Mutational studies of Asp31 suggest that, although this residue is not essential for motor rotation, a negative charge at this position contributes to optimal speed and/or efficiency of the motor.

Constraints on models for the flagellar rotary motor

Most bacteria that swim are propelled by flagellar filaments, each driven at its base by a rotary motor embedded in the cell wall and cytoplasmic membrane. A motor is about 45 nm in diameter and made up of about 20 different kinds of parts. It is assembled from the inside out. It is powered by a proton (or in some species, a sodium-ion) flux. It steps at least 400 times per revolution. At low speeds and high torques, about 1000 protons are required per revolution, speed is proportional to protonmotive force, and torque varies little with temperature or hydrogen isotope. At high speeds and low torques, torque increases with temperature and is sensitive to hydrogen isotope. At room temperature, torque varies remarkably little with speed from about -100 Hz (the present limit of measurement) to about 200 Hz, and then it declines rapidly reaching zero at about 300 Hz. These are facts that motor models should explain. None of the existing models for the flagellar rotary motor completely do so.

Cysteine-scanning mutagenesis of the periplasmic loop regions of PomA, a putative channel component of the sodium-driven flagellar motor in Vibrio alginolyticus

The sodium-driven motor consists of the products of at least four genes, pomA, pomB, motX, and motY, in Vibrio alginolyticus. PomA and PomB, which are homologous to the MotA and MotB components of proton-driven motors, have four transmembrane segments and one transmembrane segment, respectively, and are thought to form an ion channel. In PomA, two periplasmic loops were predicted at positions 21 to 36 between membrane segments 1 and 2 (loop(1-2)) and at positions 167 to 180 between membrane segments 3 and 4 (loop(3-4)). To characterize the two periplasmic loop regions, which may have a role as an ion entrance for the channel, we carried out cysteine-scanning mutagenesis. The T186 residue in the fourth transmembrane segment and the D71, D148, and D202 residues in the predicted cytoplasmic portion of PomA were also replaced with Cys. Only two mutations, M179C and T186C, conferred a nonmotile phenotype. Many mutations in the periplasmic loops and all of the cytoplasmic mutations did not abolish motility, though the five successive substitutions from M169C to K173C of loop(3-4) impaired motility. In some mutants that retained substantial motility, motility was inhibited by the thiol-modifying reagents dithionitrobenzoic acid and N-ethylmaleimide. The profiles of inhibition by the reagents were consistent with the membrane topology predicted from the hydrophobicity profiles. Furthermore, from the profiles of labeling by biotin maleimide, we predicted more directly the membrane topology of loop(3-4). None of the loop(1-2) residues were labeled, suggesting that the environments around the two loops are very different. A few of the mutations were characterized further. The structure and function of the loop regions are discussed.

Torque-generating units of the flagellar motor of Escherichia coli have a high duty ratio

Rotation of the bacterial flagellar motor is driven by an ensemble of torque-generating units containing the proteins MotA and MotB. Here, by inducing expression of MotA in motA- cells under conditions of low viscous load, we show that the limiting speed of the motor is independent of the number of units: at vanishing load, one unit turns the motor as rapidly as many. This result indicates that each unit may remain attached to the rotor for most of its mechanochemical cycle, that is, that it has a high duty ratio. Thus, torque generators behave more like kinesin, the protein that moves vesicles along microtubules, than myosin, the protein that powers muscle. However, their translation rates, stepping frequencies and power outputs are much higher, being greater than 30 microm s(-1), 12 kHz and 1.5 x 10(5) pN nm s(-1), respectively.

Hybrid motor with H(+)- and Na(+)-driven components can rotate Vibrio polar flagella by using sodium ions

The bacterial flagellar motor is a molecular machine that converts ion flux across the membrane into flagellar rotation. The coupling ion is either a proton or a sodium ion. The polar flagellar motor of the marine bacterium Vibrio alginolyticus is driven by sodium ions, and the four protein components, PomA, PomB, MotX, and MotY, are essential for motor function. Among them, PomA and PomB are similar to MotA and MotB of the proton-driven motors, respectively. PomA shows greatest similarity to MotA of the photosynthetic bacterium Rhodobacter sphaeroides. MotA is composed of 253 amino acids, the same length as PomA, and 40% of its residues are identical to those of PomA. R. sphaeroides MotB has high similarity only to the transmembrane region of PomB. To examine whether the R. sphaeroides motor genes can function in place of the pomA and pomB genes of V. alginolyticus, we constructed plasmids including both motA and motB or motA alone and transformed them into missense and null pomA-paralyzed mutants of V. alginolyticus. The transformants from both strains showed restored motility, although the swimming speeds were low. On the other hand, pomB mutants were not restored to motility by any plasmid containing motA and/or motB. Next, we tested which ions (proton or sodium) coupled to the hybrid motor function. The motor did not work in sodium-free buffer and was inhibited by phenamil and amiloride, sodium motor-specific inhibitors, but not by a protonophore. Thus, we conclude that the proton motor component, MotA, of R. sphaeroides can generate torque by coupling with the sodium ion flux in place of PomA of V. alginolyticus.

Suppression by the DNA fragment of the motX promoter region on long flagellar mutants of Vibrio alginolyticus

The axial length of the polar flagellum (Pof) of Vibrio alginolyticus is about 5 microm. We previously isolated mutants that make abnormally long flagella. The swarm sizes of these mutants in a soft agar plate are smaller than that of a wild-type strain. We cloned a DNA fragment into the pMF209 plasmid that restored the swarming ability of the long-Pof strain V10578. The swimming speed and flagellar length of these transformants were almost equal to the wild-type values. The amounts of PF47 flagellin and PF60 sheath-associated protein, which increased in the long-Pof mutants, were retrieved to almost the wild-type level in the transformants. The plasmid pMF209 contained only a 143 bp chromosomal fragment whose sequence is about 80% similar to that of the motX promoter region of V parahaemolyticus. We speculate that this sequence interacts with a regulatory protein that controls Pof expression. The mutation causing the long-Pof phenotype may be in the gene encoding this protein or in the control region of a structural gene that is regulated by this protein.

Na+-driven flagellar motor resistant to phenamil, an amiloride analog, caused by mutations in putative channel components

The rotation of the Na+-driven flagellar motor is specifically and strongly inhibited by phenamil, an amiloride analog. Here, we provide the first evidence that phenamil interacts directly with the Na+-channel components (PomA and PomB) of the motor. The alterations in Mpar (motility resistant to phenamil) strains were mapped to the pomA and/or pomB genes. We cloned and sequenced pomA and pomB from two Mpar strains, NMB205 and NMB201, and found a substitution in pomA (Asp148 to Tyr; NMB205) and in pomB (Pro16 to Ser; NMB201). Both residues are predicted to be near the cytoplasmic ends of the putative transmembrane segments. Mutational analyses at PomA-Asp148 and PomB-Pro16 suggest that a certain structural change around these residues affects the sensitivity of the motor to phenamil. Co-expression of the PomA D148Y and PomB P16S proteins resulted in an Mpar phenotype which seemed to be less sensitive to phenamil than either of the single mutants, although motility was more severely impaired in the absence of inhibitors. These results support the idea that PomA and PomB interact with each other and suggest that multiple residues, including Asp148 of PomA and Pro16 of PomB, constitute a high-affinity phenamil-binding site at the inner face of the PomA/PomB channel complex.

Deletion analysis of MotA and MotB, components of the force-generating unit in the flagellar motor of Salmonella

MotA and MotB are cytoplasmic membrane proteins that form the force-generating unit of the flagellar motor in Salmonella typhimurium and many other bacteria. Many missense mutations in both proteins are known to cause slow motor rotation (slow-motile phenotype) or no rotation at all (non-motile or paralysed phenotype). However, large stretches of sequence in the cytoplasmic regions of MotA and in the periplasmic region of MotB have failed to yield these types of mutations. In this study, we have investigated the effect of a series of 10-amino-acid deletions in these phenotypically silent regions. In the case of MotA, we found that only the C-terminal 5 amino acids were completely dispensable; an adjacent 10 amino acids were partially dispensable. In the cytoplasmic loop region of MotA, deletions made the protein unstable. For MotB, we found that two large segments of the periplasmic region were dispensable: the results with individual deletions showed that the first consisted of six deletions between the sole transmembrane span and the peptidoglycan binding motif, whereas the second consisted of four deletions at the C-terminus. We also found that deletions in the MotB cytoplasmic region at the N-terminus impaired motility but did not abolish it. Further investigations in MotB were carried out by combining dispensable deletion segments. The most extreme version of MotB that still retained some degree of function lacked a total of 99 amino acids in the periplasmic region, beginning immediately after the transmembrane span. These results indicate that the deleted regions in the MotA cytoplasmic loop region are essential for stability; they may or may not be directly involved in torque generation. Part of the MotA C-terminal cytoplasmic region is not essential for torque generation. MotB can be divided into three regions: an N-terminal region of about 30 amino acids in the cytoplasm, a transmembrane span and about 260 amino acids in the periplasm, including a peptidoglycan binding motif. In the periplasmic region, we suggest that the first of the two dispensable stretches in MotB may comprise part of a linker between the transmembrane span of MotB and its attachment point to the peptidoglycan layer, and that the length or specific sequence of much of that linker sequence is not critical. About 40 residues at the C-terminus are also unimportant.

Cloning of a Vibrio alginolyticus rpoN gene that is required for polar flagellar formation

A fragment of DNA was cloned which complemented a polar flagellum-defective (pof) mutation of Vibrio alginolyticus. The fragment contained two complete and two partial open reading frames (ORFs) (ORF2 and -3 and ORF1 and -4, respectively). The presumed product of ORF2 has an amino acid sequence with a high degree of similarity to that of RpoN, which is an alternative sigma factor (sigma54) for other microorganisms. The other ORFs are also homologous to the genes adjacent to other rpoN genes. Deletion analysis suggests that ORF2 complements the pof mutation. These results demonstrate that RpoN is involved in the expression of polar flagellar genes.

Putative channel components for the fast-rotating sodium-driven flagellar motor of a marine bacterium

The polar flagellum of Vibrio alginolyticus rotates remarkably fast (up to 1,700 revolutions per second) by using a motor driven by sodium ions. Two genes, motX and motY, for the sodium-driven flagellar motor have been identified in marine bacteria, Vibrio parahaemolyticus and V. alginolyticus. They have no similarity to the genes for proton-driven motors, motA and motB, whose products constitute a proton channel. MotX was proposed to be a component of a sodium channel. Here we identified additional sodium motor genes, pomA and pomB, in V. alginolyticus. Unexpectedly, PomA and PomB have similarities to MotA and MotB, respectively, especially in the predicted transmembrane regions. These results suggest that PomA and PomB may be sodium-conducting channel components of the sodium-driven motor and that the motor part consists of the products of at least four genes, pomA, pomB, motX, and motY. Furthermore, swimming speed was controlled by the expression level of the pomA gene, suggesting that newly synthesized PomA proteins, which are components of a force-generating unit, were successively integrated into the defective motor complexes. These findings imply that Na+-driven flagellar motors may have similar structure and function as proton-driven motors, but with some interesting differences as well, and it is possible to compare and study the coupling mechanisms of the sodium and proton ion flux for the force generation.

Characterization of polar-flagellar-length mutants in Vibrio alginolyticus

Vibrio alginolyticus has two types of flagella, polar (Pof) and lateral (Laf). From a Laf-defective mutant (Pof+Laf-), polar-flagellar-length mutants which have short Pof and long Pof were isolated. The mean lengths of the helical axis in wild-type, short and long Pof were 5.5.0.9 μm, 2.5.0.6 μm and 11.2.3.6 μm, respectively. The swimming speeds of the short- and long-Pof mutants were slower than that of the wild-type strain. The relationship between swimming speed and flagellar length in a population of mutant cells was examined. In the short-Pof mutant, the decrease of swimming speed seemed to be derived from the decrease in flagellar length. In the long-Pof mutant, there was almost no correlation between swimming speed and flagellar length, and the slow swimming was explained by the helical shape of the flagella, whose pitch and radius were 1.4 μm and 0.062 μm, respectively, whereas those of the wild-type flagella were 1.5 μm and 0.16 μm. The relative amounts of the various molecular components of the long Pof were different from those of the wild-type or the short Pof. This seems to be the reason for the difference in flagellar shape and length, though the mutation may be pleiotropic and affect flagellar function or regulation.

Vibrio alginolyticus mutants resistant to phenamil, a specific inhibitor of the sodium-driven flagellar motor

The polar flagella of Vibrio alginolyticus are driven by sodium motive force and those motors are specifically and strongly inhibited by phenamil, an amiloride analog that is thought to interact with a sodium channel of the flagellar motor. To study the sodium ion coupling site, we isolated motility mutants resistant to phenamil and named the phenotype Mpa(r) for motility resistant to phenamil. The motility of the wild-type (Mpa(s)) was inhibited by 50 microM phenamil, whereas Mpa(r) strains were still motile in the presence of 200 microM phenamil. The Ki value for phenamil in the Mpa(r) strain was estimated to be five times larger than that in the Mpa(s) strain. However, the sensitivities to amiloride or benzamil, another amiloride analog, were not distinctly changed in the Mpa(r) strain. The rotation rate of the wild-type Na+-driven motor fluctuates greatly in the presence of phenamil, which can be explained in terms of a relatively slow dissociation rate of phenamil from the motor. We therefore studied the stability of the rotation of the Mpa(r) and Mpa(s) motors by phenamil. The speed fluctuations of the Mpa(r) motors were distinctly reduced relative to the Mpas motors. The steadier rotation of the Mpa(r) motors can be explained by an increase in the phenamil dissociation rate from a sodium channel of the motor, which suggests that a phenamil-specific binding site of the motor is mutated in the Mpa(r) strain.

The energy transmission in ATP synthase: from the gamma-c rotor to the alpha 3 beta 3 oligomer fixed by OSCP-b stator via the beta DELSEED sequence

ATP synthase (F0F1) is driven by an electrochemical potential of H+ (delta microH+). F0F1 is composed of an ion-conducting portion (F0) and a catalytic portion (F1). The subunit composition of F1 is a alpha 3 beta 3 gamma delta epsilon. The active alpha 3 beta 3 oligomer, characterized by X-ray crystallography, has been obtained only from thermophilic F1 (TF1). We proposed in 1984 that ATP is released from the catalytic site (C site) by a conformational change induced by the beta DELSEED sequence via gamma delta epsilon-F0. In fact, cross-linking of beta DELSEED to gamma stopped the ATP-driven rotation of gamma in the center of alpha 3 beta 3. The torque of the rotation is estimated to be 420 pN x A from the delta microH+ and H(+)-current through F0F1. The angular velocity (omega) of gamma is the rate-limiting step, because delta microH+ increased the Vmax of H+ current through F0, but not the Km(ATP). The rotational unit of F0 (= ab2c10) is pi/5, while that in alpha 3 beta 3 is 2 pi/3. This difference is overcome by an analog-digital conversion via elasticity around beta DELSEED with a threshold to release ATP. The alpha beta distance at the C site is about 9.6 A (2,8-diN3-ATP), and tight Mg-ATP binding in alpha 3 beta 3 gamma was shown by ESR. The rotational relaxation of TF1 is too rapid (phi = 100 nsec), but the rate of AT(D)P-induced conformational change of alpha 3 beta 3 measured with a synchrotron is close to omega. The ATP bound between the P-loop and beta E188 is released by the shift of beta DELSEED from gamma RGL. Considering the viscosity resistance and inertia of the free rotor (gamma-c), there may be a stator containing OSCP (= delta of TF1) and F0-d to hold free rotation of alpha 3 beta 3.

New insights into the workings of the ultimate swimming machine

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Effect of viscosity on swimming by the lateral and polar flagella of Vibrio alginolyticus

By using mutants of Vibrio alginolyticus with only a polar flagellum (Pof+ Laf-) or only lateral flagella (Pof- Laf+), we examined the relationship between swimming speed and the viscosity of the medium for each flagellar system. Pof+ Laf- cells could not swim in the high-viscosity environment (ca. 200 cP) in which Pof- Laf+ cells swam at 20 microns/s. The Pof- Laf+ cells swam at about 20 microns/s at normal viscosity (1 cP) without the viscous agent, and the speed increased to 40 microns/s at about 5 cP and then decreased gradually as the viscosity was increased further. These results show the functional difference between polar and lateral flagella in viscous environments.

The sodium-driven polar flagellar motor of marine Vibrio as the mechanosensor that regulates lateral flagellar expression

Certain marine Vibrio species swim in sea water, propelled by a polar flagellum, and swarm over surfaces using numerous lateral flagella. The polar and the lateral flagellar motors are powered by sodium- and proton-motive forces, respectively. The lateral flagella are produced in media of high viscosity, and the relevant viscosity sensor is the polar flagellum. The cell might monitor either the rotation rate of the flagellar motor or the mechanical force applied against the flagellum. To test these possibilities, we examined the effects of amiloride and its derivatives, which inhibit the rotation of the sodium-driven motor, on lateral flagellar gene (laf) expression in Vibrio parahaemolyticus. Phenamil, an amiloride analogue that inhibits swimming at micromolar concentrations, induced laf transcription in media devoid of viscous agents in a dose-dependent manner. The relationship between the average swimming speed and laf induction in the presence of various concentrations of phenamil was very similar to that observed when viscosity was changed. These results indicate that marine Vibrio sense a decrease in the rotation rate of (or the sodium influx through) the polar flagellar motor as a trigger for laf induction. Alternative mechanisms for laf induction are also discussed.

Cloning and characterization of motY, a gene coding for a component of the sodium-driven flagellar motor in Vibrio alginolyticus

The bacterial flagellar motor is a molecular machine that couples proton or sodium influx to force generation for driving rotation of the helical flagellar filament. In this study, we cloned a gene (motY) encoding a component of the sodium-driven polar flagellar motor in Vibrio alginolyticus. Nucleotide sequence analysis revealed that the gene encodes a 293-amino-acid polypeptide with a single putative transmembrane segment that is very similar (94.5% identity) to the recently described MotY of V. parahaemolyticus. Their C-terminal domains were similar to the C-terminal domains of many peptidoglycan-interacting proteins, e.g., Escherichia coli MotB and OmpA, suggesting that MotY may interact with peptidoglycan for anchoring the motor. By using the lac promoter-repressor system, motY expression was controlled in V. alginolyticus cells. Swimming ability increased with increasing concentrations of the inducer isopropyl-beta-D-thiogalactopyranoside, and the swimming fraction increased after induction. These results are consistent with the notion that MotY is a component of the force-generating unit. V. alginolyticus motY complemented the motY mutation of V. parahaemolyticus. However, motY appeared to lack a region corresponding to the proposed motY promoter of V. parahaemolyticus. Instead, sequences similar to the sigma54 consensus were found in the upstream regions of both species. We propose that they are transcribed from the sigma54 -specific promoters.