Unveiling molecular scaffolds of the type IV secretion system

Inhibiting bacterial secretion systems in the fight against antibiotic resistance

Antimicrobial resistance is a mounting global health crisis that threatens a resurgence of life-threatening bacterial infections. Despite intensive drug discovery efforts, the rate of antimicrobial resistance outpaces the discovery of new antibiotic agents. One of the major mechanisms driving the rapid propagation of antibiotic resistance is bacterial conjugation mediated by the versatile type IV secretion system (T4SS). The search for therapeutic compounds that prevent the spread of antibiotic resistance via T4SS-dependent mechanisms has identified several promising molecular scaffolds that disrupt resistance determinant dissemination. In this brief review, we highlight the progress and potential of conjugation inhibitors and anti-virulence compounds that target diverse T4SS machineries. These studies provide a solid foundation for the future development of potent, dual-purpose molecular scaffolds that can be used as biochemical tools to probe type IV secretion mechanisms and target bacterial conjugation in clinical settings to prevent the dissemination of antibiotic resistance throughout microbial populations.

Anti-inflammatory effects of Kaempferol on Helicobacter pylori-induced inflammation

Inflammation induced by Helicobacter pylori infection related to gastric carcinogenesis. In this study, we have investigated the anti-inflammatory effect and its mechanism of kaempferol in the inflammatory response caused by H. pylori infection in vitro. We found that kaempferol reduced the expression of pro-inflammatory cytokines (TNF-α, IL-1β, and IL-8) and production of IL-8 in AGS cells. In addition, kaempferol suppressed translocation of cytotoxin-associated gene A (CagA) and vacuolating cytotoxin A (VacA) of H. pylori to AGS cells. It was due to decreased transcription of type IV secretion system (T4SS) components involved in CagA injection and secretion system subunit protein A (SecA) of type V secretion system (T5SS) involved in VacA secretion by kaempferol. In conclusion, kaempferol shows the anti-inflammatory effect by suppressing the translocation of CagA and VacA proteins and leading to the down-regulation of pro-inflammatory cytokines. Abbreviations: CagA: cytotoxin-associated gene A; VacA: vacuolating cytotoxin A; T4SS: type IV secretion systems; SecA: secretion system subunit protein A; T5SS: type V secretion system.

Cellular Stress Responses and Monitored Cellular Activities

To survive, organisms require mechanisms that enable them to sense changes in the outside environment, introduce necessary responses, and resist unfavorable distortion. Consequently, through evolutionary adaptation, cells have become equipped with the apparatus required to monitor their fundamental intracellular processes and the mechanisms needed to try to offset malfunction without receiving any direct signals from the outside environment. It has been shown recently that eukaryotic cells are equipped with a special mechanism that monitors their fundamental cellular functions and that some pathogenic proteobacteria can override this monitoring mechanism to cause harm. The monitored cellular activities involved in the stressed intracellular response have been researched extensively in Caenorhabditis elegans, where discovery of an association between key mitochondrial activities and innate immune responses was named "cellular associated detoxification and defenses (cSADD)." This cellular surveillance pathway (cSADD) oversees core cellular activities such as mitochondrial respiration and protein transport into mitochondria, detects xenobiotics and invading pathogens, and activates the endocrine pathways controlling behavior, detoxification, and immunity. The cSADD pathway is probably associated with cellular responses to stress in human inflammatory diseases. In the critical care field, the pathogenesis of lethal inflammatory syndromes (e.g., respiratory distress syndromes and sepsis) involves the disturbance of mitochondrial respiration leading to cell death. Up-to-date knowledge about monitored cellular activities and cSADD, especially focusing on mitochondrial involvement, can probably help fill a knowledge gap regarding the pathogenesis of lethal inflammatory syndromes in the critical care field.

Genome Organization of Sphingobium indicum B90A: An Archetypal Hexachlorocyclohexane (HCH) Degrading Genotype

Among sphingomonads, Sphingobium indicum B90A is widely investigated for its ability to degrade a manmade pesticide, γ-hexachlorocyclohexane (γ-HCH) and its isomers (α-, β-, δ-, and ε-HCH). In this study, complete genome of strain B90A was constructed using Single Molecule Real Time Sequencing (SMRT) and Illumina platform. The complete genome revealed that strain B90A harbors four replicons: one chromosome (3,654,322 bp) and three plasmids designated as pSRL1 (139,218 bp), pSRL2 (108,430 bp) and pSRL3 (43,761 bp). The study determined the precise location of lin genes (genes associated with the degradation of HCH isomers), for example, linA2, linB, linDER, linF, linGHIJ, and linKLMN on the chromosome; linA1, linC, and linF on pSRL1 and linDEbR on pSRL3. Strain B90A contained 26 copies of IS6100 element and most of them (15 copies) was found to be associated with lin genes. Duplication of several lin genes including linA, linDER, linGHIJ, and linF along with two variants of linE, that is, linEa (hydroquinone 1,2-dioxygenase) and linEb (chlorohydroquinone/hydroquinone 1,2-dioxygenase) were identified. This suggests that strain B90A not only possess efficient machinery for upper and lower HCH degradation pathways but it can also act on both hydroquinone and chlorohydroquinone metabolites produced during γ-HCH degradation. Synteny analysis revealed the duplication and transposition of linA gene (HCH dehydrochlorinase) between the chromosome and pSRL1, possibly through homologous recombination between adjacent IS6100 elements. Further, in silico analysis and laboratory experiments revealed that incomplete tyrosine metabolism was responsible for the production of extracellular brown pigment which distinguished strain B90A from other HCH degrading sphingomonads. The precise localization of lin genes, and transposable elements (IS6100) on different replicons now opens up several experimental avenues to elucidate the functions and regulatory mechanism of lin genes acquisition and transfer that were not completely known among the bacterial population inhabiting the HCH contaminated environment.

A Comparison of 14 Erythrobacter Genomes Provides Insights into the Genomic Divergence and Scattered Distribution of Phototrophs

Aerobic anoxygenic phototrophic bacteria (AAPB) are bacteriochlorophyll a (Bchl a)-containing microbial functional population. Erythrobacter is the first genus that was identified to contain AAPB species. Here, we compared 14 Erythrobacter genomes: seven phototrophic strains and seven non- phototrophic strains. Interestingly, AAPB strains are scattered in this genus based on their phylogenetic relationships. All 14 strains could be clustered into three groups based on phylo-genomic analysis, average genomic nucleotide identity and the phylogeny of signature genes (16S rRNA and virB4 genes). The AAPB strains were distributed in three groups, and gain and loss of phototrophic genes co-occurred in the evolutionary history of the genus Erythrobacter. The organization and structure of photosynthesis gene clusters (PGCs) in seven AAPB genomes displayed high synteny of major regions except for few insertions. The 14 Erythrobacter genomes had a large range of genome sizes, from 2.72 to 3.60 M, and the sizes of the core and pan- genomes were 1231 and 8170 orthologous clusters, respectively. Integrative and conjugative elements (ICEs) were frequently identified in genomes we studied, which might play significant roles in shaping or contributing to the pan-genome of Erythrobacter. Our findings suggest the ongoing evolutionary divergence of Erythrobacter genomes and the scattered distribution characteristic of PGC.

Peptidomimetic Small Molecules Disrupt Type IV Secretion System Activity in Diverse Bacterial Pathogens

Bacteria utilize complex type IV secretion systems (T4SSs) to translocate diverse effector proteins or DNA into target cells. Despite the importance of T4SSs in bacterial pathogenesis, the mechanism by which these translocation machineries deliver cargo across the bacterial envelope remains poorly understood, and very few studies have investigated the use of synthetic molecules to disrupt T4SS-mediated transport. Here, we describe two synthetic small molecules (C10 and KSK85) that disrupt T4SS-dependent processes in multiple bacterial pathogens. Helicobacter pylori exploits a pilus appendage associated with the cag T4SS to inject an oncogenic effector protein (CagA) and peptidoglycan into gastric epithelial cells. In H. pylori, KSK85 impedes biogenesis of the pilus appendage associated with the cag T4SS, while C10 disrupts cag T4SS activity without perturbing pilus assembly. In addition to the effects in H. pylori, we demonstrate that these compounds disrupt interbacterial DNA transfer by conjugative T4SSs in Escherichia coli and impede vir T4SS-mediated DNA delivery by Agrobacterium tumefaciens in a plant model of infection. Of note, C10 effectively disarmed dissemination of a derepressed IncF plasmid into a recipient bacterial population, thus demonstrating the potential of these compounds in mitigating the spread of antibiotic resistance determinants driven by conjugation. To our knowledge, this study is the first report of synthetic small molecules that impair delivery of both effector protein and DNA cargos by diverse T4SSs.

Many human and plant pathogens utilize complex nanomachines called type IV secretion systems (T4SSs) to transport proteins and DNA to target cells. In addition to delivery of harmful effector proteins into target cells, T4SSs can disseminate genetic determinants that confer antibiotic resistance among bacterial populations. In this study, we sought to identify compounds that disrupt T4SS-mediated processes. Using the human gastric pathogen H. pylori as a model system, we identified and characterized two small molecules that prevent transfer of an oncogenic effector protein to host cells. We discovered that these small molecules also prevented the spread of antibiotic resistance plasmids in E. coli populations and diminished the transfer of tumor-inducing DNA from the plant pathogen A. tumefaciens to target cells. Thus, these compounds are versatile molecular tools that can be used to study and disarm these important bacterial machines.

Genes required for assembly of pili associated with the Helicobacter pylori cag type IV secretion system

Helicobacter pylori causes numerous alterations in gastric epithelial cells through processes that are dependent on activity of the cag type IV secretion system (T4SS). Filamentous structures termed "pili" have been visualized at the interface between H. pylori and gastric epithelial cells, and previous studies suggested that pilus formation is dependent on the presence of the cag pathogenicity island (PAI). Thus far, there has been relatively little effort to identify specific genes that are required for pilus formation, and the role of pili in T4SS function is unclear. In this study, we selected 7 genes in the cag PAI that are known to be required for T4SS function and investigated whether these genes were required for pilus formation. cagT, cagX, cagV, cagM, and cag3 mutants were defective in both T4SS function and pilus formation; complemented mutants regained T4SS function and the capacity for pilus formation. cagY and cagC mutants were defective in T4SS function but retained the capacity for pilus formation. These results define a set of cag PAI genes that are required for both pilus biogenesis and T4SS function and reveal that these processes can be uncoupled in specific mutant strains.

Evidence of the presence of a functional Dot/Icm type IV-B secretion system in the fish bacterial pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is a fish bacterial pathogen that has severely challenged the sustainability of the Chilean salmon industry since its appearance in 1989. As this Gram-negative bacterium has been poorly characterized, relevant aspects of its life cycle, virulence and pathogenesis must be identified in order to properly design prophylactic procedures. This report provides evidence of the functional presence in P. salmonis of four genes homologous to those described for Dot/Icm Type IV Secretion Systems. The Dot/Icm System, the major virulence mechanism of phylogenetically related pathogens Legionella pneumophila and Coxiella burnetii, is responsible for their intracellular survival and multiplication, conditions that may also apply to P. salmonis. Our results demonstrate that the four P. salmonis dot/icm homologues (dotB, dotA, icmK and icmE) are expressed both during in vitro tissue culture cells infection and growing in cell-free media, suggestive of their putative constitutive expression. Additionally, as it happens in other referential bacterial systems, temporal acidification of cell-free media results in over expression of all four P. salmonis genes, a well-known strategy by which SSTIV-containing bacteria inhibit phagosome-lysosome fusion to survive. These findings are very important to understand the virulence mechanisms of P. salmonis in order to design new prophylactic alternatives to control the disease.

Expression of CagL from Helicobacter pylori and Preliminary Study of its Biological Function

Helicobacter pylori (H. pylori) is a highly successful human-specific gastric pathogen, infecting over half the world's population. Virulent H. pylori isolates harbour the cytotoxin-associated genes pathogenicity island (cag-PAI), the majority of which have no known function. In this study, we used cell infection assay and reverse transcriptase PCR, identified that CagL recombinant protein, one of the cag-PAI proteins, induced GES-1 cells to express cytokine IL-8. Then we performed western blot and translocation assay. Our result showed CagL polyclonal antibody counteracted translocation of CagA. This will provide a foundation for the further studies on its biological function.

Gain and loss of phototrophic genes revealed by comparison of two Citromicrobium bacterial genomes

Proteobacteria are thought to have diverged from a phototrophic ancestor, according to the scattered distribution of phototrophy throughout the proteobacterial clade, and so the occurrence of numerous closely related phototrophic and chemotrophic microorganisms may be the result of the loss of genes for phototrophy. A widespread form of bacterial phototrophy is based on the photochemical reaction center, encoded by puf and puh operons that typically are in a ‘photosynthesis gene cluster’ (abbreviated as the PGC) with pigment biosynthesis genes. Comparison of two closely related Citromicrobial genomes (98.1% sequence identity of complete 16S rRNA genes), Citromicrobium sp. JL354, which contains two copies of reaction center genes, and Citromicrobium strain JLT1363, which is chemotrophic, revealed evidence for the loss of phototrophic genes. However, evidence of horizontal gene transfer was found in these two bacterial genomes. An incomplete PGC (pufLMC-puhCBA) in strain JL354 was located within an integrating conjugative element, which indicates a potential mechanism for the horizontal transfer of genes for phototrophy.

Regulation of the type IV secretion ATPase TrwD by magnesium: implications for catalytic mechanism of the secretion ATPase superfamily

TrwD, the VirB11 homologue in conjugative plasmid R388, is a member of the large secretion ATPase superfamily, which includes ATPases from bacterial type II and type IV secretion systems, type IV pilus, and archaeal flagellae assembly. Based on structural studies of the VirB11 homologues in Helicobacter pylori and Brucella suis and the archaeal type II secretion ATPase GspE, a unified mechanism for the secretion ATPase superfamily has been proposed. Here, we have found that the ATP turnover of TrwD is down-regulated by physiological concentrations of magnesium. This regulation is exerted by increasing the affinity for ADP, hence delaying product release. Circular dichroism and limited proteolysis analysis indicate that magnesium induces conformational changes in the protein that promote a more rigid, but less active, form of the enzyme. The results shown here provide new insights into the catalytic mechanism of the secretion ATPase superfamily.

Helicobacter pylori exploits a unique repertoire of type IV secretion system components for pilus assembly at the bacteria-host cell interface

Colonization of the human stomach by Helicobacter pylori is an important risk factor for development of gastric cancer. The H. pylori cag pathogenicity island (cag PAI) encodes components of a type IV secretion system (T4SS) that translocates the bacterial oncoprotein CagA into gastric epithelial cells, and CagL is a specialized component of the cag T4SS that binds the host receptor α5β1 integrin. Here, we utilized a mass spectrometry-based approach to reveal co-purification of CagL, CagI (another integrin-binding protein), and CagH (a protein with weak sequence similarity to CagL). These three proteins are encoded by contiguous genes in the cag PAI, and are detectable on the bacterial surface. All three proteins are required for CagA translocation into host cells and H. pylori-induced IL-8 secretion by gastric epithelial cells; however, these proteins are not homologous to components of T4SSs in other bacterial species. Scanning electron microscopy analysis reveals that these proteins are involved in the formation of pili at the interface between H. pylori and gastric epithelial cells. ΔcagI and ΔcagL mutant strains fail to form pili, whereas a ΔcagH mutant strain exhibits a hyperpiliated phenotype and produces pili that are elongated and thickened compared to those of the wild-type strain. This suggests that pilus dimensions are regulated by CagH. A conserved C-terminal hexapeptide motif is present in CagH, CagI, and CagL. Deletion of these motifs results in abrogation of CagA translocation and IL-8 induction, and the C-terminal motifs of CagI and CagL are required for formation of pili. In summary, these results indicate that CagH, CagI, and CagL are components of a T4SS subassembly involved in pilus biogenesis, and highlight the important role played by unique constituents of the H. pylori cag T4SS.

Helicobacter pylori persistently colonizes the stomach in approximately half of the human population. People who are infected with H. pylori strains harboring the cag pathogenicity island (PAI) have an increased risk of developing gastric cancer. The cag PAI encodes a type IV secretion system (T4SS) that is utilized by the bacteria to inject the bacterial oncoprotein CagA into gastric epithelial cells. Related T4SSs found in several other bacteria have been studied in detail, but thus far there has been very little study of the H. pylori cag T4SS. Here, we utilized a mass spectrometry-based approach to reveal co-purification of three constituents of the H. pylori T4SS (CagH, CagI, and CagL) that lack homology to components of T4SSs in other bacterial species. These proteins are essential for CagA translocation into host cells, and scanning electron microscope studies reveal that the proteins are involved in the formation of pili at the bacterial-host cell interface. A conserved C-terminal motif present in CagH, CagI, and CagL is essential for functionality of the T4SS. This study highlights the important role played by unique constituents of the H. pylori cag T4SS, and illustrates the marked variation that exists among bacterial T4SSs.

pO157_Sal, a novel conjugative plasmid detected in outbreak isolates of Escherichia coli O157:H7

In addition to the large virulence plasmid pO157, a novel 38-kb conjugative plasmid, pO157_Sal, was identified and sequenced from an Escherichia coli O157:H7 outbreak-associated Chinese isolate that shares high similarity with a plasmid in Salmonella enterica serovar Agona. The plasmid was found in 15 of 326 isolates, 12 of which were of the same pulsed-field gel electrophoresis type.

Two distinct regions in the model protein Peb1 are critical for its heterologous transport out of Escherichia coli

Background

Escherichia coli is frequently the first-choice host organism in expression of heterologous recombinant proteins in basic research as well as in production of commercial, therapeutic polypeptides. Especially the secretion of proteins into the culture medium of E. coli is advantageous compared to intracellular production due to the ease in recovery of the recombinant protein. Since E. coli naturally is a poor secretor of proteins, a few strategies for optimization of extracellular secretion have been described. We have previously reported efficient secretion of the diagnostically interesting model protein Peb1 of Campylobacter jejuni into the growth medium of Escherichia coli strain MKS12 (ΔfliCfliD). To generate a more detailed understanding of the molecular mechanisms behind this interesting heterologous secretion system with biotechnological implications, we here analyzed further the transport of Peb1 in the E. coli host.

Results

When mature Peb1 was expressed without its SecA-YEG -dependent signal sequence and without the putative signal peptidase II recognition sequence in E. coli MKS111ΔHBB lacking the flagellar secretion complex, the protein was found in the periplasm and growth medium which indicated a flagellum-independent translocation. We assessed the Peb1 secretion proficiency by an exhaustive search for transport-affecting regions using a transposition-based scanning mutagenesis strategy. Strikingly, insertion mutagenesis of only two segments, called TAR1 (residues 42 and 43) and TAR2 (residues 173 to 180), prevented Peb1 secretion individually. We confirmed the importance of TAR regions by subsequent site-specific mutagenesis and verified that the secretion deficiency of Peb1 mutants was not due to insolubility or aggregation of the proteins in the cytoplasm. We found by cell fractionation that the mutant proteins were present in the periplasm as well as in the cytoplasm of MKS12. Hence, mutagenesis of TAR regions did not affect export of Peb1 across the cytoplasmic membrane, whereas its export over the outer membrane was markedly impaired.

Conclusions

We propose that the localization of the model protein Peb1 in the growth medium of E. coli is due to active secretion by a still unknown pathway of E. coli. The secretion apparently is a two-step process involving a periplasmic step and the TAR regions.

Molecular mechanisms underlying roseobacter-phytoplankton symbioses

Members of the Roseobacter clade of alpha-proteobacteria are among the most abundant and ecologically relevant marine bacteria. Bacterial isolates and gene sequences derived from this taxonomic lineage have been retrieved from marine environments ranging from sea ice to open ocean mixed layer to tropical coral reefs, and in ecological niches ranging from free-living plankton to sponge symbiont to biofilm pioneer. Although roseobacters are cosmopolitan in the marine environment, their numbers and activity significantly rise with increases in the population density of phytoplankton [1,2], suggesting that these bacteria are highly adapted to engage in these symbioses. This review examines the molecules and phenotypes of roseobacters that are important in establishing and maintaining the symbioses between roseobacters and phytoplankton.

Two-step and one-step secretion mechanisms in Gram-negative bacteria: contrasting the type IV secretion system and the chaperone-usher pathway of pilus biogenesis

Gram-negative bacteria have evolved diverse secretion systems/machineries to translocate substrates across the cell envelope. These various machineries fulfil a wide variety of functions but are also essential for pathogenic bacteria to infect human or plant cells. Secretion systems, of which there are seven, utilize one of two secretion mechanisms: (i) the one-step mechanism, whereby substrates are translocated directly from the bacterial cytoplasm to the extracellular medium or into the eukaryotic target cell; (ii) the two-step mechanism, whereby substrates are first translocated across the bacterial inner membrane; once in the periplasm, substrates are targeted to one of the secretion systems that mediate transport across the outer membrane and released outside the bacterial cell. The present review provides an example for each of these two classes of secretion systems and contrasts the various solutions evolved to secrete substrates.

Complete sequence of three plasmids from Bacillus thuringiensis INTA-FR7-4 environmental isolate and comparison with related plasmids from the Bacillus cereus group

Bacillus thuringiensis is an insect pathogen used worldwide as a bioinsecticide. It belongs to the Bacillus cereus sensu lato group as well as Bacillus anthracis and B. cereus. Plasmids from this group of organisms have been implicated in pathogenicity as they carry the genes responsible for different types of diseases that affect mammals and insects. Some plasmids, like pAW63 and pBT9727, encode a functional conjugation machinery allowing them to be transferred to a recipient cell. They also share extensive homology with the non-functional conjugation apparatus of pXO2 from B. anthracis. In this study we report the complete sequence of three plasmids from an environmental B. thuringiensis isolate from Argentina, obtained by a shotgun sequencing method. We obtained the complete nucleotide sequence of plasmids pFR12 (12,095bp), pFR12.5 (12,459bp) and pFR55 (55,712bp) from B. thuringiensis INTA-FR7-4. pFR12 and pFR12.5 were classified as cryptic as they do not code for any obvious functions besides replication and mobilization. Both small plasmids were classified as RCR plasmids due to similarities with the replicases they encode. Plasmid pFR55 showed a structural organization similar to that observed for plasmids pAW63, pBT9727 and pXO2. pFR55 also shares a tra region with these plasmids, containing genes related to T4SS and conjugation. A comparison between pFR55 and conjugative plasmids led to the postulation that pFR55 is a conjugative plasmid. Genes related to replication functions in pFR55 are different to those described for plasmids with known complete sequences. pFR55 is the first completely sequenced plasmid with a replication machinery related to that of ori44. The analysis of the complete sequence of plasmids from an environmental isolate of B. thuringiensis permitted the identification of a near complete conjugation apparatus in pFR55, resembling those of plasmids pAW63, pBT9727 and pXO2. The availability of this sequence is a step forward in the study of the molecular basis of the conjugative process in Gram positive bacteria, particularly due to the similarity with known conjugation systems. It is also a contribution to the expansion of the non-pathogenic B. cereus plasmid gene pool.

Distribution, diversity, and potential mobility of extrachromosomal elements related to the Bacillus anthracis pXO1 and pXO2 virulence plasmids

The presence of a pXO1- and/or pXO2-like plasmid(s) in clinical isolates of Bacillus cereus sensu stricto and in strains of the biopesticide Bacillus thuringiensis has been reported recently, and the pXO2-like plasmid pBT9727 and another pXO2-like plasmid, pAW63, were found to be conjugative. In this study, a total of 1,000 B. cereus group isolates were analyzed for the presence of pXO1- and pXO2-like replicons and for the presence of pXO2-related conjugative modules. pXO1- and pXO2-like replicons were present in ca. 6.6% and 7.7% of random environmental samples, respectively, and ca. 1.54% of the strains were positive for pXO2-like transfer module genes. Only the strains harboring a pXO2-like replicon also contained the corresponding transfer genes. For the strains which contained a pXO1- and/or pXO2-like replicon(s), a large plasmid(s) whose size was similar to that of pXO1-like and/or pXO2-like plasmids was also observed, but none of these isolates were found to carry the Bacillus anthracis toxin or capsule virulence genes. Furthermore, 17 of 22 pXO2-like plasmids containing the transfer modules were able to self-transfer and to mobilize small plasmids. No pXO1- or pXO2-like plasmid lacking the cognate transfer modules has been found to have transfer potential. In the strains possessing the putative pXO2-like conjugative apparatus, variations in the presence of the group II introns B.th.I.1 and B.th.I.2 were observed, suggesting that there is important flexibility in the conjugation modules and their regulation. There was no consistent correlation between a pXO2-like repA dendrogram and the presence of the tra region or between a virB4 dendrogram and transfer ability. Discrepancies between pXO2-like repA and virB4 dendrograms were also observed, indicating that the evolution of pXO2 is an active process.

An anomalous type IV secretion system in Rickettsia is evolutionarily conserved

BACKGROUND

Bacterial type IV secretion systems (T4SSs) comprise a diverse transporter family functioning in conjugation, competence, and effector molecule (DNA and/or protein) translocation. Thirteen genome sequences from Rickettsia, obligate intracellular symbionts/pathogens of a wide range of eukaryotes, have revealed a reduced T4SS relative to the Agrobacterium tumefaciens archetype (vir). However, the Rickettsia T4SS has not been functionally characterized for its role in symbiosis/virulence, and none of its substrates are known.

RESULTS

Superimposition of T4SS structural/functional information over previously identified Rickettsia components implicate a functional Rickettsia T4SS. virB4, virB8 and virB9 are duplicated, yet only one copy of each has the conserved features of similar genes in other T4SSs. An extraordinarily duplicated VirB6 gene encodes five hydrophobic proteins conserved only in a short region known to be involved in DNA transfer in A. tumefaciens. virB1, virB2 and virB7 are newly identified, revealing a Rickettsia T4SS lacking only virB5 relative to the vir archetype. Phylogeny estimation suggests vertical inheritance of all components, despite gene rearrangements into an archipelago of five islets. Similarities of Rickettsia VirB7/VirB9 to ComB7/ComB9 proteins of epsilon-proteobacteria, as well as phylogenetic affinities to the Legionella lvh T4SS, imply the Rickettsiales ancestor acquired a vir-like locus from distantly related bacteria, perhaps while residing in a protozoan host. Modern modifications of these systems likely reflect diversification with various eukaryotic host cells.

CONCLUSION

We present the rvh (Rickettsiales vir homolog) T4SS, an evolutionary conserved transporter with an unknown role in rickettsial biology. This work lays the foundation for future laboratory characterization of this system, and also identifies the Legionella lvh T4SS as a suitable genetic model.

Remodelling of VipA/VipB tubules by ClpV-mediated threading is crucial for type VI protein secretion

The recently identified type VI secretion systems (T6SS) have a crucial function in the virulence of various proteobacteria, including the human pathogen Vibrio cholerae. T6SS are encoded by a conserved gene cluster comprising approximately 15 open reading frames, mediating the appearance of Hcp and VgrG proteins in cell culture supernatants. Here, we analysed the function of the V. cholerae T6SS member ClpV, a specialized AAA+ protein. ClpV is crucial for a functional T6SS and interacts through its N-terminal domain with the VipA/VipB complex that is composed of two conserved and essential members of T6SS. Transferring ClpV substrate specificity to a distinct AAA+ protein involved in proteolysis caused degradation of VipA but not Hcp or VgrG2, suggesting that VipA rather than Hcp/VgrG2 functions as a primary ClpV substrate. Strikingly, VipA/VipB form tubular, cogwheel-like structures that are converted by a threading activity of ClpV into small complexes. ClpV-mediated remodelling of VipA/VipB tubules represents a crucial step in T6S, illuminating an unexpected role of an ATPase component in protein secretion.

Four VirB6 paralogs and VirB9 are expressed and interact in Ehrlichia chaffeensis-containing vacuoles

The type IV secretion system is an important virulence factor in several host cell-associated pathogens, as it delivers various bacterial macromolecules to target eukaryotic cells. Genes homologous to several virB genes and virD4 of Agrobacterium tumefaciens are found in an intravacuolar pathogen Ehrlichia chaffeensis, the tick-borne causative agent of human monocytic ehrlichiosis. In particular, despite its small genome size, E. chaffeensis has four tandem virB6 paralogs (virB6-1, -2, -3, and -4) that are 3- to 10-fold larger than A. tumefaciens virB6. The present study for the first time illustrates the relevance of the larger quadruple VirB6 paralogs by demonstrating the protein expression and interaction in E. chaffeensis. All four virB6 paralogs were cotranscribed in THP-1 human leukemia and ISE6 tick cell cultures. The four VirB6 proteins and VirB9 were expressed by E. chaffeensis in THP-1 cells, and amounts of these five proteins were similar in isolated E. chaffeensis-containing vacuoles and vacuole-free E. chaffeensis. In addition, an 80-kDa fragment of VirB6-2 was detected, which was strikingly more prevalent in E. chaffeensis-containing vacuoles than in vacuole-free E. chaffeensis. Coimmunoprecipitation analysis revealed VirB9 interaction with VirB6-1 and VirB6-2; VirB6-4 interaction with VirB6-1, VirB6-2, and VirB6-3; and VirB6-2 80-kDa fragment interaction with VirB6-3 and VirB6-4. The interaction of VirB9 and VirB6-2 was confirmed by far-Western blotting. The results suggest that E. chaffeensis VirB9, the quadruple VirB6 proteins, and the VirB6-2 80-kDa fragment form a unique molecular subassembly to cooperate in type IV secretion.

Prokaryote-eukaryote interactions identified by using Caenorhabditis elegans

Prokaryote-eukaryote interactions are ubiquitous and have important medical and environmental significance. Despite this, a paucity of data exists on the mechanisms and pathogenic consequences of bacterial-fungal encounters within a living host. We used the nematode Caenorhabditis elegans as a substitute host to study the interactions between two ecologically related and clinically troublesome pathogens, the prokaryote, Acinetobacter baumannii, and the eukaryote, Candida albicans. After co-infecting C. elegans with these organisms, we observed that A. baumannii inhibits filamentation, a key virulence determinant of C. albicans. This antagonistic, cross-kingdom interaction led to attenuated virulence of C. albicans, as determined by improved nematode survival when infected with both pathogens. In vitro coinfection assays in planktonic and biofilm environments supported the inhibitory effects of A. baumannii toward C. albicans, further showing a predilection of A. baumannii for C. albicans filaments. Interestingly, we demonstrate a likely evolutionary defense by C. albicans against A. baumannii, whereby C. albicans inhibits A. baumannii growth once a quorum develops. This counteroffensive is at least partly mediated by the C. albicans quorum-sensing molecule farnesol. We used the C. elegans-A. baumannii-C. albicans coinfection model to screen an A. baumannii mutant library, leading to the identification of several mutants attenuated in their inhibitory activity toward C. albicans. These findings present an extension to the current paradigm of studying monomicrobial pathogenesis in C. elegans and by use of genetic manipulation, provides a whole-animal model system to investigate the complex dynamics of a polymicrobial infection.

VirB2 and VirB5 proteins: specialized adhesins in bacterial type-IV secretion systems?

Many type-IV secretion systems (T4SSs) of plant and human pathogens assemble a pilus used to inject virulence molecules (effectors) into host target cells. The T4SS of Agrobacterium tumefaciens consists of VirB1-VirB11 and VirD4 proteins. Whether targeting of T4SSs to the host requires a T4SS-adhesin that specifically engages host receptors for delivery of effectors has, until recently, remained unclear. Recent data of Agrobacterium and Helicobacter indicate that two classes of T4SS components, VirB2 and VirB5, might function as adhesins that mediate host-cell targeting through binding to specific host receptors. Here, we discuss this important issue and recent progress in the field.

The bacterial type VI secretion machine: yet another player for protein transport across membranes

Several secretion systems have evolved that are widespread among Gram-negative bacteria. Recently, a new secretion system was recognized, which is named the type VI secretion system (T6SS). The T6SS components are encoded within clusters of genes initially identified as IAHP for IcmF-associated homologous proteins, since they were all found to contain a gene encoding an IcmF-like component. IcmF was previously reported as a component of the type IV secretion system (T4SS). However, with the exception of DotU, other T4SS components are not encoded within T6SS loci. Thus, the T6SS is probably a novel kind of complex multi-component secretion machine, which is often involved in interaction with eukaryotic hosts, be it a pathogenic or a symbiotic relationship. The expression of T6SS genes has been reported to be mostly induced in vivo. Interestingly, expression and assembly of T6SSs are tightly controlled at both the transcriptional and the post-translational level. This may allow a timely control of T6SS assembly and function. Two types of proteins, generically named Hcp and VgrG, are secreted via these systems, but it is not entirely clear whether they are truly secreted effector proteins or are actually components of the T6SS. The precise role and mode of action of the T6SS is still unknown. This review describes current knowledge about the T6SS and summarizes its hallmarks and its differences from other secretion systems.

The specialized secretory apparatus ESX-1 is essential for DNA transfer in Mycobacterium smegmatis

Conjugal DNA transfer in Mycobacterium smegmatis occurs by a mechanism distinct from plasmid-mediated DNA transfer. Previously, we had shown that the secretory apparatus, ESX-1, negatively regulated DNA transfer from the donor strain; ESX-1 donor mutants are hyper-conjugative. Here, we describe a genome-wide transposon mutagenesis screen to isolate recipient mutants. Surprisingly, we find that a majority of insertions map within the esx-1 locus, which encodes the secretory apparatus. Thus, in contrast to its role in donor function, ESX-1 is essential for recipient function; recipient ESX-1 mutants are hypo-conjugative. In addition to esx-1 genes, our screen identifies novel non-esx-1 loci in the M. smegmatis genome that are required for both DNA transfer and ESX-1 activity. DNA transfer therefore provides a simple molecular genetic assay to characterize ESX-1, which, in Mycobacterium tuberculosis, is necessary for full virulence. These findings reinforce the functional intertwining of DNA transfer and ESX-1 secretion, first described in the M. smegmatis donor. Moreover, our observation that ESX-1 has such diametrically opposed effects on transfer in the donor and recipient, forces us to consider how proteins secreted by the ESX-1 apparatus can function so as to modulate two seemingly disparate processes, M. smegmatis DNA transfer and M. tuberculosis virulence.

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.

A novel pathway for exotoxin delivery by an intracellular pathogen

Fundamental to the biology of many bacterial pathogens are bacterial proteins with the capacity to modulate host cellular functions. These bacterial proteins are delivered to the host's molecular targets by a great diversity of mechanisms of varying complexity. The different delivery mechanisms are adapted to the specific biology of the pathogen. Here we focus our attention on a recently described delivery pathway adapted to the biology of an intracellular pathogen, in which an exotoxin is delivered from an intracellular location to its molecular target through autocrine and paracrine pathways.

The VirB5 protein localizes to the T-pilus tips in Agrobacterium tumefaciens

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) mediates the transfer of single-stranded DNA and protein virulence factors into plant cells, and also determines the assembly of the T-pilus, which is believed to play a role in host recognition. The T-pilus is composed of the major component VirB2 and the minor component VirB5. Using immuno-electron microscopy we detected the major component VirB2 along the entire length of detached T-pili, but not on cell-bound T-pili or on the cell surface. In contrast, the minor T-pilus component VirB5 was detected on the tips of cell-bound T-pili as well as on the ends of detached T-pili and on the cell surface. To gain further insights into the role of VirB5 we introduced changes at its C terminus. C-terminal deletions of up to four amino acids and alanine replacements did not abolish T-pilus formation and incorporation of the VirB5 variants at the tip, although they did impact the length of T-pili. Also, these changes differentially affected the ability of the T4SS to transfer DNA into plant and bacterial recipients, suggesting differential effects on host-cell specificity. The data presented here suggest that VirB5 localizes at the T-pilus tip, and provide novel insights into its role during the type IV secretion process.

Interactions of pathogenic mycobacteria with host macrophages

Tuberculosis, caused by the bacterium Mycobacterium tuberculosis, is one of the most deadly infectious diseases across the globe. The success of M. tuberculosis is related to its capacity to survive and replicate in macrophages, cells of the host innate immune system that are designed to detect and eliminate pathogens [1,2]. In this review, we will focus on the mechanisms used by the innate system of the host to detect and eliminate mycobacteria and the strategies used by M. tuberculosis to overcome host responses to establish a successful infection.

Combating bacteria and drug resistance by inhibiting mechanisms of persistence and adaptation

Antibiotics have revolutionized the treatment of infectious disease but have also rapidly selected for the emergence of resistant pathogens. Traditional methods of antibiotic discovery have failed to keep pace with the evolution of this resistance, which suggests that new strategies to combat bacterial infections may be required. An improved understanding of bacterial stress responses and evolution suggests that in some circumstances, the ability of bacteria to survive antibiotic therapy either by transiently tolerating antibiotics or by evolving resistance requires specific biochemical processes that may themselves be subject to intervention. Inhibiting these processes may prolong the efficacy of current antibiotics and provide an alternative to escalating the current arms race between antibiotics and bacterial resistance. Though these approaches are not clinically validated and will certainly face their own set of challenges, their potential to protect our ever-shrinking arsenal of antibiotics merits their investigation. This Review summarizes the early efforts toward this goal.

Cloning and sequence analysis of Helicobacter pylori NCTC 11637 cagT gene

AIM: To clone and analyze H. pylori cagT gene.

METHODS: H. pylori cagT gene was amplified from the genome DNA by polymerase chain reaction (PCR). The PCR product was inserted into pGEM-T vector and then transformed into E.coli DH5α. The positive recombinant clone was analyzed by digestion of restriction endonuclease. Next, the cagT gene fragment was inserted directionally into vector pQE30 to construct recombinant clone of cagT, which was sequenced finally.

RESULTS: A cagT gene consisting of 843 base pairs, which encoded a product of 280 amino acids, was obtained using PCR method and was cloned into pGEM-T vector successfully. Its GenBank accession number was EF114758. Sequence analysis showed that cagT gene had shared 97%-99% homology with other strains in GenBank.

CONCLUSION: The correct cagT gene is successfully cloned, which established a basis for further investigation of its biological function.

NMR structure of a complex between the VirB9/VirB7 interaction domains of the pKM101 type IV secretion system

Type IV secretion (T4S) systems translocate DNA and protein effectors through the double membrane of Gram-negative bacteria. The paradigmatic T4S system in Agrobacterium tumefaciens is assembled from 11 VirB subunits and VirD4. Two subunits, VirB9 and VirB7, form an important stabilizing complex in the outer membrane. We describe here the NMR structure of a complex between the C-terminal domain of the VirB9 homolog TraO (TraO(CT)), bound to VirB7-like TraN from plasmid pKM101. TraO(CT) forms a beta-sandwich around which TraN winds. Structure-based mutations in VirB7 and VirB9 of A. tumefaciens show that the heterodimer interface is conserved. Opposite this interface, the TraO structure shows a protruding three-stranded beta-appendage, and here, we supply evidence that the corresponding region of VirB9 of A. tumefaciens inserts in the membrane and protrudes extracellularly. This complex structure elucidates the molecular basis for the interaction between two essential components of a T4S system.

Environmental mimics and the Lvh type IVA secretion system contribute to virulence-related phenotypes of Legionella pneumophila

Legionella pneumophila, the causative organism of Legionnaires' disease, is a fresh-water bacterium and intracellular parasite of amoebae. This study examined the effects of incubation in water and amoeba encystment on L. pneumophila strain JR32 and null mutants in dot/icm genes encoding a type IVB secretion system required for entry, delayed acidification of L. pneumophila-containing phagosomes, and intracellular multiplication when stationary-phase bacteria infect amoebae and macrophages. Following incubation of stationary-phase cultures in water, mutants in dotA and dotB, essential for function of the type IVB secretion system, exhibited entry and delay of phagosome acidification comparable to that of strain JR32. Following encystment in Acanthamoeba castellanii and reversion of cysts to amoeba trophozoites, dotA and dotB mutants exhibited intracellular multiplication in amoebae. The L. pneumophila Lvh locus, encoding a type IVA secretion system homologous to that in Agrobacterium tumefaciens, was required for restoration of entry and intracellular multiplication in dot/icm mutants following incubation in water and amoeba encystment and was required for delay of phagosome acidification in strain JR32. These data support a model in which the Dot/Icm type IVB secretion system is conditionally rather than absolutely required for L. pneumophila virulence-related phenotypes. The data suggest that the Lvh type IVA secretion system, previously thought to be dispensable, is involved in virulence-related phenotypes under conditions mimicking the spread of Legionnaires' disease from environmental niches. Since environmental amoebae are implicated as reservoirs for an increasing number of environmental pathogens and for drug-resistant bacteria, the environmental mimics developed here may be useful in virulence studies of other pathogens.

Inhibitors of Helicobacter pylori ATPase Cagα block CagA transport and cag virulence

With the steadily increasing occurrence of antibiotic resistance in bacteria, there is a great need for new antibacterial compounds. The approach described here involves targeting virulence-related bacterial type IV secretion systems (TFSSs) with small-molecule inhibitors. The cag TFSS of Helicobacter pylori was chosen as a model, and novel inhibitors directed against the cag VirB11-type ATPase Cagα were identified. The cag genes encode proteins that are components of a contact-dependent secretion system used by the bacterium to translocate the effector molecule CagA into host cells. Translocated CagA is associated with severe gastritis, and carcinoma. Furthermore, functional TFSSs and immunodominant CagA play a role in interleukin (IL)-8 induction, which is an important factor for chronic inflammation. Inhibitors of Cagα were identified by high-throughput screening of chemical libraries that comprised 524 400 small molecules. The ATPase activity of Cagα was inhibited by the selected compounds in an in vitro enzymic assay using the purified enzyme. The most active compound, CHIR-1, reduced TFSS function to an extent that cellular effects on AGS cells mediated by CagA were virtually undetectable, while reduced levels of IL-8 induction were observed. Gastric colonization by CHIR-1-pre-treated bacteria was found to be impaired in a dose-dependent manner using a mouse model of infection. Small-molecule Cagα inhibitors, the first described inhibitors of a TFSS, are potential candidates for the development of new antibacterial compounds that may lead to alternative medical treatments. The compounds are expected to impose weak selective pressure, since they target virulence functions. Moreover, the targeted virulence protein is conserved in a variety of bacterial pathogens. Additionally, TFSS inhibitors are potent tools to study the biology of TFSSs.

Protein-protein interactions among Helicobacter pylori cag proteins

Many Helicobacter pylori isolates contain a 40-kb region of chromosomal DNA known as the cag pathogenicity island (PAI). The risk for development of gastric cancer or peptic ulcer disease is higher among humans infected with cag PAI-positive H. pylori strains than among those infected with cag PAI-negative strains. The cag PAI encodes a type IV secretion system that translocates CagA into gastric epithelial cells. To identify Cag proteins that are expressed by H. pylori during growth in vitro, we compared the proteomes of a wild-type H. pylori strain and an isogenic cag PAI deletion mutant using two-dimensional difference gel electrophoresis (2D-DIGE) in multiple pH ranges. Seven Cag proteins were identified by this approach. We then used a yeast two-hybrid system to detect potential protein-protein interactions among 14 Cag proteins. One heterotypic interaction (CagY/7 with CagX/8) and two homotypic interactions (involving H. pylori VirB11/ATPase and Cag5) were similar to interactions previously reported to occur among homologous components of the Agrobacterium tumefaciens type IV secretion system. Other interactions involved Cag proteins that do not have known homologues in other bacterial species. Biochemical analysis confirmed selected interactions involving five of the proteins that were identified by 2D-DIGE. Protein-protein interactions among Cag proteins are likely to have an important role in the assembly of the H. pylori type IV secretion apparatus.

A virulence locus of Pseudomonas aeruginosa encodes a protein secretion apparatus

Bacterial pathogens frequently use protein secretion to mediate interactions with their hosts. Here we found that a virulence locus (HSI-I) of Pseudomonas aeruginosa encodes a protein secretion apparatus. The apparatus assembled in discrete subcellular locations and exported Hcp1, a hexameric protein that forms rings with a 40 angstrom internal diameter. Regulatory patterns of HSI-I suggested that the apparatus functions during chronic infections. We detected Hcp1 in pulmonary secretions of cystic fibrosis (CF) patients and Hcp1-specific antibodies in their sera. Thus, HSI-I likely contributes to the pathogenesis of P. aeruginosa in CF patients. HSI-I-related loci are widely distributed among bacterial pathogens and may play a general role in mediating host interactions.

Type IV secretion systems and their effectors in bacterial pathogenesis

Type IV secretion systems (T4SSs) are membrane-associated transporter complexes used by various bacteria to deliver substrate molecules to a wide range of target cells. T4SSs are involved in horizontal DNA transfer to other bacteria and eukaryotic cells, in DNA uptake from or release into the extracellular milieu, in toxin secretion and in the injection of virulence factors into eukaryotic host target cells by several mammalian pathogens. Rapid progress has been made towards defining the structures and functions of T4SSs, identifying the translocated effector molecules and elucidating the mechanisms by which the effectors subvert eukaryotic cellular processes during infection. These findings have had an important impact on our understanding of how these pathogens manipulate host cell functions to trigger bacterial uptake, facilitate intracellular growth and suppress defence mechanisms, thus facilitating bacterial colonization and disease development.

Interaction with CagF is required for translocation of CagA into the host via the Helicobacter pylori type IV secretion system

Development of severe gastric diseases is strongly associated with those strains of Helicobacter pylori that contain the cag pathogenicity island (PAI) inserted into the chromosome. The cag PAI encodes a type IV secretion system that translocates the major disease-associated virulence protein, CagA, into the host epithelial cell. CagA then affects host signaling pathways, leading to cell elongations and inflammation. Since the precise mechanism by which the CagA toxin is translocated by the type IV secretion system remained elusive, we used fusion proteins and immunoprecipitation studies to identify CagA-interacting secretion components. Here we demonstrate that CagA, in addition to other yet-unidentified proteins, interacts with CagF, presumably at the inner bacterial membrane. This interaction is required for CagA translocation, since an isogenic nonpolar cagF mutant was translocation deficient. Our results suggest that CagF may be a protein with unique chaperone-like function that is involved in the early steps of CagA recognition and delivery into the type IV secretion channel.

Interaction of pathogenic mycobacteria with the host immune system

Pathogenic mycobacteria, in particular Mycobacterium tuberculosis, the causative agent of tuberculosis, have the remarkable capacity to circumvent destruction within one of the most hostile cell types of a vertebrate host: the macrophage. The ability of pathogenic mycobacteria to survive inside macrophages has been known for more than 30 years; yet, only recently have advances in molecular genetics, biochemistry, immunology, as well as global analysis of gene expression, started to unravel the strategies utilized by these pathogens for intracellular persistence. In addition, the definition of key molecules that are important for intracellular survival opens the possibility to develop new drugs to combat mycobacterial diseases.

The putative lytic transglycosylase VirB1 from Brucella suis interacts with the type IV secretion system core components VirB8, VirB9 and VirB11

VirB1-like proteins are believed to act as lytic transglycosylases, which facilitate the assembly of type IV secretion systems via localized lysis of the peptidoglycan. This paper presents the biochemical analysis of interactions of purified Brucella suis VirB1 with core components of the type IV secretion system. Genes encoding VirB1, VirB8, VirB9, VirB10 and VirB11 were cloned into expression vectors; the affinity-tagged proteins were purified from Escherichia coli, and analyses by gel filtration chromatography showed that they form monomers or homo-multimers. Analysis of protein-protein interactions by affinity precipitation revealed that VirB1 bound to VirB9 and VirB11. The results of bicistron expression experiments followed by gel filtration further supported the VirB1-VirB9 interaction. Peptide array mapping identified regions of VirB1 that interact with VirB8, VirB9 and VirB11 and underscored the importance of the C-terminus, especially for the VirB1-VirB9 interaction. The binding sites were localized on a structure model of VirB1, suggesting that different portions of VirB1 may interact with other VirB proteins during assembly of the type IV secretion machinery.

Structural and dynamic properties of bacterial type IV secretion systems (review)

The type IV secretion systems (T4SS) are widely distributed among the gram-negative and -positive bacteria. These systems mediate the transfer of DNA and protein substrates across the cell envelope to bacterial or eukaryotic cells generally through a process requiring direct cell-to-cell contact. Bacteria have evolved T4SS for survival during establishment of pathogenic or symbiotic relationships with eukaryotic hosts. The Agrobacterium tumefaciens VirB/D4 T4SS and related conjugation machines serve as models for detailed mechanistic studies aimed at elucidating the nature of translocation signals, machine assembly pathways and architectures, and the dynamics of substrate translocation. The A. tumefaciens VirB/D4 T4SS are polar-localized organelles composed of a secretion channel and an extracellular T pilus. These T4SS are assembled from 11 or more subunits. whose membrane topologies, intersubunit contacts and, in some cases, 3-dimensional structures are known. Recently, powerful in vivo assays have identified C-terminal translocation signals, defined for the first time the translocation route for a DNA substrate through a type IV secretion channel, and supplied evidence that ATP energy consumption contributes to a late stage of machine morphogenesis. Together, these recent findings describe the mechanics of type IV secretion in unprecedented detail.

The Trojan horse: survival tactics of pathogenic mycobacteria in macrophages

Mycobacterium tuberculosis, the causative agent of tuberculosis, has infected billions of people worldwide. A key to the success of M. tuberculosis and related pathogenic mycobacteria lies in their ability to persist within the hostile environment of the host macrophage. After internalization by macrophages, most microbes are rapidly transported to lysosomes in which they are destroyed. By contrast, pathogenic mycobacteria prevent fusion of phagosomes with lysosomes, thereby surviving intracellularly. Recent progress in understanding the molecular biology of host-mycobacteria interactions is providing insights into these survival tactics.

Conjugative plasmid pAW63 brings new insights into the genesis of the Bacillus anthracis virulence plasmid pXO2 and of the Bacillus thuringiensis plasmid pBT9727

BACKGROUND

Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis belong to the genetically close-knit Bacillus cereus sensu lato group, a family of rod-shaped Gram-positive bacteria. pAW63 is the first conjugative plasmid from the B. cereus group to be completely sequenced.

RESULTS

The 71,777 bp nucleotide sequence of pAW63 reveals a modular structure, including a 42 kb tra region encoding homologs of the Type IV secretion systems components VirB11, VirB4 and VirD4, as well as homologs of Gram-positive conjugation genes from Enterococcus, Lactococcus, Listeria, Streptococcus and Staphylococcus species. It also firmly establishes the existence of a common backbone between pAW63, pXO2 from Bacillus anthracis and pBT9727 from the pathogenic Bacillus thuringiensis serovar konkukian strain 97-27. The alignment of these three plasmids highlights the presence of well conserved segments, in contrast to distinct regions of high sequence plasticity. The study of their specific differences has provided a three-point reference framework that can be exploited to formulate solid hypotheses concerning the functionalities and the molecular evolution of these three closely related plasmids. This has provided insight into the chronology of their divergence, and led to the discovery of two Type II introns on pAW63, matching copies of the mobile element IS231L in different loci of pXO2 and pBT9727, and the identification on pXO2 of a 37 kb pathogenicity island (PAI) containing the anthrax capsule genes.

CONCLUSION

The complete sequence determination of pAW63 has led to a functional map of the plasmid yielding insights into its conjugative apparatus, which includes T4SS-like components, as well as its resemblance to other large plasmids of Gram-positive bacteria. Of particular interest is the extensive homology shared between pAW63 and pXO2, the second virulence plasmid of B. anthracis, as well as pBT9727 from the pathogenic strain B. thuringiensis serovar konkukian strain 97-27.

Agrobacterium tumefaciens VirB9, an outer-membrane-associated component of a type IV secretion system, regulates substrate selection and T-pilus biogenesis

Agrobacterium tumefaciens translocates DNA and protein substrates between cells via a type IV secretion system (T4SS) whose channel subunits include the VirD4 coupling protein, VirB11 ATPase, VirB6, VirB8, VirB2, and VirB9. In this study, we used linker insertion mutagenesis to characterize the contribution of the outer-membrane-associated VirB9 to assembly and function of the VirB/D4 T4SS. Twenty-five dipeptide insertion mutations were classified as permissive for intercellular substrate transfer (Tra+), completely transfer defective (Tra-), or substrate discriminating, e.g., selectively permissive for transfer only of the oncogenic transfer DNA and the VirE2 protein substrates or of a mobilizable IncQ plasmid substrate. Mutations inhibiting transfer of DNA substrates did not affect formation of close contacts of the substrate with inner membrane channel subunits but blocked formation of contacts with the VirB2 and VirB9 channel subunits, which is indicative of a defect in assembly or function of the distal portion of the secretion channel. Several mutations in the N- and C-terminal regions disrupted VirB9 complex formation with the outer-membrane-associated lipoprotein VirB7 or the inner membrane energy sensor VirB10. Several VirB9.i2-producing Tra+ strains failed to elaborate T pilus at detectable levels (Pil-), and three such Tra+ Pil- mutant strains were rendered Tra- upon deletion of virB2, indicating that the cellular form of pilin protein is essential for substrate translocation. Our findings, together with computer-based analyses, support a model in which distinct domains of VirB9 contribute to substrate selection and translocation, establishment of channel subunit contacts, and T-pilus biogenesis.

Identification of the VirB4-VirB8-VirB5-VirB2 pilus assembly sequence of type IV secretion systems

Type IV secretion systems mediate the translocation of virulence factors (proteins and/or DNA) from Gram-negative bacteria into eukaryotic cells. A complex of 11 conserved proteins (VirB1-VirB11) spans the inner and the outer membrane and assembles extracellular T-pili in Agrobacterium tumefaciens. Here we report a sequence of protein interactions required for the formation of complexes between VirB2 and VirB5, which precedes their incorporation into pili. The NTPase Walker A active site of the inner membrane protein VirB4 is required for virulence, but an active site VirB4 variant stabilized VirB3 and VirB8 and enabled T-pilus formation. Analysis of VirB protein complexes extracted from the membranes with mild detergent revealed that VirB2-VirB5 complex formation depended on VirB4, which identified a novel T-pilus assembly step. Bicistron expression demonstrated direct interaction of VirB4 with VirB8, and analyses with purified proteins showed that VirB5 bound to VirB8 and VirB10. VirB4 therefore localizes at the basis of a trans-envelope interaction sequence, and by stabilization of VirB8 it mediates the incorporation of VirB5 and VirB2 into extracellular pili.

The Icm/Dot type-IV secretion systems of Legionella pneumophila and Coxiella burnetii

Type-IV secretion systems are devices present in a wide range of bacteria (including bacterial pathogens) that deliver macromolecules (proteins and single-strand-DNA) across kingdom barriers (as well as between bacteria and into the surroundings). The type-IV secretion systems were divided into two subgroups and Legionella pneumophila and Coxiella burnetii are the only two bacteria known today to utilize a type-IVB secretion system for pathogenesis. In this review we summarized the available information concerning the icm/dot type-IVB secretion systems by comparing the two bacteria that possess this system, the proteins components of their systems as well as the homology of proteins from type-IVB secretion systems to proteins from type-IVA secretion systems. In addition, the phenotypes associated with mutants in the L. pneumophila icm/dot genes, their relations to properties of specific Icm/Dot proteins as well as the protein substrates delivered by this system are described.