Sodium-driven motor of the polar flagellum in marine bacteria Vibrio

Bacterial Electrophysiology

Bacterial ion fluxes are involved in the generation of energy, transport, and motility. As such, bacterial electrophysiology is fundamentally important for the bacterial life cycle, but it is often neglected and consequently, by and large, not understood. Arguably, the two main reasons for this are the complexity of measuring relevant variables in small cells with a cell envelope that contains the cell wall and the fact that, in a unicellular organism, relevant variables become intertwined in a nontrivial manner. To help give bacterial electrophysiology studies a firm footing, in this review, we go back to basics. We look first at the biophysics of bacterial membrane potential, and then at the approaches and models developed mostly for the study of neurons and eukaryotic mitochondria. We discuss their applicability to bacterial cells. Finally, we connect bacterial membrane potential with other relevant (electro)physiological variables and summarize methods that can be used to both measure and influence bacterial electrophysiology.

Lighting the way: how the Vibrio fischeri model microbe reveals the complexity of Earth's "simplest" life forms

Vibrio (Aliivibrio) fischeri's initial rise to fame derived from its alluring production of blue-green light. Subsequent studies to probe the mechanisms underlying this bioluminescence helped the field discover the phenomenon now known as quorum sensing. Orthologs of quorum-sensing regulators (i.e., LuxR and LuxI) originally identified in V. fischeri were subsequently uncovered in a plethora of bacterial species, and analogous pathways were found in yet others. Over the past three decades, the study of this microbe has greatly expanded to probe the unique role of V. fischeri as the exclusive symbiont of the light organ of the Hawaiian bobtail squid, Euprymna scolopes. Buoyed by this optically amenable host and by persistent and insightful researchers who have applied novel and cross-disciplinary approaches, V. fischeri has developed into a robust model for microbe-host associations. It has contributed to our understanding of how bacteria experience and respond to specific, often fluxing environmental conditions and the mechanisms by which bacteria impact the development of their host. It has also deepened our understanding of numerous microbial processes such as motility and chemotaxis, biofilm formation and dispersal, and bacterial competition, and of the relevance of specific bacterial genes in the context of colonizing an animal host. Parallels in these processes between this symbiont and bacteria studied as pathogens are readily apparent, demonstrating functional conservation across diverse associations and permitting a reinterpretation of "pathogenesis." Collectively, these advances built a foundation for microbiome studies and have positioned V. fischeri to continue to expand the frontiers of our understanding of the microbial world inside animals.

Deciphering the genomes of motility-deficient mutants of Vibrio alginolyticus 138-2

The motility of Vibrio species plays a pivotal role in their survival and adaptation to diverse environments and is intricately associated with pathogenicity in both humans and aquatic animals. Numerous mutant strains of Vibrio alginolyticus have been generated using UV or EMS mutagenesis to probe flagellar motility using molecular genetic approaches. Identifying these mutations promises to yield valuable insights into motility at the protein structural physiology level. In this study, we determined the complete genomic structure of 4 reference specimens of laboratory V. alginolyticus strains: a precursor strain, V. alginolyticus 138-2, two strains showing defects in the lateral flagellum (VIO5 and YM4), and one strain showing defects in the polar flagellum (YM19). Subsequently, we meticulously ascertained the specific mutation sites within the 18 motility-deficient strains related to the polar flagellum (they fall into three categories: flagellar-deficient, multi-flagellar, and chemotaxis-deficient strains) by whole genome sequencing and mapping to the complete genome of parental strains VIO5 or YM4. The mutant strains had an average of 20.6 (±12.7) mutations, most of which were randomly distributed throughout the genome. However, at least two or more different mutations in six flagellar-related genes were detected in 18 mutants specifically selected as chemotaxis-deficient mutants. Genomic analysis using a large number of mutant strains is a very effective tool to comprehensively identify genes associated with specific phenotypes using forward genetics.

Ion selectivity and rotor coupling of the Vibrio flagellar sodium-driven stator unit

Bacteria swim using a flagellar motor that is powered by stator units. Vibrio spp. are highly motile bacteria responsible for various human diseases, the polar flagella of which are exclusively driven by sodium-dependent stator units (PomAB). However, how ion selectivity is attained, how ion transport triggers the directional rotation of the stator unit, and how the stator unit is incorporated into the flagellar rotor remained largely unclear. Here, we have determined by cryo-electron microscopy the structure of Vibrio PomAB. The electrostatic potential map uncovers sodium binding sites, which together with functional experiments and molecular dynamics simulations, reveal a mechanism for ion translocation and selectivity. Bulky hydrophobic residues from PomA prime PomA for clockwise rotation. We propose that a dynamic helical motif in PomA regulates the distance between PomA subunit cytoplasmic domains, stator unit activation, and torque transmission. Together, our study provides mechanistic insights for understanding ion selectivity and rotor incorporation of the stator unit of the bacterial flagellum.

Ion-Powered Rotary Motors: Where Did They Come from and Where They Are Going?

Molecular motors are found in many living organisms. One such molecular machine, the ion-powered rotary motor (IRM), requires the movement of ions across a membrane against a concentration gradient to drive rotational movement. The bacterial flagellar motor (BFM) is an example of an IRM which relies on ion movement through the stator proteins to generate the rotation of the flagella. There are many ions which can be used by the BFM stators to power motility and different ions can be used by a single bacterium expressing multiple stator variants. The use of ancestral sequence reconstruction (ASR) and functional analysis of reconstructed stators shows promise for understanding how these proteins evolved and when the divergence in ion use may have occurred. In this review, we discuss extant BFM stators and the ions that power them as well as recent examples of the use of ASR to study ion-channel selectivity and how this might be applied to further study of the BFM stator complex.

Differential Bending Stiffness of the Bacterial Flagellar Hook under Counterclockwise and Clockwise Rotations

The bacterial hook, as a universal joint coupling rotation of the flagellar motor and the filament, is an important component of the flagellum that propels the bacteria to swim. The mechanical properties of the hook are essential for the flagellum to achieve normal functions. In multiflagellated bacteria such as Escherichia coli, the hook must be compliant so that it can bend for the filaments to form a coherently rotating bundle to generate the thrust when the motor rotates counterclockwise (CCW), yet it also must be rigid so that the bundle can disrupt for the bacteria to tumble to change swimming direction when the motor rotates clockwise (CW). Here, by combining an elastic rod model with high-resolution bead assay to accurately measure the bending stiffness of the hook under CCW or CW rotation in vivo, we elucidate how the hook accomplishes this dual functionality: the hook stiffens under CW rotation, with bending stiffness under CW rotation twice as large as that under CCW rotation. This enables a robust run-and-tumble swimming motility for multiflagellated bacteria.

Measurements of the Ion Channel Activity of the Transmembrane Stator Complex in the Bacterial Flagellar Motor

The bacterial flagellum is driven by a rotational motor located at the base of the flagellum. The stator unit complex conducts cations such as protons (H⁺) and sodium ions (Na⁺) along the electrochemical potential across the cytoplasmic membrane and interacts with the rotor to generate the rotational force. Escherichia coli and Salmonella have the H⁺-type stator complex, which serves as a transmembrane H⁺ channel that couples H⁺ flow through an ion channel to torque generation whereas Vibrio and some Bacillus species have the Na⁺-type stator complex. In this chapter, we describe how to measure the ion conductivity of the transmembrane stator complex over-expressed in E. coli cells using fluorescent indicators. Intensity measurements of fluorescent indicators using either a fluorescence spectrophotometer or microscope allow quantitative detection of changes in the intracellular ion concentrations due to the ion channel activity of the transmembrane protein complex.

The Vibrio Polar Flagellum: Structure and Regulation

Here we discuss the structure and regulation of the Vibrio flagellum and its role in the virulence of pathogenic species. We will cover some of the novel insights into the structure of this nanomachine that have recently been enabled by cryoelectron tomography. We will also highlight the recent genetic studies that have increased our understanding in flagellar synthesis specifically at the bacterial cell pole, temporal regulation of flagellar genes, and how the flagellum enables directional motility through Run-Reverse-Flick cycles.

Dynamic Hybrid Flagellar Motors-Fuel Switch and More

Flagellar motors are intricate rotating nanomachines that are powered by transmembrane ion gradients. The stator complexes are the powerhouses of the flagellar motor: They convert a transmembrane ion gradient, mainly of H⁺ or Na⁺, into rotation of the helical flagellar filament. They are thus essential for motor function. The number of stators synchronously engaged in the motor is surprisingly dynamic and depends on the load and the environmental concentration of the corresponding coupling ion. Thus, the rotor-stator interactions determine an important part of the properties of the motor. Numerous bacteria have been identified as possessing more than one set of stators, and some species have been demonstrated to use these different stators in various configurations to modify motor functions by dynamic in-flight swapping. Here, we review knowledge of the properties, the functions, and the evolution of these hybrid motors and discuss questions that remain unsolved.

Oligomerization of the FliF Domains Suggests a Coordinated Assembly of the Bacterial Flagellum MS Ring

The bacterial flagellum is a complex, self-assembling macromolecular machine that powers bacterial motility. It plays diverse roles in bacterial virulence, including aiding in colonization and dissemination during infection. The flagellum consists of a filamentous structure protruding from the cell, and of the basal body, a large assembly that spans the cell envelope. The basal body is comprised of over 20 different proteins forming several concentric ring structures, termed the M- S- L- P- and C-rings, respectively. In particular, the MS rings are formed by a single protein FliF, which consists of two trans-membrane helices anchoring it to the inner membrane and surrounding a large periplasmic domain. Assembly of the MS ring, through oligomerization of FliF, is one of the first steps of basal body assembly. Previous computational analysis had shown that the periplasmic region of FliF consists of three structurally similar domains, termed Ring-Building Motif (RBM)1, RBM2, and RBM3. The structure of the MS-ring has been reported recently, and unexpectedly shown that these three domains adopt different symmetries, with RBM3 having a 34-mer stoichiometry, while RBM2 adopts two distinct positions in the complex, including a 23-mer ring. This observation raises some important question on the assembly of the MS ring, and the formation of this symmetry mismatch within a single protein. In this study, we analyze the oligomerization of the individual RBM domains in isolation, in the Salmonella enterica serovar Typhimurium FliF ortholog. We demonstrate that the periplasmic domain of FliF assembles into the MS ring, in the absence of the trans-membrane helices. We also report that the RBM2 and RBM3 domains oligomerize into ring structures, but not RBM1. Intriguingly, we observe that a construct encompassing RBM1 and RBM2 is monomeric, suggesting that RBM1 interacts with RBM2, and inhibits its oligomerization. However, this inhibition is lifted by the addition of RBM3. Collectively, this data suggest a mechanism for the controlled assembly of the MS ring.

A slight bending of an α-helix in FliM creates a counterclockwise-locked structure of the flagellar motor in Vibrio

Many bacteria swim by rotating flagella. The chemotaxis system controls the direction of flagellar rotation. Vibrio alginolyticus, which has a single polar flagellum, swims smoothly by rotating the flagellar motor counterclockwise (CCW) in response to attractants. In response to repellents, the motor frequently switches its rotational direction between CCW and clockwise (CW). We isolated a mutant strain that swims with a CW-locked rotation of the flagellum, which pulls rather than pushes the cell. This CW phenotype arises from a R49P substitution in FliM, which is the component in the C-ring of the motor that binds the chemotaxis signaling protein, phosphorylated CheY. However, this phenotype is independent of CheY, indicating that the mutation produces a CW conformation of the C-ring in the absence of CheY. The crystal structure of FliM with the R49P substitution showed a conformational change in the N-terminal α-helix of the middle domain of FliM (FliMM). This helix should mediates FliM-FliM interaction. The structural models of wild-type and mutant C-ring showed that the relatively small conformational change in FliMM induces a drastic rearrangement of the conformation of the FliMM domain that generates a CW conformation of the C-ring.

Ancestral Sequence Reconstructions of MotB Are Proton-Motile and Require MotA for Motility

The bacterial flagellar motor (BFM) is a nanomachine that rotates the flagellum to propel many known bacteria. The BFM is powered by ion transit across the cell membrane through the stator complex, a membrane protein. Different bacteria use various ions to run their BFM, but the majority of BFMs are powered by either proton (H+) or sodium (Na+) ions. The transmembrane (TM) domain of the B-subunit of the stator complex is crucial for ion selectivity, as it forms the ion channel in complex with TM3 and TM4 of the A-subunit. In this study, we reconstructed and engineered thirteen ancestral sequences of the stator B-subunit to evaluate the functional properties and ionic power source of the stator proteins at reconstruction nodes to evaluate the potential of ancestral sequence reconstruction (ASR) methods for stator engineering and to test specific motifs previously hypothesized to be involved in ion-selectivity. We found that all thirteen of our reconstructed ancient B-subunit proteins could assemble into functional stator complexes in combination with the contemporary Escherichia coli MotA-subunit to restore motility in stator deleted E. coli strains. The flagellar rotation of the thirteen ancestral MotBs was found to be Na+ independent which suggested that the F30/Y30 residue was not significantly correlated with sodium/proton phenotype, in contrast to what we had reported previously. Additionally, four among the thirteen reconstructed B-subunits were compatible with the A-subunit of Aquifex aeolicus and able to function in a sodium-independent manner. Overall, this work demonstrates the use of ancestral reconstruction to generate novel stators and quantify which residues are correlated with which ionic power source.

Structural Conservation and Adaptation of the Bacterial Flagella Motor

Many bacteria require flagella for the ability to move, survive, and cause infection. The flagellum is a complex nanomachine that has evolved to increase the fitness of each bacterium to diverse environments. Over several decades, molecular, biochemical, and structural insights into the flagella have led to a comprehensive understanding of the structure and function of this fascinating nanomachine. Notably, X-ray crystallography, cryo-electron microscopy (cryo-EM), and cryo-electron tomography (cryo-ET) have elucidated the flagella and their components to unprecedented resolution, gleaning insights into their structural conservation and adaptation. In this review, we focus on recent structural studies that have led to a mechanistic understanding of flagellar assembly, function, and evolution.

The flagellar motor of Vibrio alginolyticus undergoes major structural remodeling during rotational switching

The bacterial flagellar motor switches rotational direction between counterclockwise (CCW) and clockwise (CW) to direct the migration of the cell. The cytoplasmic ring (C-ring) of the motor, which is composed of FliG, FliM, and FliN, is known for controlling the rotational sense of the flagellum. However, the mechanism underlying rotational switching remains elusive. Here, we deployed cryo-electron tomography to visualize the C-ring in two rotational biased mutants in Vibrio alginolyticus. We determined the C-ring molecular architectures, providing novel insights into the mechanism of rotational switching. We report that the C-ring maintained 34-fold symmetry in both rotational senses, and the protein composition remained constant. The two structures show FliG conformational changes elicit a large conformational rearrangement of the rotor complex that coincides with rotational switching of the flagellum. FliM and FliN form a stable spiral-shaped base of the C-ring, likely stabilizing the C-ring during the conformational remodeling.

The cyclic lipopeptides suppress the motility of Vibrio alginolyticus via targeting the Na+ -driven flagellar motor component MotX

In our previous study, we found that pumilacidin-like cyclic lipopeptides (CLPs) derived from marine bacterium Bacillus sp. strain 176 significantly suppressed the mobile capability and virulence of Vibrio alginolyticus. Here, to further disclose the mechanism of CLPs inhibiting the motility of V. alginolyticus, we first applied transcriptomic analysis to V. alginolyticus treated with or without CLPs. The transcriptomic results showed that the expression of several important components of the Na⁺ -driven flagellar motor closely related to bacterial motility were markedly suppressed, suggesting that the structure and function of Na⁺ -driven flagellar motor might be disabled by CLPs. The transcriptomic data were further analysed by the protein-protein interaction network, and the results supported that MotX, one of the essential components of Na⁺ -driven flagellar motor was most likely the action target of CLPs. In combination of gene knockout, electrophoretic mobility shift assay and immunoblotting techniques, CLPs were demonstrated to affect the rotation of flagella of Vibrio alginolyticus via direct interacting with the Na⁺ -driven flagellar motor component MotX, which eventually inhibited the bacterial motility. Interestingly, homologues of MotX were found broadly distributed and highly conserved in different pathogenic species, which extends the application range of CLPs as an antibacterial drug targeting bacterial motility in many pathogens.

Assembly mechanism of a supramolecular MS-ring complex to initiate bacterial flagellar biogenesis in Vibrio species

The bacterial flagellum is an organelle responsible for motility and has a rotary motor comprising the rotor and the stator. Flagellar biogenesis is initiated by the assembly of the MS-ring, a supramolecular complex embedded in the cytoplasmic membrane. The MS-ring consists of a few dozen copies of the transmembrane FliF protein, and is an essential core structure which is a part of the rotor. The number and location of the flagella are controlled by the FlhF and FlhG proteins in some species. However, there is no clarity on the factors initiating MS-ring assembly, and contribution of FlhF/FlhG to this process. Here, we show that FlhF and a C-ring component FliG facilitate Vibrio MS-ring formation. When Vibrio FliF alone was expressed in Escherichia coli cells, MS-ring formation rarely occurred, indicating the requirement of other factors for MS-ring assembly. Consequently, we investigated if FlhF aided FliF in MS-ring assembly. We found that FlhF allowed GFP-fused FliF to localize at the cell pole in a Vibrio cell, suggesting that it increases local concentration of FliF at the pole. When FliF was co-expressed with FlhF in E. coli cells, the MS-ring was effectively formed, indicating that FlhF somehow contributes to MS-ring formation. The isolated MS-ring structure was similar to the MS-ring formed by Salmonella FliF. Interestingly, FliG facilitates MS-ring formation, suggesting that FliF and FliG assist in each other's assembly into the MS-ring and C-ring. This study aids in understanding the mechanism behind MS-ring assembly using appropriate spatial/temporal regulations.Importance Flagellar formation is initiated by the assembly of the FliF protein into the MS-ring complex, embedded in the cytoplasmic membrane. The appropriate spatial/temporal control of MS-ring formation is important for the morphogenesis of the bacterial flagellum. Here, we focus on the assembly mechanism of Vibrio FliF into the MS-ring. FlhF, a positive regulator of the number and location of flagella, recruits the FliF molecules at the cell pole and facilitates MS-ring formation. FliG also facilitates MS-ring formation. Our study showed that these factors control flagellar biogenesis in Vibrio, by initiating the MS-ring assembly. Furthermore, it also implies that flagellar biogenesis is a sophisticated system linked with the expression of certain genes, protein localization and a supramolecular complex assembly.

MotP Subunit is Critical for Ion Selectivity and Evolution of a K+-Coupled Flagellar Motor

The bacterial flagellar motor is a sophisticated nanomachine embedded in the cell envelope. The flagellar motor is driven by an electrochemical gradient of cations such as H⁺, Na⁺, and K⁺ through ion channels in stator complexes embedded in the cell membrane. The flagellum is believed to rotate as a result of electrostatic interaction forces between the stator and the rotor. In bacteria of the genus Bacillus and related species, the single transmembrane segment of MotB-type subunit protein (MotB and MotS) is critical for the selection of the H⁺ and Na⁺ coupling ions. Here, we constructed and characterized several hybrid stators combined with single Na⁺-coupled and dual Na⁺- and K⁺-coupled stator subunits, and we report that the MotP subunit is critical for the selection of K⁺. This result suggested that the K⁺ selectivity of the MotP/MotS complexes evolved from the single Na⁺-coupled stator MotP/MotS complexes. This finding will promote the understanding of the evolution of flagellar motors and the molecular mechanisms of coupling ion selectivity.

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.

Reciprocal c-di-GMP signaling: Incomplete flagellum biogenesis triggers c-di-GMP signaling pathways that promote biofilm formation

The assembly status of the V. cholerae flagellum regulates biofilm formation, suggesting that the bacterium senses a lack of movement to commit to a sessile lifestyle. Motility and biofilm formation are inversely regulated by the second messenger molecule cyclic dimeric guanosine monophosphate (c-di-GMP). Therefore, we sought to define the flagellum-associated c-di-GMP-mediated signaling pathways that regulate the transition from a motile to a sessile state. Here we report that elimination of the flagellum, via loss of the FlaA flagellin, results in a flagellum-dependent biofilm regulatory (FDBR) response, which elevates cellular c-di-GMP levels, increases biofilm gene expression, and enhances biofilm formation. The strength of the FDBR response is linked with status of the flagellar stator: it can be reversed by deletion of the T ring component MotX, and reduced by mutations altering either the Na⁺ binding ability of the stator or the Na⁺ motive force. Absence of the stator also results in reduction of mannose-sensitive hemagglutinin (MSHA) pilus levels on the cell surface, suggesting interconnectivity of signal transduction pathways involved in biofilm formation. Strains lacking flagellar rotor components similarly launched an FDBR response, however this was independent of the status of assembly of the flagellar stator. We found that the FDBR response requires at least three specific diguanylate cyclases that contribute to increased c-di-GMP levels, and propose that activation of biofilm formation during this response relies on c-di-GMP-dependent activation of positive regulators of biofilm production. Together our results dissect how flagellum assembly activates c-di-GMP signaling circuits, and how V. cholerae utilizes these signals to transition from a motile to a sessile state.

A key regulator of Vibrio cholerae physiology is the nucleotide-based, second messenger cyclic dimeric guanosine monophosphate (c-di-GMP). We found that the status of flagellar biosynthesis at different stages of flagellar assembly modulates c-di-GMP signaling in V. cholerae and identified diguanylate cyclases involved in this regulatory process. The effect of motility status on the cellular c-di-GMP level is partly dependent on the flagellar stator and Na⁺ flux through the flagellum. Finally, we showed that c-di-GMP-dependent positive regulators of biofilm formation are critical for the signaling cascade that connects motility status to biofilm formation. Our results show that in addition to c-di-GMP promoting motile to biofilm lifestyle switch, “motility status” of V. cholerae modulates c-di-GMP signaling and biofilm formation.

The Role of Treponema denticola Motility in Synergistic Biofilm Formation With Porphyromonas gingivalis

Chronic periodontitis has a polymicrobial biofilm etiology and interactions between key oral bacterial species, such as Porphyromonas gingivalis and Treponema denticola contribute to disease progression. P. gingivalis and T. denticola are co-localized in subgingival plaque and have been previously shown to exhibit strong synergy in growth, biofilm formation and virulence in an animal model of disease. The motility of T. denticola, although not considered as a classic virulence factor, may be involved in synergistic biofilm development between P. gingivalis and T. denticola. We determined the role of T. denticola motility in polymicrobial biofilm development using an optimized transformation protocol to produce two T. denticola mutants targeting the motility machinery. These deletion mutants were non-motile and lacked the gene encoding the flagellar hook protein of the periplasmic flagella (ΔflgE) or a component of the stator motor that drives the flagella (ΔmotB). The specificity of these gene deletions was determined by whole genome sequencing. Quantitative proteomic analyses of mutant strains revealed that the specific inactivation of the motility-associated gene, motB, had effects beyond motility. There were 64 and 326 proteins that changed in abundance in the ΔflgE and ΔmotB mutants, respectively. In the ΔflgE mutant, motility-associated proteins showed the most significant change in abundance confirming the phenotype change for the mutant was related to motility. However, the inactivation of motB as well as stopping motility also upregulated cellular stress responses in the mutant indicating pleiotropic effects of the mutation. T. denticola wild-type and P. gingivalis displayed synergistic biofilm development with a 2-fold higher biomass of the dual-species biofilms than the sum of the monospecies biofilms. Inactivation of T. denticola flgE and motB reduced this synergy. A 5-fold reduction in dual-species biofilm biomass was found with the motility-specific ΔflgE mutant suggesting that T. denticola periplasmic flagella are essential in synergistic biofilm formation with P. gingivalis.

Voltage vs. Ligand II: Structural insights of the intrinsic flexibility in cyclic nucleotide-gated channels

In the preceding article, we present a flexibility analysis of the voltage-gated ion channel (VGIC) superfamily. In this study, we describe in detail the flexibility profile of the voltage-sensor domain (VSD) and the pore domain (PD) concerning the evolution of 6TM ion channels. In particular, we highlight the role of flexibility in the emergence of CNG channels and describe a significant level of sequence similarity between the archetypical VSD and the TolQ proteins. A highly flexible S4-like segment exhibiting Lys instead Arg for these membrane proteins is reported. Sequence analysis indicates that, in addition to this S4-like segment, TolQ proteins also show similarity with specific motifs in S2 and S3 from typical V-sensors. Notably, S3 flexibility profiles from typical VSDs and S3-like in TolQ proteins are also similar. Interestingly, TolQ from early divergent prokaryotes are comparatively more flexible than those in modern counterparts or true V-sensors. Regarding the PD, we also found that 2TM K+-channels in early prokaryotes are considerably more flexible than the ones in modern microbes, and such flexibility is comparable to the one present in CNG channels. Voltage dependence is mainly exhibited in prokaryotic CNG channels whose VSD is rigid whereas the eukaryotic CNG channels are considerably more flexible and poorly V-dependent. The implication of the flexibility present in CNG channels, their sensitivity to cyclic nucleotides and the cation selectivity are discussed. Finally, we generated a structural model of the putative cyclic nucleotide-modulated ion channel, which we coined here as AqK, from the thermophilic bacteria Aquifex aeolicus, one of the earliest diverging prokaryotes known. Overall, our analysis suggests that V-sensors in CNG-like channels were essentially rigid in early prokaryotes but raises the possibility that this module was probably part of a very flexible stator protein of the bacterial flagellum motor complex.

Characterization of PomA periplasmic loop and sodium ion entering in stator complex of sodium-driven flagellar motor

The bacterial flagellar motor is a rotary nanomachine driven by ion flow. The flagellar stator complex, which is composed of two proteins, PomA and PomB, performs energy transduction in marine Vibrio. PomA is a four transmembrane (TM) protein and the cytoplasmic region between TM2 and TM3 (loop2–3) interacts with the rotor protein FliG to generate torque. The periplasmic regions between TM1 and TM2 (loop1–2) and TM3 and TM4 (loop3–4) are candidates to be at the entrance to the transmembrane ion channel of the stator. In this study, we purified the stator complex with cysteine replacements in the periplasmic loops and assessed the reactivity of the protein with biotin maleimide (BM). BM easily modified Cys residues in loop3–4 but hardly labelled Cys residues in loop1–2. We could not purify the plug deletion stator (ΔL stator) composed of PomBΔ41–120 and WT-PomA but could do the ΔL stator with PomA-D31C of loop1–2 or with PomB-D24N of TM. When the ion channel is closed, PomA and PomB interact strongly. When the ion channel opens, PomA interacts less tightly with PomB. The plug and loop1–2 region regulate this activation of the stator, which depends on the binding of sodium ion to the D24 residue of PomB.

Effect of PlzD, a YcgR homologue of c-di-GMP-binding protein, on polar flagellar motility in Vibrio alginolyticus

YcgR, a cyclic diguanylate (c-di-GMP)-binding protein expressed in Escherichia coli, brakes flagellar rotation by binding to the motor in a c-di-GMP dependent manner and has been implicated in triggering biofilm formation. Vibrio alginolyticus has a single polar flagellum and encodes YcgR homologue, PlzD. When PlzD or PlzD-GFP was highly over-produced in nutrient-poor condition, the polar flagellar motility of V. alginolyticus was reduced. This inhibitory effect is c-di-GMP independent as mutants substituting putative c-di-GMP-binding residues retain the effect. Moderate over-expression of PlzD-GFP allowed its localization at the flagellated cell pole. Truncation of the N-terminal 12 or 35 residues of PlzD abolished the inhibitory effect and polar localization, and no inhibitory effect was observed by deleting plzD or expressing an endogenous level of PlzD-GFP. Subcellular fractionation showed that PlzD, but not its N-terminally truncated variants, was precipitated when over-produced. Moreover, immunoblotting and N-terminal sequencing revealed that endogenous PlzD is synthesized from Met33. These results suggest that an N-terminal extension allows PlzD to localize at the cell pole but causes aggregation and leads to inhibition of motility. In V. alginolyticus, PlzD has a potential property to associate with the polar flagellar motor but this interaction is too weak to inhibit rotation.

Essential ion binding residues for Na+ flow in stator complex of the Vibrio flagellar motor

The bacterial flagellar motor is a unique supramolecular complex which converts ion flow into rotational force. Many biological devices mainly use two types of ions, proton and sodium ion. This is probably because of the fact that life originated in seawater, which is rich in protons and sodium ions. The polar flagellar motor in Vibrio is coupled with sodium ion and the energy converting unit of the motor is composed of two membrane proteins, PomA and PomB. It has been shown that the ion binding residue essential for ion transduction is the conserved aspartic acid residue (PomB-D24) in the PomB transmembrane region. To reveal the mechanism of ion selectivity, we identified essential residues, PomA-T158 and PomA-T186, other than PomB-D24, in the Na⁺-driven flagellar motor. It has been shown that the side chain of threonine contacts Na⁺ in Na⁺-coupled transporters. We monitored the Na⁺-binding specific structural changes using ATR-FTIR spectroscopy. The signals were abolished in PomA-T158A and -T186A, as well as in PomB-D24N. Molecular dynamics simulations further confirmed the strong binding of Na⁺ to D24 and showed that T158A and T186A hindered the Na⁺ binding and transportation. The data indicate that two threonine residues (PomA-T158 and PomA-T186), together with PomB-D24, are important for Na⁺ conduction in the Vibrio flagellar motor. The results contribute to clarify the mechanism of ion recognition and conversion of ion flow into mechanical force.

Studies on wound healing potential of red pigment isolated from marine Bacterium Vibrio sp

Wounds are common clinical entities of life which may be subacute or acute. Wound healing is a complex biochemical process where the cell structures are restored to normalcy, which depend on cell proliferation and migration, basically fibroblast cell. The present investigation was undertaken to evaluate the healing efficacy of red pigment isolated from marine isolate Vibrio sps on experimental wounds in albino rats. The red pigment was applied topically, twice daily for 14 days. Treatment with framycetin ointment was used as reference control. The red pigment treated group showed faster reduction in wound area in comparison with control and framycetin ointment treated groups. In conclusion, red pigment possesses significant healing potential in wounds and has a positive influence on the different phases of wound repair.

Flagellum-mediated motility in Pelotomaculum thermopropionicum SI

The basic functions of a propionate-oxidizing bacterium Pelotomaculum thermopropionicum flagellum, such as motility and chemotaxis, have not been studied. To investigate its motility, we compared with that of Syntrophobacter fumaroxidans, an aflagellar propionate-oxidizing bacterium, in soft agar medium. P. thermopropionicum cells spread, while S. fumaroxidans cells moved downward slightly, indicating flagellum-dependent motility in P. thermopropionicum SI. The motility of P. thermopropionicum was inhibited by the addition of carbonyl cyanide m-chlorophenyl hydrazone, a proton uncoupler, which is consistent with the fact that stator protein, MotB of P. thermopropionicum, shared sequence homology with proton-type stators. In addition, 5-N-ethyl-N-isopropyl amiloride, an Na⁺ channel blocker, showed no inhibitory effect on the motility. Furthermore, motAB of P. thermopropionicum complemented the defective swimming ability of Escherichia coli ∆motAB. These results suggest that the motility of P. thermopropionicum SI depends on the proton-type flagellar motor.

Rotational direction of flagellar motor from the conformation of FliG middle domain in marine Vibrio

FliG, which is composed of three distinctive domains, N-terminal (N), middle (M), and C-terminal (C), is an essential rotor component that generates torque and determines rotational direction. To determine the role of FliG in determining flagellar rotational direction, we prepared rotational biased mutants of fliG in Vibrio alginolyticus. The E144D mutant, whose residue is belonging to the EHPQR-motif in FliG_M, exhibited an increased number of switching events. This phenotype generated a response similar to the phenol-repellent response in chemotaxis. To clarify the effect of E144D mutation on the rotational switching, we combined the mutation with other che mutations (G214S, G215A and A282T) in FliG. Two of the double mutants suppressed the rotational biased phenotype. To gain structural insight into the mutations, we performed molecular dynamic simulations of the FliG_MC domain, based on the crystal structure of Thermotoga maritima FliG and nuclear magnetic resonance analysis. Furthermore, we examined the swimming behavior of the fliG mutants lacking CheY. The results suggested that the conformation of FliG in E144D mutant was similar to that in the wild type. However, that of G214S and G215A caused a steric hindrance in FliG. The conformational change in FliG_M triggered by binding CheY may lead to a rapid change of direction and may occur in both directional states.

A global genomic approach uncovers novel components for twitching motility-mediated biofilm expansion in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an extremely successful pathogen able to cause both acute and chronic infections in a range of hosts, utilizing a diverse arsenal of cell-associated and secreted virulence factors. A major cell-associated virulence factor, the Type IV pilus (T4P), is required for epithelial cell adherence and mediates a form of surface translocation termed twitching motility, which is necessary to establish a mature biofilm and actively expand these biofilms. P. aeruginosa twitching motility-mediated biofilm expansion is a coordinated, multicellular behaviour, allowing cells to rapidly colonize surfaces, including implanted medical devices. Although at least 44 proteins are known to be involved in the biogenesis, assembly and regulation of the T4P, with additional regulatory components and pathways implicated, it is unclear how these components and pathways interact to control these processes. In the current study, we used a global genomics-based random-mutagenesis technique, transposon directed insertion-site sequencing (TraDIS), coupled with a physical segregation approach, to identify all genes implicated in twitching motility-mediated biofilm expansion in P. aeruginosa. Our approach allowed identification of both known and novel genes, providing new insight into the complex molecular network that regulates this process in P. aeruginosa. Additionally, our data suggest that the flagellum-associated gene products have a differential effect on twitching motility, based on whether components are intra- or extracellular. Overall the success of our TraDIS approach supports the use of this global genomic technique for investigating virulence genes in bacterial pathogens.

The Vibrio H-Ring Facilitates the Outer Membrane Penetration of the Polar Sheathed Flagellum

The bacterial flagellum has evolved as one of the most remarkable nanomachines in nature. It provides swimming and swarming motilities that are often essential for the bacterial life cycle and pathogenesis. Many bacteria such as Salmonella and Vibrio species use flagella as an external propeller to move to favorable environments, whereas spirochetes utilize internal periplasmic flagella to drive a serpentine movement of the cell bodies through tissues. Here, we use cryo-electron tomography to visualize the polar sheathed flagellum of Vibrio alginolyticus with particular focus on a Vibrio-specific feature, the H-ring. We characterized the H-ring by identifying its two components FlgT and FlgO. We found that the majority of flagella are located within the periplasmic space in the absence of the H-ring, which are different from those of external flagella in wild-type cells. Our results not only indicate the H-ring has a novel function in facilitating the penetration of the outer membrane and the assembly of the external sheathed flagella but also are consistent with the notion that the flagella have evolved to adapt highly diverse needs by receiving or removing accessary genes.IMPORTANCE Flagellum is the major organelle for motility in many bacterial species. While most bacteria possess external flagella, such as the multiple peritrichous flagella found in Escherichia coli and Salmonella enterica or the single polar sheathed flagellum in Vibrio spp., spirochetes uniquely assemble periplasmic flagella, which are embedded between their inner and outer membranes. Here, we show for the first time that the external flagella in Vibrio alginolyticus can be changed as periplasmic flagella by deleting two flagellar genes. The discovery here may provide new insights into the molecular basis underlying assembly, diversity, and evolution of flagella.

FliL association with flagellar stator in the sodium-driven Vibrio motor characterized by the fluorescent microscopy

Bacterial flagellar motor (BFM) is a protein complex used for bacterial motility and chemotaxis that involves in energy transformation, torque generation and switching. FliL is a single-transmembrane protein associated with flagellar motor function. We performed biochemical and biophysical approaches to investigate the functional roles of FliL associated with stator-units. Firstly, we found the periplasmic region of FliL is crucial for its polar localization. Also, the plug mutation in stator-unit affected the polar localization of FliL implying the activation of stator-unit is important for FliL recruitment. Secondly, we applied single-molecule fluorescent microscopy to study the role of FliL in stator-unit assembly. Using molecular counting by photobleaching, we found the stoichiometry of stator-unit and FliL protein would be 1:1 in a functional motor. Moreover, the turnover time of stator-units are slightly increased in the absence of FliL. By further investigation of protein dynamics on membrane, we found the diffusions of stator-units and FliL are independent. Surprisingly, the FliL diffusion rate without stator-units is unexpectedly slow indicating a protein-complex forming event. Our results suggest that FliL plays a supporting role to the stator in the BFM.

Solution structure analysis of the periplasmic region of bacterial flagellar motor stators by small angle X-ray scattering

The bacterial flagellar motor drives the rotation of helical flagellar filaments to propel bacteria through viscous media. It consists of a dynamic population of mechanosensitive stators that are embedded in the inner membrane and activate in response to external load. This entails assembly around the rotor, anchoring to the peptidoglycan layer to counteract torque from the rotor and opening of a cation channel to facilitate an influx of cations, which is converted into mechanical rotation. Stator complexes are comprised of four copies of an integral membrane A subunit and two copies of a B subunit. Each B subunit includes a C-terminal OmpA-like peptidoglycan-binding (PGB) domain. This is thought to be linked to a single N-terminal transmembrane helix by a long unstructured peptide, which allows the PGB domain to bind to the peptidoglycan layer during stator anchoring. The high-resolution crystal structures of flagellar motor PGB domains from Salmonella enterica (MotB_C2) and Vibrio alginolyticus (PomB_C5) have previously been elucidated. Here, we use small-angle X-ray scattering (SAXS). We show that unlike MotB_C2, the dimeric conformation of the PomB_C5 in solution differs to its crystal structure, and explore the functional relevance by characterising gain-of-function mutants as well as wild-type constructs of various lengths. These provide new insight into the conformational diversity of flagellar motor PGB domains and experimental verification of their overall topology.

Molecular architecture of the sheathed polar flagellum in Vibrio alginolyticus

Vibrio species are Gram-negative rod-shaped bacteria that are ubiquitous and often highly motile in aqueous environments. Vibrio swimming motility is driven by a polar flagellum covered with a membranous sheath, but this sheathed flagellum is not well understood at the molecular level because of limited structural information. Here, we use Vibrio alginolyticus as a model system to study the sheathed flagellum in intact cells by combining cryoelectron tomography (cryo-ET) and subtomogram analysis with a genetic approach. We reveal striking differences between sheathed and unsheathed flagella in V. alginolyticus cells, including a novel ring-like structure at the bottom of the hook that is associated with major remodeling of the outer membrane and sheath formation. Using mutants defective in flagellar motor components, we defined a Vibrio-specific feature (also known as the T ring) as a distinctive periplasmic structure with 13-fold symmetry. The unique architecture of the T ring provides a static platform to recruit the PomA/B complexes, which are required to generate higher torques for rotation of the sheathed flagellum and fast motility of Vibrio cells. Furthermore, the Vibrio flagellar motor exhibits an intrinsic length variation between the inner and the outer membrane bound complexes, suggesting the outer membrane bound complex can shift slightly along the axial rod during flagellar rotation. Together, our detailed analyses of the polar flagella in intact cells provide insights into unique aspects of the sheathed flagellum and the distinct motility of Vibrio species.

A unique role of flagellar function in Aliivibrio salmonicida pathogenicity not related to bacterial motility in aquatic environments

Aliivibrio salmonicida is the causative agent of cold-water vibriosis, a septicemia of farmed salmonid fish. The mechanisms of disease are not well described, and few virulence factors have been identified. However, a requirement for motility in the pathogenesis has been reported. Al. salmonicida is motile by the means of lophotrichous polar flagella, consisting of multiple flagellin subunits that are expressed simultaneously. Here we show that flagellin subunit FlaA, but not FlaD, is of major importance for motility in Al. salmonicida. Deletion of flaA resulted in 62% reduction in motility, as well as a reduction in the fraction of flagellated cells and number of flagella per cell. Similarly, deletion of the gene encoding motor protein motA gave rise to an aflagellate phenotype and cessation of motility. Surprisingly, we found that Al. salmonicida does not require motility for invasion of Atlantic salmon. Nevertheless, in-frame deletion mutants defective of motA and flaA were less virulent in Atlantic salmon challenged by immersion, whereas an effect on virulence after i.p. challenge was only seen for the latter. Our results indicate a complex requirement for motility and/or flagellation in the pathogenesis of cold-water vibriosis, but the mechanisms involved remain unknown. We hypothesize that the differences in virulence observed after immersion and i.p. challenge are related to the immune response of the host.

Biochemical characterization of the flagellar stator-associated inner membrane protein FliL from Vibrio alginolyticus

The flagellar motor is embedded in the cell envelope and rotates upon interaction between the stator and the rotor. The rotation is powered by ion flow through the stator. A single transmembrane protein named FliL is associated with torque generation in the flagellar motor. We established an Escherichia coli over-expression system for FliL of Vibrio alginolyticus, a marine bacterium that has a sodium-driven polar flagellum. We successfully expressed, purified, and crystallized the ca. 17 kDa full-length FliL protein and generated a construct that expresses only the ca. 14 kDa periplasmic region of FliL (ΔTM FliL). Biochemical characterization and NMR analysis revealed that ΔTM FliL weakly interacted with itself to form an oligomer. We speculate that the observed dynamic interaction may be involved in the role of FliL in flagellar motor function.

Mutational analysis and overproduction effects of MotX, an essential component for motor function of Na+-driven polar flagella of Vibrio

The bacterial flagellar motor is a rotary motor complex composed of various proteins. The motor contains a central rod, multiple ring-like structures and stators. The Na+-driven polar flagellar motor of the marine bacterium Vibrio alginolyticus has a specific ring, called the ‘T-ring’, which consists of two periplasmic proteins, MotX and MotY. The T-ring is essential for assembly of the torque-generating unit, the PomA/PomB stator complex, into the motor. To investigate the role of the T-ring for motor function, we performed random mutagenesis of the motX gene on a plasmid. The isolated MotX mutants showed nonmotile, slow-motile, and up-motile phenotypes by the expression from the plasmid. Deletion analysis indicated that the C-terminal region and the signal peptide in MotX are not always essential for flagellar motor function. We also found that overproduction of MotX caused the delay of growth and aberrant cell shape. MotX might have unexpected roles not only in flagellar motor function but also in cell morphology control.

Mechanism of Stator Assembly and Incorporation into the Flagellar Motor

In many cases, conformational changes in proteins are related to their functions, and thereby inhibiting those changes causes functional defects. One way to perturb such conformational changes is to covalently link the regions where the changes are induced. Here, I introduce an example in which an intramolecular disulfide crosslink in the stator protein of PomB, introduced based on its crystal structure, reversibly inhibits the rotation of the flagellar motor, and I detail how we analyzed that phenotype. In this Chapter, first I describe how we monitor the motility inhibition and restoration by controlling disulfide bridge formation, and secondly how we detect intramolecular disulfide crosslinks, which are sometimes difficult to monitor by mobility shifts on SDS-PAGE gels.

Ion Selectivity of the Flagellar Motors Derived from the Alkaliphilic Bacillus and Paenibacillus Species

Many bacteria can swim using their flagella, which are filamentous organelles that extend from the cell surface. The flagellar motor is energized by either a proton (H⁺) or sodium ion (Na⁺) as the motive force. MotAB-type stators use protons, whereas MotPS- and PomAB-type stators use Na⁺ as the coupling ions. Recently, alkaliphilic Bacillus alcalophilus was shown to use potassium ions (K⁺) and rubidium ions (Rb⁺) for flagellar rotation, and the flagellar motor from Paenibacillus sp. TCA-20 uses divalent cations such as magnesium ions (Mg²⁺), calcium ions (Ca²⁺), and strontium ions (Sr²⁺) for coupling. In this chapter, we focus on how to identify the coupling ions for flagellar rotation of alkaliphilic Bacillus and Paenibacillus species.

Genetic Analysis of Collective Motility of Paenibacillus sp. NAIST15-1

Bacteria have developed various motility mechanisms to adapt to a variety of solid surfaces. A rhizosphere isolate, Paenibacillus sp. NAIST15-1, exhibited unusual motility behavior. When spotted onto 1.5% agar media, Paenibacillus sp. formed many colonies, each of which moved around actively at a speed of 3.6 μm/sec. As their density increased, each moving colony began to spiral, finally forming a static round colony. Despite its unusual motility behavior, draft genome sequencing revealed that both the composition and organization of flagellar genes in Paenibacillus sp. were very similar to those in Bacillus subtilis. Disruption of flagellar genes and flagellar stator operons resulted in loss of motility. Paenibacillus sp. showed increased transcription of flagellar genes and hyperflagellation on hard agar media. Thus, increased flagella and their rotation drive Paenibacillus sp. motility. We also identified a large extracellular protein, CmoA, which is conserved only in several Paenibacillus and related species. A cmoA mutant could neither form moving colonies nor move on hard agar media; however, motility was restored by exogenous CmoA. CmoA was located around cells and enveloped cell clusters. Comparison of cellular behavior between the wild type and cmoA mutant indicated that extracellular CmoA is involved in drawing water out of agar media and/or smoothing the cell surface interface. This function of CmoA probably enables Paenibacillus sp. to move on hard agar media.

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.

Studies on the mechanism of bacterial flagellar rotation and the flagellar number regulation

Many motile bacteria have the motility organ, the flagellum. It rotates by the rotary motor driven by the ion-motive force and is embedded in the cell surface at the base of each flagellar filament. Many researchers have been studying its rotary mechanism for years, but most of the energy conversion processes have been remained in mystery. We focused on the flagellar stator, which works at the core process of energy conversion, and found that the periplasmic region of the stator changes its conformation to be activated only when the stator units are incorporated into the motor and anchored at the cell wall. Meanwhile, the physiologically important supramolecular complex is localized in the cell at the right place and the right time with a proper amount. How the cell achieves such a proper localization is the fundamental question for life science, and we undertake this problem by analyzing the mechanism for biogenesis of a single polar flagellum of Vibrio alginolyticus. Here I describe the molecular mechanism of how the flagellum is generated at the specific place with a proper number, and also how the flagellar stator is incorporated into the motor to complete the functional motor assembly, based on our studies.

Type III secretion systems: the bacterial flagellum and the injectisome

The flagellum and the injectisome are two of the most complex and fascinating bacterial nanomachines. At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and the injectisome needle. Recent advances, combining structural biology, cryo-electron tomography, molecular genetics, in vivo imaging, bioinformatics and biophysics, have greatly increased our understanding of the T3SS, especially the structure of its transmembrane and cytosolic components, the transcriptional, post-transcriptional and functional regulation and the remarkable adaptivity of the system. This review aims to integrate these new findings into our current knowledge of the evolution, function, regulation and dynamics of the T3SS, and to highlight commonalities and differences between the two systems, as well as their potential applications.

Diverse high-torque bacterial flagellar motors assemble wider stator rings using a conserved protein scaffold

Although it is known that diverse bacterial flagellar motors produce different torques, the mechanism underlying torque variation is unknown. To understand this difference better, we combined genetic analyses with electron cryo-tomography subtomogram averaging to determine in situ structures of flagellar motors that produce different torques, from Campylobacter and Vibrio species. For the first time, to our knowledge, our results unambiguously locate the torque-generating stator complexes and show that diverse high-torque motors use variants of an ancestrally related family of structures to scaffold incorporation of additional stator complexes at wider radii from the axial driveshaft than in the model enteric motor. We identify the protein components of these additional scaffold structures and elucidate their sequential assembly, demonstrating that they are required for stator-complex incorporation. These proteins are widespread, suggesting that different bacteria have tailored torques to specific environments by scaffolding alternative stator placement and number. Our results quantitatively account for different motor torques, complete the assignment of the locations of the major flagellar components, and provide crucial constraints for understanding mechanisms of torque generation and the evolution of multiprotein complexes.

A novel type bacterial flagellar motor that can use divalent cations as a coupling ion

The bacterial flagellar motor is a sophisticated nanomachine embedded in the cell envelope and powered by an electrochemical gradient of H(+), Na(+), or K(+)across the cytoplasmic membrane. Here we describe a new member of the bacterial flagellar stator channel family (MotAB1 of Paenibacillus sp. TCA20 (TCA-MotAB1)) that is coupled to divalent cations (Ca(2+)and Mg(2+)). In the absence of divalent cations of alkaline earth metals, no swimming was observed in Paenibacillus sp. TCA20, which grows optimally in Ca(2+)-rich environments. This pattern was confirmed by swimming assays of a stator-free Bacillus subtilis mutant expressing TCA-MotAB1. Both a stator-free and major Mg(2+)uptake system-deleted B. subtilis mutant expressing TCA-MotAB1 complemented both growth and motility deficiency under low Mg(2+)conditions and exhibited [Mg(2+)]in identical to that of the wild-type. This is the first report of a flagellar motor that can use Ca(2+)and Mg(2+)as coupling ions. These findings will promote the understanding of the operating principles of flagellar motors and molecular mechanisms of ion selectivity.

An Element of Determinism in a Stochastic Flagellar Motor Switch

Marine bacterium Vibrio alginolyticus uses a single polar flagellum to navigate in an aqueous environment. Similar to Escherichia coli cells, the polar flagellar motor has two states; when the motor is counter-clockwise, the cell swims forward and when the motor is clockwise, the cell swims backward. V. alginolyticus also incorporates a direction randomization step at the start of the forward swimming interval by flicking its flagellum. To gain an understanding on how the polar flagellar motor switch is regulated, distributions of the forward Δ_f and backward Δ_b intervals are investigated herein. We found that the steady-state probability density functions, P(Δ_f) and P(Δ_b), of freely swimming bacteria are strongly peaked at a finite time, suggesting that the motor switch is not Poissonian. The short-time inhibition is sufficiently strong and long lasting, i.e., several hundred milliseconds for both intervals, which is readily observed and characterized. Treating motor reversal dynamics as a first-passage problem, which results from conformation fluctuations of the motor switch, we calculated P(Δ_f) and P(Δ_b) and found good agreement with the measurements.

Shelter in a Swarm

Flagella propel bacteria during both swimming and swarming, dispersing them widely. However, while swimming bacteria use chemotaxis to find nutrients and avoid toxic environments, swarming bacteria appear to suppress chemotaxis and to use the dynamics of their collective motion to continuously expand and acquire new territory, barrel through lethal chemicals in their path, carry along bacterial and fungal cargo that assists in exploration of new niches, and engage in group warfare for niche dominance. Here, we focus on two aspects of swarming, which, if understood, hold the promise of revealing new insights into microbial signaling and behavior, with ramifications beyond bacterial swarming. These are as follows: how bacteria sense they are on a surface and turn on programs that promote movement and how they override scarcity and adversity as dense packs.