Genetic and biochemical analysis of Salmonella typhimurium FliI, a flagellar protein related to the catalytic subunit of the F0F1 ATPase and to virulence proteins of mammalian and plant pathogens

Genomic and Virulence Characterization of Intrauterine Pathogenic Escherichia coli With Multi-Drug Resistance Isolated From Cow Uteri With Metritis

Metritis is a major disease in dairy cows causing animal death, decrease of birth rate, milk production, and economic loss. Antibiotic treatment is generally used to treat such disease but has a high failure rate of 23–35%. The reason for the treatment failure remains unclear, although antibiotic resistance is postulated as one of factors. Our study investigated the prevalence of extended spectrum β-lactamase (ESBL) producing bacteria in uterine samples of cows with metritis and characterized the isolated intrauterine pathogenic Escherichia coli (IUPEC) strains using whole genome sequencing. We found that the cows with metritis we examined had a high percentage of ESBL producing IUPEC with multi-drug resistance including ceftiofur which is commonly used for metritis treatment. The ESBL producing IUPEC strains harbored versatile antibiotic resistance genes conferring resistance against 29 antibiotic classes, suggesting that transmission of these bacteria to other animals and humans may lead to antibiotic treatment failure. Furthermore, these strains had strong adhesion and invasion activity, along with critical virulence factors, indicating that they may cause infectious diseases in not only the uterus, but also in other organs and hosts.

In Vitro Reconstitution of Functional Type III Protein Export and Insights into Flagellar Assembly

The type III secretion system (T3SS) forms the functional core of injectisomes, protein transporters that allow bacteria to deliver virulence factors into their hosts for infection, and flagella, which are critical for many pathogens to reach the site of infection. In spite of intensive genetic and biochemical studies, the T3SS protein export mechanism remains unclear due to the difficulty of accurate measurement of protein export in vivo . Here, we developed an in vitro flagellar T3S protein transport assay system using an inverted cytoplasmic membrane vesicle (IMV) for accurate and controlled measurements of flagellar protein export. We show that the flagellar T3SS in the IMV fully retains export activity. The flagellar hook was constructed inside the lumen of the IMV by adding purified component proteins externally to the IMV solution. We reproduced the hook length control and export specificity switch in the IMV consistent with that seen in the native cell. Previous in vivo analyses showed that flagellar protein export is driven by proton motive force (PMF) and facilitated by ATP hydrolysis by FliI, a T3SS-specific ATPase. Our in vitro assay recapitulated these previous in vivo observations but furthermore clearly demonstrated that even ATP hydrolysis by FliI alone can drive flagellar protein export. Moreover, this assay showed that addition of the FliH 2 /FliI complex to the assay solution at a concentration similar to that in the cell dramatically enhanced protein export, confirming that the FliH 2 /FliI complex in the cytoplasm is important for effective protein transport.

IMPORTANCE The type III secretion system (T3SS) is the functional core of the injectisome, a bacterial protein transporter used to deliver virulence proteins into host cells, and bacterial flagella, critical for many pathogens. The molecular mechanism of protein transport is still unclear due to difficulties in accurate measurements of protein transport under well-controlled conditions in vivo . We succeeded in developing an in vitro transport assay system of the flagellar T3SS using inverted membrane vesicles (IMVs). Flagellar hook formation was reproduced in the IMV, suggesting that the export apparatus in the IMV retains a protein transport activity similar to that in the cell. Using this system, we revealed that ATP hydrolysis by the T3SS ATPase can drive protein export without PMF.

A rapidly new-typed detection of norovirus based on F0F1-ATPase molecular motor biosensor

In order to adapt port rapid detection of food borne norovirus, presently we developed a new typed detection method based on F₀F₁-ATPase molecular motor biosensor. A specific probe was encompassed the conservative region of norovirus and F₀F₁-ATPase within chromatophore was constructed as a molecular motor biosensor through the "ε-subunit antibody-streptomycin-biotin-probe" system. Norovirus was captured based on probe-RNA specific binding. Our results demonstrated that the Limit of Quantification (LOQ) is 0.005 ng/mL for NV RNA and also demonstrated that this method possesses specificity and none cross-reaction for food borne virus. What's more, the experiment used this method could be accomplished in 1 h. We detected 10 samples by using this method and the results were consistent with RT-PCR results. Overall, based on F₀F₁-ATPase molecular motors biosensor system we firstly established a new typed detection method for norovirus detection and demonstrated that this method is sensitive and specific and can be used in the rapid detection for food borne virus.

Soluble components of the flagellar export apparatus, FliI, FliJ, and FliH, do not deliver flagellin, the major filament protein, from the cytosol to the export gate

Flagella, the locomotion organelles of bacteria, extend from the cytoplasm to the cell exterior. External flagellar proteins are synthesized in the cytoplasm and exported by the flagellar type III secretion system. Soluble components of the flagellar export apparatus, FliI, FliH, and FliJ, have been implicated to carry late export substrates in complex with their cognate chaperones from the cytoplasm to the export gate. The importance of the soluble components in the delivery of the three minor late substrates FlgK, FlgL (hook-filament junction) and FliD (filament-cap) has been convincingly demonstrated, but their role in the transport of the major filament component flagellin (FliC) is still unclear. We have used continuous ATPase activity measurements and quartz crystal microbalance (QCM) studies to characterize interactions between the soluble export components and flagellin or the FliC:FliS substrate-chaperone complex. As controls, interactions between soluble export component pairs were characterized providing Kd values. FliC or FliC:FliS did not influence the ATPase activity of FliI alone or in complex with FliH and/or FliJ suggesting lack of interaction in solution. Immobilized FliI, FliH, or FliJ did not interact with FliC or FliC:FliS detected by QCM. The lack of interaction in the fluid phase between FliC or FliC:FliS and the soluble export components, in particular with the ATPase FliI, suggests that cells use different mechanisms for the export of late minor substrates, and the major substrate, FliC. It seems that the abundantly produced flagellin does not require the assistance of the soluble export components to efficiently reach the export gate.

The non-flagellar type III secretion system evolved from the bacterial flagellum and diversified into host-cell adapted systems

Type 3 secretion systems (T3SSs) are essential components of two complex bacterial machineries: the flagellum, which drives cell motility, and the non-flagellar T3SS (NF-T3SS), which delivers effectors into eukaryotic cells. Yet the origin, specialization, and diversification of these machineries remained unclear. We developed computational tools to identify homologous components of the two systems and to discriminate between them. Our analysis of >1,000 genomes identified 921 T3SSs, including 222 NF-T3SSs. Phylogenomic and comparative analyses of these systems argue that the NF-T3SS arose from an exaptation of the flagellum, i.e. the recruitment of part of the flagellum structure for the evolution of the new protein delivery function. This reconstructed chronology of the exaptation process proceeded in at least two steps. An intermediate ancestral form of NF-T3SS, whose descendants still exist in Myxococcales, lacked elements that are essential for motility and included a subset of NF-T3SS features. We argue that this ancestral version was involved in protein translocation. A second major step in the evolution of NF-T3SSs occurred via recruitment of secretins to the NF-T3SS, an event that occurred at least three times from different systems. In rhizobiales, a partial homologous gene replacement of the secretin resulted in two genes of complementary function. Acquisition of a secretin was followed by the rapid adaptation of the resulting NF-T3SSs to multiple, distinct eukaryotic cell envelopes where they became key in parasitic and mutualistic associations between prokaryotes and eukaryotes. Our work elucidates major steps of the evolutionary scenario leading to extant NF-T3SSs. It demonstrates how molecular evolution can convert one complex molecular machine into a second, equally complex machine by successive deletions, innovations, and recruitment from other molecular systems.

Insights into cross-kingdom plant pathogenic bacteria

Plant and human pathogens have evolved disease factors to successfully exploit their respective hosts. Phytopathogens utilize specific determinants that help to breach reinforced cell walls and manipulate plant physiology to facilitate the disease process, while human pathogens use determinants for exploiting mammalian physiology and overcoming highly developed adaptive immune responses. Emerging research, however, has highlighted the ability of seemingly dedicated human pathogens to cause plant disease, and specialized plant pathogens to cause human disease. Such microbes represent interesting systems for studying the evolution of cross-kingdom pathogenicity, and the benefits and tradeoffs of exploiting multiple hosts with drastically different morphologies and physiologies. This review will explore cross-kingdom pathogenicity, where plants and humans are common hosts. We illustrate that while cross-kingdom pathogenicity appears to be maintained, the directionality of host association (plant to human, or human to plant) is difficult to determine. Cross-kingdom human pathogens, and their potential plant reservoirs, have important implications for the emergence of infectious diseases.

Structure of the cytoplasmic domain of FlhA and implication for flagellar type III protein export

FlhA is the largest integral membrane component of the flagellar type III protein export apparatus of Salmonella and is composed of an N-terminal transmembrane domain (FlhA(TM)) and a C-terminal cytoplasmic domain (FlhA(C)). FlhA(C) is thought to form a platform of the export gate for the soluble components to bind to for efficient delivery of export substrates to the gate. Here, we report a structure of FlhA(C) at 2.8 A resolution. FlhA(C) consists of four subdomains (A(C)D1, A(C)D2, A(C)D3 and A(C)D4) and a linker connecting FlhA(C) to FlhA(TM). The sites of temperature-sensitive (ts) mutations that impair protein export are distributed to all four domains, with half of them at subdomain interfaces. Analyses of the ts mutations and four suppressor mutations to the G368C ts mutation suggested that FlhA(C) changes its conformation for its function. Molecular dynamics simulation demonstrated an open-close motion with a 5-10 ns oscillation in the distance between A(C)D2 and A(C)D4. These results along with further mutation analyses suggest that a dynamic domain motion of FlhA(C) is essential for protein export.

Flagellar formation in C-ring-defective mutants by overproduction of FliI, the ATPase specific for flagellar type III secretion

The flagellar cytoplasmic ring (C ring), which consists of three proteins, FliG, FliM, and FliN, is located on the cytoplasmic side of the flagellum. The C ring is a multifunctional structure necessary for flagellar protein secretion, torque generation, and switching of the rotational direction of the motor. The deletion of any one of the fliG, fliM, and fliN genes results in a Fla(-) phenotype. Here, we show that the overproduction of the flagellum-specific ATPase FliI overcomes the inability of basal bodies with partial C-ring structures to produce complete flagella. Flagella made upon FliI overproduction were paralyzed, indicating that an intact C ring is essential for motor function. In FliN- or FliM-deficient mutants, flagellum production was about 10% of the wild-type level, while it was only a few percent in FliG-deficient mutants, suggesting that the size of partial C rings affects the extent of flagellation. For flagella made in C-ring mutants, the hook length varied considerably, with many being markedly shorter or longer than that of the wild type. The broad distribution of hook lengths suggests that defective C rings cannot control the hook length as tightly as the wild type even though FliK and FlhB are both intact.

Effect of flagellar mutations on Yersinia enterocolitica biofilm formation

Yersinia enterocolitica biovar 1B is one of a number of strains pathogenic to humans in the genus Yersinia. It has three different type III secretion systems, Ysc, Ysa, and the flagella. In this study, the effect of flagella on biofilm formation was evaluated. In a panel of 31 mutant Y. enterocolitica strains, we observed that mutations that abolish the structure or rotation of the flagella greatly reduce biofilm formation when the bacteria are grown under static conditions. These results were further evaluated by assessing biofilm formation under continuous culture using a flow cell chamber. The results confirmed the important contribution of flagella to the initiation of biofilm production but indicated that there are differences in the progression of biofilm development between static growth and flow conditions. Our results suggest that flagella play a critical role in biofilm formation in Y. enterocolitica.

Energy source of flagellar type III secretion

Bacterial flagella contain a specialized secretion apparatus that functions to deliver the protein subunits that form the filament and other structures to outside the membrane. This apparatus is related to the injectisome used by many gram-negative pathogens and symbionts to transfer effector proteins into host cells; in both systems this export mechanism is termed 'type III' secretion. The flagellar secretion apparatus comprises a membrane-embedded complex of about five proteins, and soluble factors, which include export-dedicated chaperones and an ATPase, FliI, that was thought to provide the energy for export. Here we show that flagellar secretion in Salmonella enterica requires the proton motive force (PMF) and does not require ATP hydrolysis by FliI. The export of several flagellar export substrates was prevented by treatment with the protonophore CCCP, with no accompanying decrease in cellular ATP levels. Weak swarming motility and rare flagella were observed in a mutant deleted for FliI and for the non-flagellar type-III secretion ATPases InvJ and SsaN. These findings show that the flagellar secretion apparatus functions as a proton-driven protein exporter and that ATP hydrolysis is not essential for type III secretion.

Type VI secretion: a beginner's guide

Type VI secretion is a newly described mechanism for protein transport across the cell envelope of Gram-negative bacteria. Components that have been partially characterised include an IcmF homologue, the ATPase ClpV, a regulatory FHA domain protein and the secreted VgrG and Hcp proteins. Type VI secretion is clearly a key virulence factor for some important pathogenic bacteria and has been implicated in the translocation of a potential effector protein into eukaryotic cells by at least one organism (Vibrio cholerae). However, type VI secretion systems (T6SSs) are widespread in nature and not confined to known pathogens. In accordance with the general rule that the expression of protein secretion systems is tightly regulated, expression of type VI secretion is controlled at both transcriptional and post-transcriptional levels.

Identification of amino acid residues within the N-terminal domain of EspA that play a role in EspA filament biogenesis and function

Enteropathogenic Escherichia coli employs a filamentous type III secretion system, made by homopolymerization of the translocator protein EspA. In this study, we have shown that the N-terminal region of EspA has a role in EspA's protein stability, interaction with the CesAB chaperone, and filament biogenesis and function.

Enzymatic characterization of the enteropathogenic Escherichia coli type III secretion ATPase EscN

Type III secretion is a transport mechanism by which bacteria secrete proteins across their cell envelope. This protein export pathway is used by two different bacterial nanomachines: the flagellum and the injectisome. An indispensable component of these secretion systems is an ATPase similar to the F1-ATPase beta subunit. Here we characterize EscN, an enteropathogenic Escherichia coli type III ATPase. A recombinant version of EscN, which was fully functional in complementation tests, was purified to homogeneity. Our results demonstrate that EscN is a Mg2+-dependent ATPase (kcat 0.35 s(-1)). We also define optimal conditions for the hydrolysis reaction. EscN displays protein concentration-dependent activity, suggesting that the specific activity changes with the oligomeric state of the protein. The presence of active oligomers was revealed by size exclusion chromatography and native gel electrophoresis.

Structural similarity between the flagellar type III ATPase FliI and F1-ATPase subunits

Construction of the bacterial flagellum in the cell exterior proceeds at its distal end by highly ordered self-assembly of many different component proteins, which are selectively exported through the central channel of the growing flagellum by the flagellar type III export apparatus. FliI is the ATPase of the export apparatus that drives the export process. Here we report the 2.4 A resolution crystal structure of FliI in the ADP-bound form. FliI consists of three domains, and the whole structure shows extensive similarities to the alpha and beta subunits of F0F1-ATPsynthase, a rotary motor that drives the chemical reaction of ATP synthesis. A hexamer model of FliI has been constructed based on the F1-ATPase structure composed of the alpha3beta3gamma subunits. Although the regions that differ in conformation between FliI and the F1-alpha/beta subunits are all located on the outer surface of the hexamer ring, the main chain structures at the subunit interface and those surrounding the central channel of the ring are well conserved. These results imply an evolutionary relation between the flagellum and F0F1-ATPsynthase and a similarity in the mechanism between FliI and F1-ATPase despite the apparently different functions of these proteins.

The FliN-FliH interaction mediates localization of flagellar export ATPase FliI to the C ring complex

FliH regulates the flagellar export ATPase FliI, preventing nonproductive ATP hydrolysis. FliH has been shown to stably associate with the C ring protein FliN. Analysis of this complex reveals that FliH is required for FliI localization to the C ring, and thus FliH not only inhibits FliI ATPase activity but also may act to target FliI to the basal body. Quantitative binding studies revealed a KD of 110 nM for FliH binding to FliN. The KD for FliH binding of a FliN variant from a temperature-sensitive nonflagellate fliN point mutant was determined to be 270 nM, suggesting a molecular explanation for its phenotype. Another variant FliN from a temperature-sensitive mutant with a different phenotype displayed binding with an intermediate affinity. Weak export activity in a fliN null mutant was greatly increased by overproduction of FliI, mimicking a previously observed FliH bypass effect and supporting the conclusion that FliN-FliH binding is important for localization of FliI to the C ring and thus the membrane-embedded export apparatus beyond. A model incorporating the present findings is presented.

Crystallization and preliminary X-ray analysis of Salmonella FliI, the ATPase component of the type III flagellar protein-export apparatus

Most of the structural components making up the bacterial flagellum are translocated through the central channel of the growing flagellar structure by the type III flagellar protein-export apparatus in an ATPase-driven manner and are assembled at the growing end. FliI is the ATPase that drives flagellar protein export using the energy of ATP hydrolysis. FliI forms an oligomeric ring structure in order to attain maximum ATPase activity. In this study, FliI(Delta1-18), an N-terminally truncated variant of FliI lacking the first 18 residues, was purified and crystallized. Crystals were obtained using the hanging-drop vapour-diffusion technique with PEG 8000 as a precipitant. FliI(Delta1-18) crystals grew in the monoclinic space group P2(1), with unit-cell parameters a = 48, b = 73, c = 126 A, beta = 94 degrees, and diffracted to 2.4 A resolution. Anomalous difference Patterson maps of Os-derivative and Pt-derivative crystals showed significant peaks in their Harker sections, indicating that both derivatives are suitable for structure determination.

Oligomerization of the bacterial flagellar ATPase FliI is controlled by its extreme N-terminal region

Salmonella FliI is the flagellar ATPase which converts the energy of ATP hydrolysis into the export of flagellar proteins. It forms a ring-shaped oligomer in the presence of ATP, its analogs, or phospholipids. The extreme N-terminal region of FliI has an unstable conformation and is responsible for the interaction with other components of the export apparatus and for regulation of the catalytic mechanism. To understand the role of this N-terminal region in more detail, we used multi-angle light-scattering, analytical ultracentrifugation, far-UV CD and biochemical methods to characterize a partially functional variant of FliI, missing its first seven amino acid residues (His-FliI(Delta1-7)), whose ATPase activity is about ten times lower than that of wild-type FliI. His-FliI(Delta1-7) is monomeric in solution. The deletion increased the content of alpha-helix, suggesting that the deletion stabilizes the unstable N-terminal region into an alpha-helical conformation. The deletion did not influence the K(m) value for ATP. However, unlike the wild-type, ATP and acidic phospholipids did not induce oligomerization of His-FliI(Delta1-7) or increase its ATPase activity. These results suggest that the deletion suppresses the oligomerization of FliI, and that a conformational change in the unstable N-terminal region is required for FliI oligomerization to effectively couple the energy of ATP hydrolysis to the translocation of flagellar proteins.

The flagellar hierarchy of Rhodobacter sphaeroides is controlled by the concerted action of two enhancer-binding proteins

The expression of the bacterial flagellar genes follows a hierarchical pattern. In Rhodobacter sphaeroides the flagellar genes encoding the hook and basal body proteins are expressed from sigma54-dependent promoters. This type of promoters is always regulated by transcriptional activators that belong to the family of the enhancer-binding proteins (EBPs). We searched for possible EBPs in the genome of R. sphaeroides and mutagenized two open reading frames (ORFs) (fleQ and fleT), which are in the vicinity of flagellar genes. The resulting mutants were non-motile and could only be complemented by the wild-type copy of the mutagenized gene. Transcriptional fusions showed that all the flagellar sigma54-dependent promoters with exception of fleTp, required both transcriptional activators for their expression. Interestingly, transcription of the fleT operon is only dependent on FleQ, and FleT has a negative effect. Both activators were capable of hydrolysing ATP, and were capable of promoting transcription from the flagellar promoters at some extent. Electrophoretic mobility shift assays suggest that only FleQ interacts with DNA whereas FleT improves binding of FleQ to DNA. A four-tiered flagellar transcriptional hierarchy and a regulatory mechanism based on the intracellular concentration of both activators and differential enhancer affinities are proposed.

Ab initio genotype-phenotype association reveals intrinsic modularity in genetic networks

Microbial species express an astonishing diversity of phenotypic traits, behaviors, and metabolic capacities. However, our molecular understanding of these phenotypes is based almost entirely on studies in a handful of model organisms that together represent only a small fraction of this phenotypic diversity. Furthermore, many microbial species are not amenable to traditional laboratory analysis because of their exotic lifestyles and/or lack of suitable molecular genetic techniques. As an adjunct to experimental analysis, we have developed a computational information-theoretic framework that produces high-confidence gene-phenotype predictions using cross-species distributions of genes and phenotypes across 202 fully sequenced archaea and eubacteria. In addition to identifying the genetic basis of complex traits, our approach reveals the organization of these genes into generic preferentially co-inherited modules, many of which correspond directly to known enzymatic pathways, molecular complexes, signaling pathways, and molecular machines.

Chaperone release and unfolding of substrates in type III secretion

Type III protein secretion systems are essential virulence factors of many bacteria pathogenic to humans, animals and plants. These systems mediate the transfer of bacterial virulence proteins directly into the host cell cytoplasm. Proteins are thought to travel this pathway in a largely unfolded manner, and a family of customized cytoplasmic chaperones, which specifically bind cognate secreted proteins, are essential for secretion. Here we show that InvC, an ATPase associated with a Salmonella enterica type III secretion system, has a critical function in substrate recognition. Furthermore, InvC induces chaperone release from and unfolding of the cognate secreted protein in an ATP-dependent manner. Our results show a similarity between the mechanisms of substrate recognition by type III protein secretion systems and AAA + ATPase disassembly machines.

HP0958 is an essential motility gene in Helicobacter pylori

Motility is an essential colonization factor for the human gastric pathogen Helicobacter pylori. The H. pylori genome encodes most known flagellar proteins, although a number of key transcription regulators, chaperones, and structural proteins have not yet been identified. Using recently published yeast two-hybrid data we identified HP0958 as a potential motility-associated protein due to its strong interactions with RpoN (sigma(54)) and FliH, a flagellar ATPase regulator. HP0958 exhibits no sequence similarity to any published flagellar genes but contains a carboxy-terminal zinc finger domain that could function in nucleic acid or protein binding. We created a HP0958 mutant by inserting a chloramphenicol resistance marker into the gene using a PCR-based allelic exchange method and the resultant mutant was non-motile as measured by a BacTracker instrument. Electron microscopic analysis revealed that the HP0958 mutant cells were aflagellate and Western blot analysis revealed a dramatic reduction in flagellin and hook protein production. The HP0958 mutant also showed decreased transcription of flgE, flaB and flaA as well as the checkpoint genes flhA and flhF. Expression of flgM was increased relative to the wild-type and both rpoN and fliA (sigma(28)) expression were unchanged. We conclude that HP0958 is essential for normal motility and flagella production, and represents a novel flagellar component in the epsilon proteobacteria.

Crystal structure of the flagellar rotor protein FliN from Thermotoga maritima

FliN is a component of the bacterial flagellum that is present at levels of more than 100 copies and forms the bulk of the C ring, a drum-shaped structure at the inner end of the basal body. FliN interacts with FliG and FliM to form the rotor-mounted switch complex that controls clockwise-counterclockwise switching of the motor. In addition to its functions in motor rotation and switching, FliN is thought to have a role in the export of proteins that form the exterior structures of the flagellum (the rod, hook, and filament). Here, we describe the crystal structure of most of the FliN protein of Thermotoga maritima. FliN is a tightly intertwined dimer composed mostly of beta sheet. Several well-conserved hydrophobic residues form a nonpolar patch on the surface of the molecule. A mutation in the hydrophobic patch affected both flagellar assembly and switching, showing that this surface feature is important for FliN function. The association state of FliN in solution was studied by analytical ultracentrifugation, which provided clues to the higher-level organization of the protein. T. maritima FliN is primarily a dimer in solution, and T. maritima FliN and FliM together form a stable FliM(1)-FliN(4) complex. Escherichia coli FliN forms a stable tetramer in solution. The arrangement of FliN subunits in the tetramer was modeled by reference to the crystal structure of tetrameric HrcQB(C), a related protein that functions in virulence factor secretion in Pseudomonas syringae. The modeled tetramer is elongated, with approximate dimensions of 110 by 40 by 35 Angstroms, and it has a large hydrophobic cleft formed from the hydrophobic patches on the dimers. On the basis of the present data and available electron microscopic images, we propose a model for the organization of FliN subunits in the C ring.

Crystallization and preliminary X-ray analysis of the C-terminal cytoplasmic domain of FlhA, a membrane-protein subunit of the bacterial flagellar type III protein-export apparatus

The axial components of the bacterial flagellum and the scaffolding proteins for its assembly are exported through the flagellar-specific type III protein-export apparatus, which is believed to be located on the cytoplasmic surface of the basal body. FlhA is an essential component of the type III export apparatus of Salmonella and consists of two major portions: an N-terminal transmembrane domain and a C-terminal cytoplasmic domain (FlhAC). FlhAC and a 38 kDa fragment of FlhAC (FlhAC38K) were purified and crystallized. The crystals were obtained by the sitting-drop vapour-diffusion technique with PEG 8000 as a precipitant. FlhAC crystals grew in the tetragonal space group I4(1)/I4(3), with unit-cell parameters a = b = 216.6, c = 65.0 A. FlhAC38K was crystallized in an orthorhombic form, with unit-cell parameters a = 53.0, b = 93.1, c = 186.5 A. X-ray diffraction data from crystals of FlhAC and the SeMet derivative of FlhAC were collected to 2.9 and 3.2 A, respectively.

Structural and functional analysis of the C-terminal cytoplasmic domain of FlhA, an integral membrane component of the type III flagellar protein export apparatus in Salmonella

FlhA is an integral membrane component of the Salmonella type III flagellar protein export apparatus. It consists of 692 amino acid residues and has two domains: the N-terminal transmembrane domain consisting of the first 327 amino acid residues, and the C-terminal cytoplasmic domain (FlhAC) comprising the remainder. Here, we have investigated the structure and function of FlhAC. DNA sequence analysis revealed that temperature-sensitive flhA mutations, which abolish flagellar protein export at the restrictive temperature, lie in FlhAC, indicating that FlhAC plays an important role in the protein export process. Limited proteolysis of purified His-FlhAC by trypsin and V8 showed that only a small part of FlhAC near its N terminus (residues 328-351) is sensitive to proteolysis. FlhAC38K, the smallest fragment produced by V8 proteolysis, is monomeric and has a spherical shape as judged by analytical gel filtration chromatography and analytical ultracentrifugation. The far-UV CD spectrum of FlhAC38K showed that it contains considerable amounts of secondary structure. FlhA(Delta328-351) missing residues 328-351 failed to complement the flhA mutant, indicating that the proteolytically sensitive region of FlhA is important for its function. FlhA(Delta328-351) was inserted into the cytoplasmic membrane, and exerted a strong dominant negative effect on wild-type cells, suggesting that it retains the ability to interact with other export components within the cytoplasmic membrane. Overproduced FlhAC38K inhibited both motility and flagellar protein export of wild-type cells to some degree, suggesting that FlhAC38K is directly involved in the translocation reaction. Amino acid residues 328-351 of FlhA appear to be a relatively flexible linker between the transmembrane domain and FlhAC38K.

Genetic analysis of the Salmonella enterica type III secretion-associated ATPase InvC defines discrete functional domains

An essential component of all type III secretion systems is a highly conserved ATPase that shares significant amino acid sequence similarity to the beta subunit of the F(0)F(1) ATPases and is thought to provide the energy for the secretion process. We have performed a genetic and functional analysis of InvC, the ATPase associated with the Salmonella enterica type III secretion system encoded within its pathogenicity island 1. Through a mutagenesis analysis, we have identified amino acid residues that are essential for specific activities of InvC, such as nucleotide hydrolysis and membrane binding. This has allowed us to define discrete domains of InvC that are specifically associated with different essential activities of this protein.

The rotary motor of bacterial flagella

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

Interactions of FliJ with the Salmonella type III flagellar export apparatus

FliJ, a 17-kDa protein, is a soluble component of the Salmonella type III flagellar protein export system that has antiaggregation properties and several other characteristics that suggest it may have a chaperone-like function. We have now examined this protein in detail. Ten-amino-acid scanning deletions covering the entire 147-amino-acid sequence were tested for complementation of a fliJ null strain; only the first and last deletions complemented. A few of the deletions, especially towards the C terminus, exerted a dominant negative effect on wild-type cells, indicating that they were actively interfering with function. Two truncated versions of FliJ, representing its N- and C-terminal halves, failed to complement and were not dominant. We tested for FliJ self-association by several techniques. Size-exclusion chromatography (Superdex 200) indicated an apparent molecular mass of around 50 kDa, which could reflect either multimerization or an elongated shape or both. Multiangle light scattering gave a peak value of 20 kDa, close to the molecular mass of the monomer. Analytical ultracentrifugation gave evidence for weak self-association as a trimer or tetramer. It was known from previous studies that FliJ interacts with the N-terminal region of FliH, a negative regulator of the ATPase FliI. Using both truncation and deletion versions of FliJ, we now show that it is its C-terminal region that is responsible for this interaction. We also show that FliJ interacts with the soluble cytoplasmic domain of the largest membrane component of the export apparatus, FlhA; although small deletions in FliJ did not interfere with the association, both truncated versions failed to associate, indicating that a substantial amount of the central region of the FliJ sequence participates in the association. We present a model summarizing these multiple interactions.

Type III protein translocase: HrcN is a peripheral ATPase that is activated by oligomerization

Type III protein secretion (TTS) is catalyzed by translocases that span both membranes of Gram-negative bacteria. A hydrophilic TTS component homologous to F1/V1-ATPases is ubiquitous and essential for secretion. We show that hrcN encodes the putative TTS ATPase of Pseudomonas syringae pathovar phaseolicola and that HrcN is a peripheral protein that assembles in clusters at the membrane. A decahistidinyl HrcN derivative was overexpressed in Escherichia coli and purified to homogeneity in a folded state. Hydrodynamic analysis, cross-linking, and electron microscopy revealed four distinct HrcN forms: I, 48 kDa (monomer); II, approximately 300 kDa (putative hexamer); III, 575 kDa (dodecamer); and IV, approximately 3.5 MDa. Form III is the predominant form of HrcN at the membrane, and its ATPase activity is dramatically stimulated (>700-fold) over the basal activity of Form I. We propose that TTS ATPases catalyze protein translocation as activated homo-oligomers at the plasma membrane.

The ATPase FliI can interact with the type III flagellar protein export apparatus in the absence of its regulator, FliH

Salmonella FliI is the ATPase that drives flagellar protein export. It normally exists as a complex together with the regulatory protein FliH. A fliH null mutant was slightly motile, with overproduction of FliI resulting in substantial improvement of its motility. Mutations in the cytoplasmic domains of FlhA and FlhB, which are integral membrane components of the type III flagellar export apparatus, also resulted in substantially improved motility, even at normal FliI levels. Thus, FliH, though undoubtedly important, is not essential.

Oligomerization and activation of the FliI ATPase central to bacterial flagellum assembly

FliI is the peripheral membrane ATPase pivotal to the type III protein export mechanism underlying the assembly of the bacterial flagellum. Gel filtration and multiangle light scattering showed that purified soluble native FliI protein was in a monomeric state but, in the presence of ATP, FliI showed a propensity to oligomerize. Electron microscopy revealed that FliI assembles to a ring structure, the yield of which was increased by the presence of a non-hydrolysable ATP analogue. Single particle analysis of the resulting electron micrograph images, to which no symmetry was applied, showed that the FliI ring structure has sixfold symmetry and an external diameter of approximately 10 nm. The oligomeric ring has a central cavity of 2.5-3.0 nm, which is comparable to the known diameter of the flagellar export channel into which export substrates feed. Enzymatic activity of the FliI ATPase showed positive co-operativity, establishing that oligomerization and enzyme activity are coupled. Escherichia coli phospholipids increased enzyme co-operativity, and in vitro cross-linking demonstrated that they promoted FliI multimerization. The data reveal central facets of the structure and action of the flagellar assembly ATPase and, by extension, the homologous ATPases of virulence-related type III export systems.

Molecular dissection of Salmonella FliH, a regulator of the ATPase FliI and the type III flagellar protein export pathway

FliH is a soluble component of the flagellar export apparatus that binds to the ATPase FliI, and negatively regulates its activity. The 235-amino-acid FliH dimerizes and interacts with FliI to form a hetero-trimeric (FliH)2FliI complex. In the present work, the importance of different regions of FliH was examined. A set of 24 scanning deletions of 10 amino acids was constructed over the entire FliH sequence, along with several combined deletions of 40 amino acids and truncations of both N- and C-termini. The mutant proteins were examined with respect to (i) complementation; (ii) dominance and multicopy effects; (iii) interaction with wild-type FliH; (iv) interaction with FliI; (v) inhibition of the ATPase activity of FliI; and (vi) interaction with the putative general chaperone FliJ. Analysis of the deletion mutants revealed a clear functional demarcation between the FliH N- and C-terminal regions. The 10-amino-acid deletions throughout most of the N-terminal half of the sequence complemented and were not dominant, whereas those throughout most of the C-terminal half did not complement and were dominant. FliI binding was disrupted by C-terminal deletions from residue 101 onwards, indicating that the C-terminal domain of FliH is essential for interaction with FliI. FliH dimerization was abolished by deletion of residues 101-140 in the centre of the sequence, as were complementation, dominance and interaction with FliI and FliJ. The importance of this region was confirmed by the fact that fragment FliHC2 (residues 99-235) interacted with FliH and FliI, whereas fragment FliHC1 (residues 119-235) did not. FliHC2 formed a relatively unstable complex with FliI and showed biphasic regulation of ATPase activity, suggesting that the FliH N-terminus stabilizes the (FliH)2FliI complex. Several of the N-terminal deletions tested permitted close to normal ATPase activity of FliI. Deletion of the last five residues of FliH caused a fivefold activation of ATPase activity, suggesting that this region of FliH governs a switch between repression and activation of FliI. Deletion of the first 10 residues of FliH abolished complementation, severely reduced its interaction with FliJ and uncoupled its role as a FliI repressor from its other export functions. Based on these data, a model is presented for the domain construction and function of FliH in complex with FliI and FliJ.

Intrinsic membrane targeting of the flagellar export ATPase FliI: interaction with acidic phospholipids and FliH

The specialised ATPase FliI is central to export of flagellar axial protein subunits during flagellum assembly. We establish the normal cellular location of FliI and its regulatory accessory protein FliH in motile Salmonella typhimurium, and ascertain the regions involved in FliH(2)/FliI heterotrimerisation. Both FliI and FliH localised to the cytoplasmic membrane in the presence and in the absence of proteins making up the flagellar export machinery and basal body. Membrane association was tight, and FliI and FliH interacted with Escherichia coli phospholipids in vitro, both separately and as the preformed FliH(2)/FliI complex, in the presence or in the absence of ATP. Yeast two-hybrid analysis and pull-down assays revealed that the C-terminal half of FliH (H105-235) directs FliH homodimerisation, and interacts with the N-terminal region of FliI (I1-155), which in turn has an intra-molecular interaction with the remainder of the protein (I156-456) containing the ATPase domain. The FliH105-235 interaction with FliI was sufficient to exert the FliH-mediated down-regulation of ATPase activity. The basal ATPase activity of isolated FliI was stimulated tenfold by bacterial (acidic) phospholipids, such that activity was 100-fold higher than when bound by FliH in the absence of phospholipids. The results indicate similarities between FliI and the well-characterised SecA ATPase that energises general protein secretion. They suggest that FliI and FliH are intrinsically targeted to the inner membrane before contacting the flagellar secretion machinery, with both FliH105-235 and membrane phospholipids interacting with FliI to couple ATP hydrolysis to flagellum assembly.

Proteolytic analysis of the FliH/FliI complex, the ATPase component of the type III flagellar export apparatus of Salmonella

The ATPase FliI of the Salmonella type III flagellar protein export apparatus is a 456 amino acid residue cytoplasmic protein consisting of two regions, an N-terminal flagellum-specific region and a C-terminal ATPase region. It forms a complex with a regulatory protein FliH in the cytoplasm. Multi-angle light-scattering studies indicate that FliH forms a homodimer, (FliH)2, and that FliH and FliI together form a heterotrimer, (FliH)2FliI. Mobility upon gel-filtration chromatography gives much higher apparent molecular masses for both species, whereas the mobility of FliI is normal. Sedimentation velocity measurements indicate that both (FliH)2 and the FliH/FliI complex are quite elongated. We have analyzed FliH, FliI and the FliH/FliI complex for proteolytic sensitivity. FliI was degraded by clostripain into two stable fragments, one of 48 kDa (FliI(CL48), missing the first seven amino acid residues) and the other of 46 kDa (FliI(CL46), missing the first 26 residues). Small amounts of two closely spaced 38 kDa fragments (FliI(CL38), missing the first 93 and 97 residues, respectively) were also detected. The FliH homodimer was insensitive to clostripain proteolysis and provided protection to FliI within the FliH/FliI complex. Neither FliI(CL48) nor FliI(CL46) could form a complex with FliH, demonstrating that the N terminus of FliI is essential for the interaction. ATP, AMP-PNP, and ADP bound forms of FliI within the FliH/FliI complex regained sensitivity to clostripain cleavage. Also, the sensitivity of the two FliI(CL38) cleavage sites was much greater in the ATP and AMP-PNP bound forms than in either the ADP bound form or nucleotide-free FliI. The ATPase domain itself was insensitive to clostripain cleavage. We suggest that the N-terminal flagellum-specific region of FliI is flexible and changes its conformation during the ATP hydrolysis cycle.

Bacterial flagella and type III secretion systems

Certain classes of pathogenic bacteria secrete virulence proteins in a Sec-independent manner, by a mechanism known as type III secretion. The main body of the export apparatus specific for virulence proteins is identified as a needle complex, which has a similar structural organization to flagella. The two structures share several proteins with highly homologous amino acid sequences. Even where the sequence identity is low among flagellar proteins from various species, the physico-chemical properties of each protein remain homologous. Therefore, by comparing the physico-chemical properties of unidentified proteins, it is possible to find homologs among type III secretion systems.

Extensive alanine scanning reveals protein-protein and protein-DNA interaction surfaces in the global regulator FlhD from Escherichia coli

FlhD and FlhC are the transcriptional activators of the flagellar regulon. The heterotetrameric complex formed by these two proteins activates the transcription of the class II flagellar genes. The flagellar regulon consists not only of flagellar genes, but also of the chemotactic genes and some receptor proteins. Recently, a connection between the flagellar regulon and some virulence genes has been found in some species. Furthermore, FlhD, but not FlhC, regulates another non-flagellar target. As a first attempt to understand the mechanism of the flagellar transcriptional activation by FlhD and FlhC, the structure of FlhD has been solved. In order to understand the mechanism of the action of FlhD when it regulates the flagellar genes, we conducted site-directed mutagenesis based on its three-dimensional structure. Six interaction surfaces in the FlhD dimer were mapped by alanine scanning mutagenesis. Two of them are surface clusters formed by residues His-2, Asp-28, Arg-35, Phe-34 and Asn-61 located at each side of the dimer core. The other four are located in the flexible arms of the dimer. The residues Ser-82, Arg-83, Val-84, His-91, Thr-92, Ile-94 and Leu-96 are located at this region. All these residues are involved in the FlhD/FlhC interaction with the exception of Ser-82, Arg-83 and Val-84. These three residues affect the DNA-binding ability of the complex. The three-dimensional topology of FlhD and the site-directed mutagenesis results support the hypothesis of FlhC as an allosteric effector that activates FlhD for the recognition of the DNA.

FliH, a soluble component of the type III flagellar export apparatus of Salmonella, forms a complex with FliI and inhibits its ATPase activity

Both FliH and the ATPase FliI are cytoplasmic components of the Salmonella type III flagellar export apparatus. Dominance and inhibition data have suggested that the N-terminus of FliI interacts with FliH and that this interaction is important for the ATPase function of the C-terminal domain of FliI. N-terminally histidine-tagged, wild-type FliI retarded untagged FliH in a Ni-NTA affinity chromatography assay, as did N-His-tagged versions of FliI carrying catalytic mutations. In contrast, N-His-tagged FliI carrying the double mutation R7C/L12P did not, further indicating that the N-terminus of FliI is responsible for interaction with FliH. Native agarose gel electrophoresis confirmed that FliH and FliI form a complex. Analytical gel filtration with in-line multiangle light scattering indicated that FliH alone forms a dimer, FliI alone remains as a monomer, and FliH and FliI together form a (FliH)2FliI complex. Ni-NTA affinity chromatography using N-His-tagged FliH and a large excess of untagged FliH confirmed that FliH forms a homodimer. The ATPase activity of the FliH-FliI complex was about 10-fold lower than that of FliI alone; the presence or absence of ATP did not affect the formation of the complex. We propose that FliH functions as a negative regulator to prevent FliI from hydrolysing ATP until the flagellar export apparatus is competent to link this hydrolysis to the translocation of export substrates across the plane of the cytoplasmic membrane into the lumen of the nascent flagellar structure.

Supramolecular structure of the Shigella type III secretion machinery: the needle part is changeable in length and essential for delivery of effectors

We investigated the supramolecular structure of the SHIGELLA: type III secretion machinery including its major components. Our results indicated that the machinery was composed of needle and basal parts with respective lengths of 45.4 +/- 3.3 and 31.6 +/- 0.3 nm, and contained MxiD, MxiG, MxiJ and MxiH. spa47, encoding a putative F(1)-type ATPase, was required for the secretion of effector proteins via the type III system and was involved in the formation of the needle. The spa47 mutant produced a defective, needle-less type III structure, which contained MxiD, MxiG and MxiJ but not MxiH. The mxiH mutant produced a defective type III structure lacking the needle and failed to secrete effector proteins. Upon overexpression of MxiH in the mxiH mutant, the bacteria produced type III structures with protruding dramatically long needles, and showed a remarkable increase in invasiveness. Our results suggest that MxiH is the major needle component of the type III machinery and is essential for delivery of the effector proteins, and that the level of MxiH affects the length of the needle.

The tripartite type III secreton of Shigella flexneri inserts IpaB and IpaC into host membranes

Bacterial type III secretion systems serve to translocate proteins into eukaryotic cells, requiring a secreton and a translocator for proteins to pass the bacterial and host membranes. We used the contact hemolytic activity of Shigella flexneri to investigate its putative translocator. Hemolysis was caused by formation of a 25-A pore within the red blood cell (RBC) membrane. Of the five proteins secreted by Shigella upon activation of its type III secretion system, only the hydrophobic IpaB and IpaC were tightly associated with RBC membranes isolated after hemolysis. Ipa protein secretion and hemolysis were kinetically coupled processes. However, Ipa protein secretion in the immediate vicinity of RBCs was not sufficient to cause hemolysis in the absence of centrifugation. Centrifugation reduced the distance between bacterial and RBC membranes beyond a critical threshold. Electron microscopy analysis indicated that secretons were constitutively assembled at 37 degrees C before any host contact. They were composed of three parts: (a) an external needle, (b) a neck domain, and (c) a large proximal bulb. Secreton morphology did not change upon activation of secretion. In mutants of some genes encoding the secretion machinery the organelle was absent, whereas ipaB and ipaC mutants displayed normal secretons.

The bacterial flagella motor

The bacterial flagellum is probably the most complex organelle found in bacteria. Although the ribosome may be made of slightly more subunits, the bacterial flagellum is a more organized and complex structure. The limited number of flagella must be targeted to the correct place on the cell membrane and a structure with cytoplasmic, cytoplasmic membrane, outer membrane and extracellular components must be assembled. The process of controlled transcription and assembly is still not fully understood. Once assembled, the motor complex in the cytoplasmic membrane rotates, driven by the transmembrane ion gradient, at speeds that can reach many 100 Hz, driving the bacterial cell at several body lengths a second. This coupling of an electrochemical gradient to mechanical rotational work is another fascinating feature of the bacterial motor. A significant percentage of a bacterium's energy may be used in synthesizing the complex structure of the flagellum and driving its rotation. Although patterns of swimming may be random in uniform environments, in the natural environment, where cells are confronted with gradients of metabolites and toxins, motility is used to move bacteria towards their optimum environment for growth and survival. A sensory system therefore controls the switching frequency of the rotating flagellum. This review deals primarily with the structure and operation of the bacterial flagellum. There has been a great deal of research in this area over the past 20 years and only some of this has been included. We apologize in advance if certain areas are covered rather thinly, but hope that interested readers will look at the excellent detailed reviews on those areas cited at those points.

Interaction of FliI, a component of the flagellar export apparatus, with flagellin and hook protein

FliI is a key component of the flagellar export apparatus in Salmonella typhimurium. It catalyzes the hydrolysis of ATP which is necessary for flagellar assembly. Affinity blotting experiments showed that purified flagellin and hook protein, two flagellar axial proteins, interact specifically with FliI. The interaction of either of the two proteins with FliI, increases the intrinsic ATPase activity. The presence of either flagellin or hook protein stimulates ATPase activity in a specific and reversible manner. A Vmax of 0.12 nmol Pi min-1 microgram-1 and a Km for MgATP of 0.35 mM was determined for the unstimulated FliI; the presence of flagellin increased the Vmax to 0.35 nmol Pi min-1 microgram-1 and the Km for MgATP to 1.1 mM. The stimulation induced by the axial proteins was fully reversible suggesting a direct link between the catalytic activity of FliI and the export process.

Molecular characterization of a flagellar export locus of Helicobacter pylori

Motility of Helicobacter species has been shown to be essential for successful colonization of the host. We have investigated the organization of a flagellar export locus in Helicobacter pylori. A 7-kb fragment of the H. pylori CCUG 17874 genome was cloned and sequenced, revealing an operon comprising an open reading frame of unknown function (ORF03), essential housekeeping genes (ileS and murB), flagellar export genes (fliI and fliQ), and a homolog to a gene implicated in virulence factor transport in other pathogens (virB11). A promoter for this operon, showing similarity to the Escherichia coli sigma70 consensus, was identified by primer extension. Cotranscription of the genes in the operon was demonstrated by reverse transcription-PCR, and transcription of virB11, fliI, fliQ, and murB was detected in human or mouse biopsies obtained from infected hosts. The genetic organization of this locus was conserved in a panel of H. pylori clinical isolates. Engineered fliI and fliQ mutant strains were completely aflagellate and nonmotile, whereas a virB11 mutant still produced flagella. The fliI and fliQ mutant strains produced reduced levels of flagellin and the hook protein FlgE. Production of OMP4, a member of the outer membrane protein family identified in H. pylori 26695, was reduced in both the virB11 mutant and the fliI mutant, suggesting related functions of the virulence factor export protein (VirB11) and the flagellar export component (FliI).

Salmonella InvG forms a ring-like multimer that requires the InvH lipoprotein for outer membrane localization

Salmonella species translocate virulence effector proteins from the bacterial cytoplasm into mammalian host cells by means of a type III secretion apparatus, encoded by the pathogenicity island-1 (SPI-1). Little is known about the assembly and structure of this secretion apparatus, but the InvG protein is essential and could be an outer membrane secretion channel for the effector proteins. We observed that in recombinant Escherichia coli, the yield of InvG was enhanced by co-expression of InvH, and showed that mutation of invH decreased the level of InvG in wild-type Salmonella typhimurium. In E. coli, InvG alone was able to form an SDS-resistant multimer, but InvG localization to the outer membrane was dependent upon InvH, a lipoprotein itself located in the outer membrane, and no other SPI-1 specific protein. InvG targeted to the outer membrane by InvH became accessible to extracellular protease. InvG and InvH did not, however, appear to form a stable complex. Electron microscopy of InvG membrane protein purified from E. coli revealed that it forms an oligomeric ring-like structure with inner and outer diameters, 7 nm and 15 nm respectively.

Bacterial flagellation and cell division

The peritrichous flagella of Salmonella are synthesized and function through many cell generations. There are two different aspects in the relationship between flagellar biogenesis and cell division. Filament growth is independent from the cell cycle and the length of filaments appear to be locally controlled at each flagellar base, whereas the number of filaments (or flagellar basal bodies) is dependent on cell cycle. We present a model to explain how the number of filaments is maintained through generations. We will also introduce a new direction for research that might directly connect flagellation and cell division; the global communication between flagellar genes and external factors of a complex regulatory network in a cell.

Identification of Mycobacterium tuberculosis signal sequences that direct the export of a leaderless beta-lactamase gene product in Escherichia coli

Proteins secreted by Mycobacterium tuberculosis may play a key role in virulence and may also constitute antigens that elicit the host immune response. However, the M. tuberculosis protein export machinery has not been characterized. A library of M. tuberculosis H37Rv genomic DNA fragments ligated into a signal sequence selection vector that contained a leaderless beta-lactamase gene and an upstream Tac promoter was constructed. Transformation of Escherichia coli with the M. tuberculosis DNA library and selection on plates containing 50-100 micrograms ampicillin ml-1 resulted in the identification of 15 Ampr clones out of a total of 14,000 transformants. Twelve of the beta-lactamase gene fusions conferred high levels of Ampr (up to 1 mg ampicillin ml-1); insert sizes ranged from 350 to 3000 bp. Of ten inserts that were completely sequenced, two were identified as fragments of the genes for M. tuberculosis antigens 85A and 85C, which are the major secreted proteins of this pathogen. Seven of the remaining inserts were > or = 97% identical to hypothetical ORFs in the M. tuberculosis genome, one of which encoded a protein with 35% identity to a low-affinity penicillin-binding protein (PBP) from Streptomyces clavuligerus. Four of the seven hypothetical ORFs encoded putative exported proteins with one or more membrane interaction elements, including lipoprotein attachment sites and type I and II transmembrane (TM) segments. All of the inserts encoded typical signal sequences, with the exception of a possible type II membrane protein. It is concluded that expression of beta-lactamase gene fusions in E. coli provides a useful system for the identification and analysis of M. tuberculosis signal-sequence-encoding genes.

Identification of the fliI and fliJ components of the Caulobacter flagellar type III protein secretion system

Caulobacter crescentus is motile by virtue of a polar flagellum assembled during the predivisional stage of the cell cycle. Three mutant strains in which flagellar assembly was blocked at an early stage were isolated. The mutations in these strains mapped to an operon of two genes, fliI and fliJ, both of which are necessary for motility. fliI encodes a 50-kDa polypeptide whose sequence is closely related to that of the Salmonella typhimurium FliI protein, an ATPase thought to energize the export of flagellar subunits across the cytoplasmic membrane through a type III protein secretion system. fliJ encodes a 16-kDa hydrophilic protein of unknown function. Epistasis experiments demonstrated that the fliIJ operon is located in class II of the C. crescentus flagellar regulatory hierarchy, suggesting that the gene products act at an early stage in flagellar assembly. The expression of fliIJ is induced midway through the cell cycle, coincident with other class II operons, but the FliI protein remains present throughout the cell cycle. Subcellular fractionation showed that FliI is present both in the cytoplasm and in association with the membrane. Mutational analysis of FliI showed that two highly conserved amino acid residues in a bipartite ATP binding motif are necessary for flagellar assembly.