Role of FlgT in anchoring the flagellum of Vibrio cholerae

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.

Achievements in bacterial flagellar research with focus on Vibrio species

In 1980's, the most genes involved in the bacterial flagellar function and formation had been isolated though many of their functions or roles were not clarified. Bacterial flagella are the primary locomotive organ and are not necessary for growing in vitro but are probably essential for living in natural condition and are involved in the pathogenicity. In vitro, the flagella-deficient strains can grow at rates similar to wild-type strains. More than 50 genes are responsible for flagellar function, and the flagellum is constructed by more than 20 structural proteins. The maintenance cost of flagellum is high as several genes are required for its development. The fact that it evolved as a motor organ even with such the high cost shows that the motility is indispensable to survive under the harsh environment of Earth. In this review, we focus on flagella-related research conducted by the authors for about 40 years and flagellar research focused on Vibrio spp. This article is protected by copyright. All rights reserved.

The polar flagellar transcriptional regulatory network in Vibrio campbellii deviates from canonical Vibrio species

Swimming motility is a critical virulence factor in pathogenesis for numerous Vibrio species. Vibrio campbellii DS40M4 is a wild isolate that has been recently established as a highly tractable model strain for bacterial genetics studies. We sought to exploit the tractability and relevance of this strain for characterization of flagellar gene regulation in V. campbellii. Using comparative genomics, we identified homologs of V. campbellii flagellar and chemotaxis genes conserved in other members of the Vibrionaceae and determined the transcriptional profile of these loci using differential RNA-seq. We systematically deleted all 63 predicted flagellar and chemotaxis genes in V. campbellii and examined their effects on motility and flagellum production. We specifically focused on the core regulators of the flagellar hierarchy established in other vibrios: RpoN (σ⁵⁴), FlrA, FlrC, and FliA. Our results show that V. campbellii transcription of flagellar and chemotaxis genes is governed by a multi-tiered regulatory hierarchy similar to other motile Vibrio species. However, there are several critical differences in V. campbellii: (i) the σ⁵⁴-dependent regulator FlrA is dispensable for motility, (ii) the flgA, fliEFGHIJ, flrA, and flrBC operons do not require σ⁵⁴ for expression, and (iii) FlrA and FlrC co-regulate class II genes. Our model proposes that the V. campbellii flagellar transcriptional hierarchy has three classes of genes, in contrast to the four-class hierarchy in Vibrio cholerae. Our genetic and phenotypic dissection of the V. campbellii flagellar regulatory network highlights the differences that have evolved in flagellar regulation across the Vibrionaceae. Importance Vibrio campbellii is a Gram-negative bacterium that is free-living and ubiquitous in marine environments and is an important global pathogen of fish and shellfish. Disruption of the flagellar motor significantly decreases host mortality of V. campbellii, suggesting that motility is a key factor in pathogenesis. Using this model organism, we identified >60 genes that encode proteins with predicted structural, mechanical, or regulatory roles in function of the single polar flagellum in V. campbellii. We systematically tested strains containing single deletions of each gene to determine the impact on motility and flagellum production. Our studies have uncovered differences in the regulatory network and function of several genes in V. campbellii as compared to established systems in Vibrio cholerae and Vibrio parahaemolyticus.

The Structure, Composition, and Role of Periplasmic Stator Scaffolds in Polar Bacterial Flagellar Motors

In the bacterial flagellar motor, the cell-wall-anchored stator uses an electrochemical gradient across the cytoplasmic membrane to generate a turning force that is applied to the rotor connected to the flagellar filament. Existing theoretical concepts for the stator function are based on the assumption that it anchors around the rotor perimeter by binding to peptidoglycan (P). The existence of another anchoring region on the motor itself has been speculated upon, but is yet to be supported by binding studies. Due to the recent advances in electron cryotomography, evidence has emerged that polar flagellar motors contain substantial proteinaceous periplasmic structures next to the stator, without which the stator does not assemble and the motor does not function. These structures have a morphology of disks, as is the case with Vibrio spp., or a round cage, as is the case with Helicobacter pylori. It is now recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica. This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function.

[Flagellar related genes and functions in Vibrio]

Bacteria can move or swim by flagella. On the other hand, the motile ability is not necessary to live at all. In laboratory, the flagella-deficient strains can grow just like the wild-type strains. The flagellum is assembled from more than 20 structural proteins and there are more than 50 genes including the structural genes to regulate or support the flagellar formation. The cost to construct the flagellum is so expensive. The fact that it evolved as a motor organ means even at such the large cost shows that the flagellum is essential for survival in natural condition. In this review, we would like to focus on the flagella-related researches conducted by the authors and the flagellar research on Vibrio spp.

Virulence Regulation and Innate Host Response in the Pathogenicity of Vibrio cholerae

The human pathogen Vibrio cholerae is the causative agent of severe diarrheal disease known as cholera. Of the more than 200 "O" serogroups of this pathogen, O1 and O139 cause cholera outbreaks and epidemics. The rest of the serogroups, collectively known as non-O1/non-O139 cause sporadic moderate or mild diarrhea and also systemic infections. Pathogenic V. cholerae circulates between nutrient-rich human gut and nutrient-deprived aquatic environment. As an autochthonous bacterium in the environment and as a human pathogen, V. cholerae maintains its survival and proliferation in these two niches. Growth in the gastrointestinal tract involves expression of several genes that provide bacterial resistance against host factors. An intricate regulatory program involving extracellular signaling inputs is also controlling this function. On the other hand, the ability to store carbon as glycogen facilitates bacterial fitness in the aquatic environment. To initiate the infection, V. cholerae must colonize the small intestine after successfully passing through the acid barrier in the stomach and survive in the presence of bile and antimicrobial peptides in the intestinal lumen and mucus, respectively. In V. cholerae, virulence is a multilocus phenomenon with a large functionally associated network. More than 200 proteins have been identified that are functionally linked to the virulence-associated genes of the pathogen. Several of these genes have a role to play in virulence and/or in functions that have importance in the human host or the environment. A total of 524 genes are differentially expressed in classical and El Tor strains, the two biotypes of V. cholerae serogroup O1. Within the host, many immune and biological factors are able to induce genes that are responsible for survival, colonization, and virulence. The innate host immune response to V. cholerae infection includes activation of several immune protein complexes, receptor-mediated signaling pathways, and other bactericidal proteins. This article presents an overview of regulation of important virulence factors in V. cholerae and host response in the context of pathogenesis.

Current and future perspectives for controlling Vibrio biofilms in the seafood industry: a comprehensive review

The contamination of seafood with Vibrio species can have severe repercussions in the seafood industry. Vibrio species can form mature biofilms and persist on the surface of several seafoods such as crabs, oysters, mussels, and shrimp, for extended duration. Several conventional approaches have been employed to inhibit the growth of planktonic cells and prevent the formation of Vibrio biofilms. Since Vibrio biofilms are mostly resistant to these control measures, novel alternative methods need to be urgently developed. In this review, we propose environmentally friendly approaches to suppress Vibrio biofilm formation using a hypothesized mechanism of action.

Vibrio Flagellar Synthesis

Vibrio spp. are highly motile Gram-negative bacteria, ubiquitously found in aquatic environments. Some Vibrios are responsible for disease and morbidity of marine invertebrates and humans, while others are studied for their symbiotic interactions. Vibrio spp. are motile due to synthesis of flagella that rotate and propel the bacteria. Many Vibrio spp. synthesize monotrichous polar flagella (e.g., V. cholerae, V. alginolyticus); however, some synthesize peritrichous or lophotrichous flagella. Flagellar-mediated motility is intimately connected to biological and cellular processes such as chemotaxis, biofilm formation, colonization, and virulence of Vibrio spp. This review focuses on the polar flagellum and its regulation in regard to Vibrio virulence and environmental persistence.

Characterization of FlgP, an Essential Protein for Flagellar Assembly in Rhodobacter sphaeroides

The flagellar lipoprotein FlgP has been identified in several species of bacteria, and its absence provokes different phenotypes. In this study, we show that in the alphaproteobacterium Rhodobacter sphaeroides, a ΔflgP mutant is unable to assemble the hook and the filament. In contrast, the membrane/supramembrane (MS) ring and the flagellar rod appear to be assembled. In the absence of FlgP a severe defect in the transition from rod to hook polymerization occurs. In agreement with this idea, we noticed a reduction in the amount of intracellular flagellin and the chemotactic protein CheY4, both encoded by genes dependent on σ²⁸ This suggests that in the absence of flgP the switch to export the anti-sigma factor, FlgM, does not occur. The presence of FlgP was detected by Western blot in samples of isolated wild-type filament basal bodies, indicating that FlgP is an integral part of the flagellar structure. In this regard, we show that FlgP interacts with FlgH and FlgT, indicating that FlgP should be localized closely to the L and H rings. We propose that FlgP could affect the architecture of the L ring, which has been recently identified to be responsible for the rod-hook transition.IMPORTANCE Flagellar based motility confers a selective advantage on bacteria by allowing migration to favorable environments or in pathogenic species to reach the optimal niche for colonization. The flagellar structure has been well established in Salmonella However, other accessory components have been identified in other species. Many of these have been implied in adapting the flagellar function to enable faster rotation, or higher torque. FlgP has been proposed to be the main component of the basal disk located underlying the outer membrane in Campylobacter jejuni and Vibrio fischeri Its role is still unclear, and its absence impacts motility differently in different species. The study of these new components will bring a better understanding of the evolution of this complex organelle.

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.

Vibrio cholerae O1 secretes an extracellular matrix in response to antibody-mediated agglutination

Vibrio cholerae O1 is one of two serogroups responsible for epidemic cholera, a severe watery diarrhea that occurs after the bacterium colonizes the human small intestine and secretes a potent ADP-ribosylating toxin. Immunity to cholera is associated with intestinal anti-lipopolysaccharide (LPS) antibodies, which are known to inhibit V. cholerae motility and promote bacterial cell-cell crosslinking and aggregation. Here we report that V. cholerae O1 classical and El Tor biotypes produce an extracellular matrix (ECM) when forcibly immobilized and agglutinated by ZAC-3 IgG, an intestinally-derived monoclonal antibody (MAb) against the core/lipid A region of LPS. ECM secretion, as demonstrated by crystal violet staining and scanning electron microscopy, occurred within 30 minutes of antibody exposure and peaked by 3 hours. Non-motile mutants of V. cholerae did not secrete ECM following ZAC-3 IgG exposure, even though they were susceptible to agglutination. The ECM was enriched in O-specific polysaccharide (OSP) but not Vibrio polysaccharide (VPS). Finally, we demonstrate that ECM production by V. cholerae in response to ZAC-3 IgG was associated with bacterial resistant to a secondary complement-mediated attack. In summary, we propose that V. cholerae O1, upon encountering anti-LPS antibodies in the intestinal lumen, secretes an ECM (or O-antigen capsule) possibly as a strategy to shield itself from additional host immune factors and to exit an otherwise inhospitable host environment.

Structural differences in the bacterial flagellar motor among bacterial species

The bacterial flagellum is a supramolecular motility machine consisting of the basal body as a rotary motor, the hook as a universal joint, and the filament as a helical propeller. Intact structures of the bacterial flagella have been observed for different bacterial species by electron cryotomography and subtomogram averaging. The core structures of the basal body consisting of the C ring, the MS ring, the rod and the protein export apparatus, and their organization are well conserved, but novel and divergent structures have also been visualized to surround the conserved structure of the basal body. This suggests that the flagellar motors have adapted to function in various environments where bacteria live and survive. In this review, we will summarize our current findings on the divergent structures of the bacterial flagellar motor.

Oligomeric lipoprotein PelC guides Pel polysaccharide export across the outer membrane of Pseudomonas aeruginosa

Secreted polysaccharides are important functional and structural components of bacterial biofilms. The opportunistic pathogen Pseudomonas aeruginosa produces the cationic exopolysaccharide Pel, which protects bacteria from aminoglycoside antibiotics and contributes to biofilm architecture through ionic interactions with extracellular DNA. A bioinformatics analysis of genome databases suggests that gene clusters for Pel biosynthesis are present in >125 bacterial species, yet little is known about how this biofilm exopolysaccharide is synthesized and exported from the cell. In this work, we characterize PelC, an outer membrane lipoprotein essential for Pel production. Crystal structures of PelC from Geobacter metallireducens and Paraburkholderia phytofirmans coupled with structure-guided disulfide cross-linking in P. aeruginosa suggest that PelC assembles into a 12- subunit ring-shaped oligomer. In this arrangement, an aromatic belt in proximity to its lipidation site positions the highly electronegative surface of PelC toward the periplasm. PelC is structurally similar to the Escherichia coli amyloid exporter CsgG; however, unlike CsgG, PelC does not possess membrane-spanning segments required for polymer export across the outer membrane. We show that the multidomain protein PelB with a predicted C-terminal β-barrel porin localizes to the outer membrane, and propose that PelC functions as an electronegative funnel to guide the positively charged Pel polysaccharide toward an exit channel formed by PelB. Together, our findings provide insight into the unique molecular architecture and export mechanism of the Pel apparatus, a widespread exopolysaccharide secretion system found in environmental and pathogenic bacteria.

Expression of flagellin and key regulatory flagellar genes in the non-motile bacterium Piscirickettsia salmonis

The Piscirickettsia salmonis genome was screened to evaluate potential flagella-related open reading frames, as well as their genomic organization and eventual expression. A complete and organized set of flagellar genes was found for P. salmonis, although no structural flagellum has ever been reported for this bacterium. To gain further understanding, the hierarchical flagellar cascade described for Legionella pneumophila was used as a reference model for putative analysis in P. salmonis. Specifically, 5 of the most relevant genes from this cascade were chosen, including 3 regulatory genes (fleQ, triggers the cascade; fliA, regulates the σ28-coding gene; and rpoN, an RNA polymerase-dependent gene) and 2 terminal structural genes (flaA and flaB, flagellin and a flagellin-like protein, respectively). Kinetic experiments evaluated gene expressions over time, with P. salmonis assessed in 2 liquid, cell-free media and during infection of the SHK-1 fish cell line. Under all conditions, the 5 target genes were primarily expressed during early growth/infection and were differentially expressed when bacteria encountered environmental stress (i.e. a high-salt concentration). Intriguingly, the flagellin monomer was fully expressed under all growth conditions and was located near the bacterial membrane. While no structural flagellum was detected under any condition, the recombinant flagellin monomer induced a proinflammatory response in SHK-1 cells, suggesting a possible immunomodulatory function. The potential implications of these observations are discussed in the context of P. salmonis biology and pathogenic potential.

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system.

Bacterial pathogens utilize a number of highly complex and sophisticated molecular systems to colonize their hosts and alter them, creating customized niches in which to reproduce. One such system is the Type IV pilus system, made up of dozens of proteins that form a macromolecular machine to polymerize small pilin proteins into long thin filaments that are displayed on the bacterial surface. These pili have a remarkable array of functions that rely on their ability to (i) adhere to many substrates, including host cell surfaces, pili from nearby bacteria, DNA and bacterial viruses (bacteriophage), and (ii) to depolymerize or retract, which pulls the bacteria along mucosal surfaces, pulls them close together in protective aggregates, and can even draw in substrates like DNA and bacteriophage for nutrition and genetic variation. For most Type IV pilus systems, retraction is an energy-driven process facilitated by a retraction ATPase. We show here that in the simplest of the Type IV pilus systems, the Vibrio cholerae toxin-coregulated pilus, a pilin-like protein initiates pilus retraction by what appears to be mechanical rather than enzymatic means. Our results provide a framework for understanding more complex Type IV pili and the related Type II secretion systems, which represent targets for novel highly specific antibiotics.

The FlgT Protein Is Involved in Aeromonas hydrophila Polar Flagella Stability and Not Affects Anchorage of Lateral Flagella

Aeromonas hydrophila sodium-driven polar flagellum has a complex stator-motor. Consist of two sets of redundant and non-exchangeable proteins (PomA/PomB and PomA₂/PomB₂), which are homologs to other sodium-conducting polar flagellum stator motors; and also two essential proteins (MotX and MotY), that they interact with one of those two redundant pairs of proteins and form the T-ring. In this work, we described an essential protein for polar flagellum stability and rotation which is orthologs to Vibrio spp. FlgT and it is encoded outside of the A. hydrophila polar flagellum regions. The flgT was present in all mesophilic Aeromonas strains tested and also in the non-motile Aeromonas salmonicida. The A. hydrophila ΔflgT mutant is able to assemble the polar flagellum but is more unstable and released into the culture supernatant from the cell upon completion assembly. Presence of FlgT in purified polar hook-basal bodies (HBB) of wild-type strain was confirmed by Western blotting and electron microscopy observations showed an outer ring of the T-ring (H-ring) which is not present in the ΔflgT mutant. Anchoring and motility of proton-driven lateral flagella was not affected in the ΔflgT mutant and specific antibodies did not detect FlgT in purified lateral HBB of wild type strain.

Architecture of the type IVa pilus machine

Type IVa pili are filamentous cell surface structures observed in many bacteria. They pull cells forward by extending, adhering to surfaces, and then retracting. We used cryo-electron tomography of intact Myxococcus xanthus cells to visualize type IVa pili and the protein machine that assembles and retracts them (the type IVa pilus machine, or T4PM) in situ, in both the piliated and nonpiliated states, at a resolution of 3 to 4 nanometers. We found that T4PM comprises an outer membrane pore, four interconnected ring structures in the periplasm and cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. By systematically imaging mutants lacking defined T4PM proteins or with individual proteins fused to tags, we mapped the locations of all 10 T4PM core components and the minor pilins, thereby providing insights into pilus assembly, structure, and function.

Plant-based production of two chimeric monoclonal IgG antibodies directed against immunodominant epitopes of Vibrio cholerae lipopolysaccharide

We have produced and characterized two chimeric human IgG1 monoclonal antibodies that bind different immunodominant epitopes on Vibrio cholerae lipopolysaccharide (LPS). MAb 2D6 IgG1 recognizes Ogawa O-polysaccharide antigen, while mAb ZAC-3 IgG1 recognizes core/lipid A moiety of Ogawa and Inaba LPS. Both antibodies were expressed using a Nicotiana benthamiana-based rapid antibody-manufacturing platform (RAMP) and evaluated in vitro for activities associated with immunity to V. cholerae, including vibriocidal activity, bacterial agglutination and motility arrest.

The bacterial flagellar motor and its structural diversity

The bacterial flagellum is a reversible rotary motor powered by an electrochemical-potential difference of specific ions across the cytoplasmic membrane. The H(+)-driven motor of Salmonella spins at ∼300 Hz, whereas the Na(+)-driven motor of marine Vibrio spp. can rotate much faster, up to 1700 Hz. A highly conserved motor structure consists of the MS ring, C ring, rod, and export apparatus. The C ring and the export apparatus show dynamic properties for exerting their functional activities. Various additional structures surrounding the conserved motor structure are observed in different bacterial species. In this review we summarize our current understanding of the structure, function, and assembly of the flagellar motor in Salmonella and marine Vibrio.

A distant homologue of the FlgT protein interacts with MotB and FliL and is essential for flagellar rotation in Rhodobacter sphaeroides

In this work, we describe a periplasmic protein that is essential for flagellar rotation in Rhodobacter sphaeroides. This protein is encoded upstream of flgA, and its expression is dependent on the flagellar master regulator FleQ and on the class III flagellar activator FleT. Sequence comparisons suggest that this protein is a distant homologue of FlgT. We show evidence that in R. sphaeroides, FlgT interacts with the periplasmic regions of MotB and FliL and with the flagellar protein MotF, which was recently characterized as a membrane component of the flagellum in this bacterium. In addition, the localization of green fluorescent protein (GFP)-MotF is completely dependent on FlgT. The Mot(-) phenotype of flgT cells was weakly suppressed by point mutants of MotB that presumably keep the proton channel open and efficiently suppress the Mot(-) phenotype of motF and fliL cells, indicating that FlgT could play an additional role beyond the opening of the proton channel. The presence of FlgT in purified filament-hook-basal bodies of the wild-type strain was confirmed by Western blotting, and the observation of these structures under an electron microscope showed that the basal bodies from flgT cells had lost the ring that covers the LP ring in the wild-type structure. Moreover, MotF was detected by immunoblotting in the basal bodies obtained from the wild-type strain but not from flgT cells. From these results, we suggest that FlgT forms a ring around the LP ring, which anchors MotF and stabilizes the stator complex of the flagellar motor.

Insight into the assembly mechanism in the supramolecular rings of the sodium-driven Vibrio flagellar motor from the structure of FlgT

Flagellar motility is a key factor for bacterial survival and growth in fluctuating environments. The polar flagellum of a marine bacterium, Vibrio alginolyticus, is driven by sodium ion influx and rotates approximately six times faster than the proton-driven motor of Escherichia coli. The basal body of the sodium motor has two unique ring structures, the T ring and the H ring. These structures are essential for proper assembly of the stator unit into the basal body and to stabilize the motor. FlgT, which is a flagellar protein specific for Vibrio sp., is required to form and stabilize both ring structures. Here, we report the crystal structure of FlgT at 2.0-Å resolution. FlgT is composed of three domains, the N-terminal domain (FlgT-N), the middle domain (FlgT-M), and the C-terminal domain (FlgT-C). FlgT-M is similar to the N-terminal domain of TolB, and FlgT-C resembles the N-terminal domain of FliI and the α/β subunits of F1-ATPase. To elucidate the role of each domain, we prepared domain deletion mutants of FlgT and analyzed their effects on the basal-body ring formation. The results suggest that FlgT-N contributes to the construction of the H-ring structure, and FlgT-M mediates the T-ring association on the LP ring. FlgT-C is not essential but stabilizes the H-ring structure. On the basis of these results, we propose an assembly mechanism for the basal-body rings and the stator units of the sodium-driven flagellar motor.

Fluorescence imaging of GFP-fused periplasmic components of Na+-driven flagellar motor using Tat pathway in Vibrio alginolyticus

The twin-arginine translocation (Tat) system works to export folded proteins across the cytoplasmic membrane via specific signal peptides harbouring a twin-arginine motif. In Escherichia coli, a functional GFP is exported to the periplasm through the Tat pathway by fusion of the signal peptide of TorA, which is one of the periplasmic proteins exported by the Tat pathway. In this study, we fused the signal peptide of Vibrio alginolyticus TorA (TorASP) to GFP and demonstrate the export of functional GFP to the periplasm of V. alginolyticus. We also made fusions of TorASP-GFP with MotX, MotY and FlgT, which are periplasmic components of the Na(+)-driven flagellar motor. Those fusion proteins were localized to the flagellar motor independent of the Na(+) concentration in the environment.

Novel genes involved in Pseudomonas fluorescens Pf0-1 motility and biofilm formation

AdnA in Pseudomonas fluorescens, an ortholog of FleQ in P. aeruginosa, regulates both motility and flagellum-mediated attachment to various surfaces. A whole-genome microarray determined the AdnA transcriptome by comparing the gene expression pattern of wild-type Pf0-1 to that of Pf0-2x (adnA deletion mutant) in broth culture. In the absence of AdnA, expression of 92 genes was decreased, while 11 genes showed increased expression. Analysis of 16 of these genes fused to lacZ confirmed the microarray results. Several genes were further evaluated for their role in motility and biofilm formation. Two genes, Pfl01_1508 and Pfl01_1517, affected motility and had different effects on biofilm formation in Pf0-1. These two genes are predicted to specify proteins similar to the glycosyl transferases FgtA1 and FgtA2, which have been shown to be involved in virulence and motility in P. syringae. Three other genes, Pfl01_1516, Pfl01_1572, and Pfl01_1573, not previously associated with motility and biofilm formation in Pseudomonas had similar effects on biofilm formation in Pf0-1. Deletion of each of these genes led to different motility defects. Our data revealed an additional level of complexity in the control of flagellum function beyond the core genes known to be required and may yield insights into processes important for environmental persistence of P. fluorescens Pf0-1.

Specificity of motor components in the dual flagellar system of Shewanella putrefaciens CN-32

Bacterial flagellar motors are intricate nanomachines in which the stator units and rotor component FliM may be dynamically exchanged during function. Similar to other bacterial species, the gammaproteobacterium Shewanella putrefaciens CN-32 possesses a complete secondary flagellar system along with a corresponding stator unit. Expression of the secondary system occurs during planktonic growth in complex media and leads to the formation of a subpopulation with one or more additional flagella at random positions in addition to the primary polar system. We used physiological and phenotypic characterizations of defined mutants in concert with fluorescent microscopy on labelled components of the two different systems, the stator proteins PomB and MotB, the rotor components FliM(1) and FliM(2), and the auxiliary motor components MotX and MotY, to determine localization, function and dynamics of the proteins in the flagellar motors. The results demonstrate that the polar flagellum is driven by a Na(+)-dependent FliM(1)/PomAB/MotX/MotY flagellar motor while the secondary system is rotated by a H(+)-dependent FliM(2)/MotAB motor. The components were highly specific for their corresponding motor and are unlikely to be extensively swapped or shared between the two flagellar systems under planktonic conditions. The results have implications for both specificity and dynamics of flagellar motor components.

Genetic and molecular characterization of flagellar assembly in Shewanella oneidensis

Shewanella oneidensis is a highly motile organism by virtue of a polar flagellum. Unlike most flagellated bacteria, it contains only one major chromosome segment encoding the components of the flagellum with the exception of the motor proteins. In this region, three genes encode flagellinsaccording to the original genome annotation. However, we find that only flaA and flaB encode functional filament subunits. Although these two genesare under the control of different promoters, they are actively transcribed and subsequently translated, producing a considerable number of flagellin proteins. Additionally, both flagellins are able to interact with their chaperon FliS and are subjected to feedback regulation. Furthermore, FlaA and FlaB are glycosylated by a pathwayinvolving a major glycosylating enzyme,PseB, in spite of the lack of the majority of theconsensus glycosylation sites. In conclusion, flagellar assembly in S. oneidensis has novel features despite the conservation of homologous genes across taxa.

Burkholderia glumae: next major pathogen of rice?

Burkholderia glumae causes bacterial panicle blight of rice, which is an increasingly important disease problem in global rice production. Toxoflavin and lipase are known to be major virulence factors of this pathogen, and their production is dependent on the TofI/TofR quorum-sensing system, which is mediated by N-octanoyl homoserine lactone. Flagellar biogenesis and a type III secretion system are also required for full virulence of B. glumae. Bacterial panicle blight is thought to be caused by seed-borne B. glumae; however, its disease cycle is not fully understood. In spite of its economic importance, neither effective control measures for bacterial panicle blight nor rice varieties showing complete resistance to the disease are currently available. A better understanding of the molecular mechanisms underlying B. glumae virulence and of the rice defence mechanisms against the pathogen would lead to the development of better methods of disease control for bacterial panicle blight.

TAXONOMY

Bacteria; Proteobacteria; Betaproteobacteria; Burkholderiales; Burkholderiaceae; Burkholderia.

MICROBIOLOGICAL PROPERTIES

Gram-negative, capsulated, motile, lophotrichous flagella, pectolytic.

DISEASE SYMPTOMS

Aborted seed, empty grains as a result of failure of grain filling, brown spots on panicles, seedling rot.

DISEASE CONTROL

Seed sterilization, planting partially resistant lines (no completely resistant line is available). KNOWN VIRULENCE FACTORS: Toxoflavin, lipase, type III effectors.

The flagellar basal body-associated protein FlgT is essential for a novel ring structure in the sodium-driven Vibrio motor

In Vibrio alginolyticus, the flagellar motor can rotate at a remarkably high speed, ca. three to four times faster than the Escherichia coli or Salmonella motor. Here, we found a Vibrio-specific protein, FlgT, in the purified flagellar basal body fraction. Defects of FlgT resulted in partial Fla⁻ and Mot⁻ phenotypes, suggesting that FlgT is involved in formation of the flagellar structure and generating flagellar rotation. Electron microscopic observation of the basal body of ΔflgT cells revealed a smaller LP ring structure compared to the wild type, and most of the T ring was lost. His₆-tagged FlgT could be coisolated with MotY, the T-ring component, suggesting that FlgT may interact with the T ring composed of MotX and MotY. From these lines of evidence, we conclude that FlgT associates with the basal body and is responsible to form an outer ring of the LP ring, named the H ring, which can be distinguished from the LP ring formed by FlgH and FlgI. Vibrio-specific structures, e.g., the T ring and H ring might contribute the more robust motor structure compared to that of E. coli and Salmonella.