The virulence plasmid of Yersinia, an antihost genome

A distinctive role for the Yersinia protein kinase: actin binding, kinase activation, and cytoskeleton disruption

The bacterial pathogens of the genus Yersinia deliver several virulence factors into target cells using a type III secretion system. We demonstrate that Yersinia protein kinase A (YpkA), an essential bacterial virulence factor, is produced as an inactive serine/threonine kinase. The inactive kinase is activated within the host cell by a cytosolic eukaryotic activator. Using biochemical purification techniques, we demonstrate that actin is a cellular activator of YpkA. This stimulation of YpkA kinase activity by actin depends on the presence of the C-terminal twenty amino acids of YpkA, because deletion of these 20 aa not only obliterates YpkA activity, but it also destroys the interaction between YpkA and actin. Activated YpkA functions within cultured epithelial cells to disrupt the actin cytoskeleton. The disruption of the actin cytoskeleton by YpkA would be expected to inhibit macrophage function and phagocytosis of Yersinia.

Invasion of epithelial cells by Yersinia pestis: evidence for a Y. pestis-specific invasin

The causative agent of plague, Yersinia pestis, is regarded as being noninvasive for epithelial cells and lacks the major adhesins and invasins of its enteropathogenic relatives Yersinia enterocolitica and Yersinia pseudotuberculosis. However, there are studies indicating that Y. pestis invades and causes systemic infection from ingestive and aerogenic routes of infection. Accordingly, we developed a gentamicin protection assay and reexamined invasiveness of Y. pestis for HeLa cells. By optimizing this assay, we discovered that Y. pestis is highly invasive. Several factors, including the presence of fetal bovine serum, the configuration of the tissue culture plate, the temperature at which the bacteria are grown, and the presence of the plasminogen activator protease Pla-encoding plasmid pPCP1, were found to influence invasiveness strongly. Suboptimal combinations of these factors may have contributed to negative findings by previous studies attempting to demonstrate invasion by Y. pestis. Invasion of HeLa cells was strongly inhibited by cytochalasin D and modestly inhibited by colchicine, indicating strong and modest respective requirements for microfilaments and microtubules. We found no significant effect of the iron status of yersiniae or of the pigmentation locus on invasion and likewise no significant effect of the Yops regulon. However, an unidentified thermally induced property (possibly the Y. pestis-specific capsular protein Caf1) did inhibit invasiveness significantly, and the plasmid pPCP1, unique to Y. pestis, was essential for highly efficient invasion. pPCP1 encodes an invasion-promoting factor and not just an adhesin, because Y. pestis lacking this plasmid still adhered to HeLa cells. These studies have enlarged our picture of Y. pestis biology and revealed the importance of properties that are unique to Y. pestis.

Pathogen-induced apoptosis of macrophages: a common end for different pathogenic strategies

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. Several bacterial pathogens induce apoptosis specifically in macrophages, but the mechanisms by which it occurs differ, and the resulting pathology can take different courses. Macrophage death caused by Shigella flexneri and Salmonella spp. has been shown to result in the release of pro-inflammatory cytokines. Conversely, Yersinia spp. induce apoptosis by suppressing the signalling pathways that lead to the production of tumour necrosis factor (TNF)-alpha, a cytokine essential for the control of this infection. It is likely that there are a variety of reasons why macrophages are particularly susceptible to pathogen-induced apoptosis. One reason may be the expression of surface receptors that recognize highly conserved bacterial components, such as lipopolysaccharide (LPS) and bacterial lipoproteins (BLPs). These receptors have recently been shown to activate pro-apoptotic signalling pathways. The roles of macrophage apoptosis in different disease processes are discussed.

Molecular and cell biology aspects of plague

A 70-kb virulence plasmid (sometimes called pYV) enables Yersinia spp. to survive and multiply in the lymphoid tissues of their host. It encodes the Yop virulon, a system consisting of secreted proteins called Yops and their dedicated type III secretion apparatus called Ysc. The Ysc apparatus forms a channel composed of 29 proteins. Of these, 10 have counterparts in almost every type III system. Secretion of some Yops requires the assistance, in the bacterial cytosol, of small individual chaperones called the Syc proteins. These chaperones act as bodyguards or secretion pilots for their partner Yop. Yop proteins fall into two categories. Some are intracellular effectors, whereas the others are "translocators" needed to deliver the effectors across the eukaryotic plasma membrane, into eukaryotic cells. The translocators (YopB, YopD, LcrV) form a pore of 16-23 A in the eukaryotic cell plasma membrane. The effector Yops are YopE, YopH, YpkA/YopO, YopP/YopJ, YopM, and YopT. YopH is a powerful phosphotyrosine phosphatase playing an antiphagocytic role by dephosphorylating several focal adhesion proteins. YopE and YopT contribute to antiphagocytic effects by inactivating GTPases controlling cytoskeleton dynamics. YopP/YopJ plays an anti-inflammatory role by preventing the activation of the transcription factor NF-kappaB. It also induces rapid apoptosis of macrophages. Less is known about the role of the phosphoserine kinase YopO/YpkA and YopM.

The RhoGAP activity of the Yersinia pseudotuberculosis cytotoxin YopE is required for antiphagocytic function and virulence

A variety of pathogenic bacteria use type III secretion pathways to translocate virulence proteins into host eukaryotic cells. YopE is an important virulence factor that is translocated into mammalian cells via a plasmid-encoded type III system in Yersinia spp. YopE action in mammalian cells promotes the disruption of actin filaments, cell rounding and blockage of phagocytosis. It was reported recently that two proteins with sequence similarity to YopE, SptP of Salmonella typhimurium and ExoS of Pseudomonas aeruginosa, function as GTPase-activating proteins (GAPs) for Rho GTPases. YopE contains an 'arginine finger' motif that is present in SptP, ExoS and other Rho GAPs and is essential for catalysis by this class of proteins. We show here that a GST-YopE fusion protein stimulated in vitro GTP hydrolysis by the Rho family members Cdc42, RhoA and Rac1, but not by Ras. Conversion of the essential arginine in the arginine finger motif to alanine (R144A) eliminated the in vitro GAP activity of GST-YopE. Infection assays carried out with a Yersinia pseudotuberculosis strain producing YopER144A demonstrated that GAP function was essential for the disruption of actin filaments, cell rounding and inhibition of phagocytosis by YopE in HeLa cells. Furthermore, the GAP function of YopE was important for Y. pseudotuberculosis pathogenesis in a mouse infection assay. Transfection of HeLa cells with a vector that produces a constitutively active form of RhoA (RhoA-V14) prevented the disruption of actin filaments and cell rounding by YopE. Production of an activated form of Rac1 (Rac1-V12), but not RhoA-V14, in HeLa cells interfered with YopE antiphagocytic activity. These results demonstrate that YopE functions as a RhoGAP to downregulate multiple Rho GTPases, leading to the disruption of actin filaments and inhibition of bacterial uptake into host cells.

Motility is required to initiate host cell invasion by Yersinia enterocolitica

Invasin-mediated invasion of host cells by the pathogen Yersinia enterocolitica was shown to be affected by flagellar-dependent motility. Motility appears to be required to ensure the bacterium migrates to and contacts the host cell. Nonmotile strains of Y. enterocolitica were less invasive than motile strains, but the reduction in invasion could be overcome by artificially bringing the bacteria into host cell contact by centrifugation. Mutations in known regulatory genes of the flagellar regulon, flhDC and fliA, resulted in less inv expression but did not have a significant effect on invasin levels. However, invasin levels were reduced for strains that harbored flhDC on a multicopy plasmid, apparently as a result of increased proteolysis of invasin.

LcrQ/YscM1, regulators of the Yersinia yop virulon, are injected into host cells by a chaperone-dependent mechanism

Pathogenic Yersinia species employ type III machines to secrete YopBDR into the extracellular milieu. After attaching to host cells, yersiniae transform the type III machinery into an injection device and target YopEHMNOPT into eukaryotic cells. Yersinia pseudotuberculosis LcrQ is a transcriptional regulator that prevents the expression of yop genes. We report that LcrQ is injected into eukaryotic cells. YscM1, the transciptional regulator of Yersinia enterocolitica, is also injected into eukaryotic cells, whereas the related YscM2 protein remains associated with bacterial cells. Type III targeting of YscM1 requires binding to the SycH chaperone. Chaperone binding as well as depletion of YscM1 and YscM2 from the cytoplasm of Y. enterocolitica causes an increase in yop expression, whereas a block in regulator export reduces expression. We propose a model whereby the chaperone-mediated injection of LcrQ/YscM1 functions as a regulatory switch for bacteria that are attached to host cells, triggering the expression of Yops that travel the type III targeting pathway.

Evaluation of a Yersinia adhesion gene (yadA) specific PCR for the identification of enteropathogenic Yersinia enterocolitica

A total of 101 Yersinia enterocolitica strains was investigated with a PCR assay [Blais and Phillipe, Food Control, 6 (1995) 211-214] targeting the Yersinia adhesin gene (yadA) responsible for autoagglutination. Compared to the autoagglutination test the PCR assay has a specificity of 100% but a sensitivity of only 70%. This failure might be caused by the sequence heterogeneity of yadA.

Shigella flexneri IpaH(7.8) facilitates escape of virulent bacteria from the endocytic vacuoles of mouse and human macrophages

The behavior of Shigella flexneri ipaH mutants was studied in human monocyte-derived macrophages (HMDM), in 1-day-old human monocytes, and in J774 mouse macrophage cell line. In HMDM, strain pWR700, an ipaH(7.8) deletion mutant of S. flexneri 2a strain 2457T, behaved like the wild-type strain 2457T. This strain caused rapid host cell death by oncosis, and few bacterial CFU were recovered after incubation in the presence of gentamicin as previously described for 2457T-infected HMDM. However, analysis of bacterial compartmentalization within endocytic vacuoles with gentamicin and chloroquine indicated that more pWR700 than 2457T was present within the endocytic vacuoles of HMDM, suggesting that ipaH(7.8) deletion mutant transited more slowly from the vacuoles to the cytoplasm. In contrast to findings with HMDM, CFU recovered from pWR700-infected mouse J774 cells were 2 to 3 logs higher than CFU from 2457T-infected J774 cells. These values exceeded CFU recovered after infection of J774 cells with plasmid-cured avirulent strain M4243A1. Incubation with gentamicin and chloroquine clearly showed that pWR700 within J774 cells was mostly present within the endocytic vacuoles. This distribution pattern was similar to that seen with M4243A1 and contrasted with the pattern seen with 2457T. Complementation of pWR700 with a recombinant clone expressing ipaH(7. 8) restored the intracellular distribution of bacteria to that seen with the wild-type strain. Strains with deletions in ipaH(4.5) or ipaH(9.8), however, behaved like 2457T in both HMDM and J774 cells. The distribution profile of pWR700 in 1-day-old monocytes was similar to that seen in J774 cells. Like infected J774 cells, 1-day-old human monocytes demonstrated apoptosis upon infection with virulent Shigella. These results suggest that a role of the ipaH(7. 8) gene product is to facilitate the escape of the virulent bacteria from the phagocytic vacuole of monocytes and macrophages.

Yersinia enterocolitica TyeA, an intracellular regulator of the type III machinery, is required for specific targeting of YopE, YopH, YopM, and YopN into the cytosol of eukaryotic cells

Pathogenic Yersinia species employ type III machines to target effector Yops into the cytosol of eukaryotic cells. Yersinia tyeA mutants are thought to be defective in the targeting of YopE and YopH without affecting the injection of YopM, YopN, YopO, YopP, and YopT into the cytosol of eukaryotic cells. One model suggests that TyeA may form a tether between YopN (LcrE) and YopD on the bacterial surface, a structure that may translocate YopE and YopH across the plasma membrane of eukaryotic cells (M. Iriarte, M. P. Sory, A. Boland, A. P. Boyd, S. D. Mills, I. Lambermont, and G. R. Cornelis, EMBO J. 17:1907-1918, 1998). We have examined the injection of Yop proteins by tyeA mutant yersiniae with the digitonin fractionation technique. We find that tyeA mutant yersiniae not only secreted YopE, YopH, YopM, and YopN into the extracellular medium but also targeted these polypeptides into the cytosol of HeLa cells. Protease protection, cell fractionation, and affinity purification experiments suggest that TyeA is located intracellularly and binds to YopN or YopD. We propose a model whereby TyeA functions as a negative regulator of the type III targeting pathway in the cytoplasm of yersiniae, presumably by preventing the export of YopN.

Signaling and invasin-promoted uptake via integrin receptors

The invasin protein encoded by enteropathogenic Yersinia allows entry of bacteria into intestinal M cells by binding to integrin receptors. In cultured cells, invasin-mediated uptake requires proteins involved in endocytosis and signaling to the cell cytoskeleton. At least four different factors have been demonstrated to play a role in regulating the efficiency of invasin-promoted uptake. These include receptor-ligand affinity, receptor clustering, signaling through focal adhesion kinase, and stimulation of cytoskeletal rearrangements by small GTP binding proteins.

Unravelling the mysteries of virulence gene regulation in Salmonella typhimurium

Salmonella typhimurium, which causes gastroenteritis in calves and humans as well as a typhoid-like disease in mice, uses numerous virulence factors to infect its hosts. Genes encoding these factors are regulated by many environmental conditions and regulatory pathways in vitro. Many virulence genes are specifically induced at particular sites during infection or in cultured host cells. The complex regulation of virulence genes observed in vitro may be necessary to restrict their expression to specific locations within the host. In vitro and in vivo studies provide clues about how virulence genes might be regulated in vivo. Future studies must assess the actual environmental signals and regulators that modulate each virulence gene in vivo and determine how multiple regulatory pathways are integrated to co-ordinate the appropriate expression of virulence factors at specific sites in vivo.

Questions about the behaviour of bacterial pathogens in vivo

Bacterial pathogens cause disease in man and animals. They have unique biological properties, which enable them to colonize mucous surfaces, penetrate them, grow in the environment of the host, inhibit or avoid host defences and damage the host. The bacterial products responsible for these five biological requirements are the determinants of pathogenicity (virulence determinants). Current knowledge comes from studies in vitro, but now interest is increasing in how bacteria behave and produce virulence determinants within the infected host. There are three aspects to elucidate: bacterial activities, the host factors that affect them and the metabolic interactions between the two. The first is relatively easy to accomplish and, recently, new methods for doing this have been devised. The second is not easy because of the complexity of the environment in vivo and its ever-changing face. Nevertheless, some information can be gained from the literature and by new methodology. The third aspect is very difficult to study effectively unless some events in vivo can be simulated in vitro. The objectives of the Discussion Meeting were to describe the new methods and to show how they, and conventional studies, are revealing the activities of bacterial pathogens in vivo. This paper sets the scene by raising some questions and suggesting, with examples, how they might be answered. Bacterial growth in vivo is the primary requirement for pathogenicity. Without growth, determinants of the other four requirements are not formed. Results from the new methods are underlining this point. The important questions are as follows. What is the pattern of a developing infection and the growth rates and population sizes of the bacteria at different stages? What nutrients are present in vivo and how do they change as infection progresses and relate to growth rates and population sizes? How are these nutrients metabolized and by what bacterial mechanisms? Which bacterial processes handle nutrient deficiencies and antagonistic conditions that may arise? Conventional and new methods can answer the first question and part of the second; examples are described. The difficulties of trying to answer the last two are discussed. Turning to production in vivo of determinants of mucosal colonization, penetration, interference with host defence and damage to the host, here are the crucial questions. Are putative determinants, which have been recognized by studies in vitro, produced in vivo and are they relevant to virulence? Can hitherto unknown virulence determinants be recognized by examining bacteria grown in vivo? Does the complement of virulence determinants change as infection proceeds? Are regulatory processes recognized in vitro, such as ToxR/ToxS, PhoP/PhoQ, quorum sensing and type III secretion, operative in vivo? What environmental factors affect virulence determinant production in vivo and by what metabolic processes? Examples indicate that the answers to the first four questions are 'yes' in most but not all cases. Attempts to answer the last, and most difficult, question are also described. Finally, sialylation of the lipopolysaccharide of gonococci in vivo by host-derived cytidine 5'-mono-phospho-N-acetyl neuraminic acid, and the effect of host lactate are described. This investigation revealed a new bacterial component important in pathogenicity, the host factors responsible for its production and the metabolism involved.

Type III secretion: a bacterial device for close combat with cells of their eukaryotic host

Salmonella, Shigella, Yersinia, Pseudomonas aeruginosa, enteropathogenic Escherichia coli and several plant-pathogenic Gram-negative bacteria use a new type of systems called 'type III secretion' to attack their host. These systems are activated by contact with a eukaryotic cell membrane and they allow bacteria to inject bacterial proteins across the two bacterial membranes and the eukaryotic cell membrane to reach a given compartment and destroy or subvert the target cell. These systems consist of a secretion apparatus made up of about 25 individual proteins and a set of proteins released by this apparatus. Some of these released proteins are 'effectors' that are delivered by extracellular bacteria into the cytosol of the target cell while the others are 'translocators' that help the 'effectors' to cross the membrane of the eukaryotic cell. Most of the 'effectors' act on the cytoskeleton or on intracellular signalling cascades. One of the proteins injected by the enteropathogenic E. coli serves as a membrane receptor for the docking of the bacterium itself at the surface of the cell.

Investigation into the role of the serine protease HtrA in Yersinia pestis pathogenesis

The HtrA stress response protein has been shown to play a role in the virulence of a number of pathogens. For some organisms, htrA mutants are attenuated in the animal model and can be used as live vaccines. A Yersinia pestis htrA orthologue was identified, cloned and sequenced, showing 86% and 87% similarity to Escherichia coli and Salmonella typhimurium HtrAs. An isogenic Y. pestis htrA mutant was constructed using a reverse genetics approach. In contrast to the wild-type strain, the mutant failed to grow at an elevated temperature of 39 degrees C, but showed only a small increase in sensitivity to oxidative stress and was only partially attenuated in the animal model. However, the mutant exhibited a different protein expression profile to that of the wild-type strain when grown at 28 degrees C to simulate growth in the flea.

Type III machines of Gram-negative bacteria: delivering the goods

Many Gram-negative pathogens use a type III secretion machine to translocate protein toxins across the bacterial cell envelope. Pathogenic Yersinia spp. export at least 14 Yop proteins via a type III machine, which recognizes secretion substrates by signals encoded in yop mRNA or chaperones bound to unfolded Yop proteins. During infection, substrate recognition appears to be regulated in a manner that allows the Yersinia type III pathway to direct Yops to the bacterial envelope, the extracellular medium or into the cytosol of host cells.

Yersinia enterocolitica invasin protein triggers differential production of interleukin-1, interleukin-8, monocyte chemoattractant protein 1, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor alpha in epithelial cells: implications for understanding the early cytokine network in Yersinia infections

Yersinia enterocolitica infection of epithelial cells results in interleukin-8 (IL-8) mRNA expression. Herein we demonstrate that besides IL-8, increased mRNA levels of five other cytokines, IL-1alpha, IL-1beta, monocyte chemoattractant protein 1 (MCP-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor alpha (TNF-alpha), can be detected upon infection of HeLa cells with Yersinia. Yersinia-triggered cytokine production was not affected by blocking phosphatidylinositol-3-phosphate kinase with wortmannin, which inhibited bacterial invasion. Comparable cytokine mRNA responses were triggered by Escherichia coli expressing Yersinia inv, while no response was triggered by an inv-deficient Yersinia mutant. Moreover, cytokine responses were independent from metabolic activity of the bacteria, as killed bacterial cells were sufficient for triggering cytokine responses in HeLa cells. Semiquantitative reverse transcription-PCR analysis was used to assess the kinetics of cytokine mRNA expression in infected HeLa cells. IL-8, IL-1alpha, IL-1beta, MCP-1, GM-CSF, and TNF-alpha mRNA expression increased within 1 h postinfection, reached a maximum after 3 to 4 h, and then declined to preinfection levels within 3 h. IL-8, MCP-1, and GM-CSF were secreted by HeLa cells, whereas IL-1alpha and IL-1beta were not secreted and thus were found exclusively intracellularly. TNF-alpha protein could not be detected in cell lysates or supernatants. Stimulation of HeLa cells with IL-1alpha was followed by increased IL-8 mRNA expression, whereas stimulation with IL-8 did not induce cytokine production. Likewise, MCP-1 and GM-CSF did not induce significant cytokine responses in HeLa cells. Our results implicate that the initial host response to Yersinia infection might be sustained by IL-8, MCP-1, and GM-CSF produced by epithelial cells.

Pseudomonas aeruginosa cystic fibrosis isolates induce rapid, type III secretion-dependent, but ExoU-independent, oncosis of macrophages and polymorphonuclear neutrophils

Pseudomonas aeruginosa, an opportunistic pathogen responsible most notably for severe infections in cystic fibrosis (CF) patients, utilizes the type III secretion system for eukaryotic cell intoxication. The CF clinical isolate CHA shows toxicity towards human polymorphonuclear neutrophils (PMNs) which is dependent on the type III secretion system but independent of the cytotoxin ExoU. In the present study, the cytotoxicity of this strain toward human and murine macrophages was demonstrated. In low-multiplicity infections (multiplicity of infection, 10), approximately 40% of the cells die within 60 min. Analysis of CHA-infected cells by transmission electron microscopy, DNA fragmentation assay, and Hoechst staining revealed the hallmarks of oncosis: cellular and nuclear swelling, disintegration of the plasma membrane, and absence of DNA fragmentation. A panel of 29 P. aeruginosa CF isolates was screened for type III system genotype, protein secretion profile, and cytotoxicity toward PMNs and macrophages. This study showed that six CF isolates were able to induce rapid ExoU-independent oncosis on phagocyte cells.

Genome plasticity in Enterobacteriaceae

The comparative analysis of multiple representatives of the genomes of particular species are leading us away from a view of bacterial genomes as static, monolithic structures towards the view that they are relatively variable, fluid structures. This plasticity is mainly the result of the rearrangement of genes within the genome and the acquisition of novel genes by horizontal transfer systems, e. g. plasmids, bacteriophages, transposons or gene cassettes. These mechanisms often act in concert thus generating a complex genetic structure. Genomic variations are not a phenomenon at the DNA level alone, they influence the phenotype of a bacterium as well and can render a formerly harmless organism into a hazardous pathogen. This review deals not only with the mechanisms of genome rearrangements and the horizontal transfer of genes in Enterobacteriaceae but also points out that mobile genetic elements themselves are subjected to variation.

Superantigen YPMa exacerbates the virulence of Yersinia pseudotuberculosis in mice

Yersinia pseudotuberculosis, a gram-negative bacterium responsible for enteric and systemic infection in humans, produces a superantigenic toxin designated YPMa (Y. pseudotuberculosis-derived mitogen). To assess the role of YPMa in the pathogenesis of Y. pseudotuberculosis, we constructed a superantigen-deficient mutant and compared its virulence in a mouse model of infection to the virulence of the wild-type strain. Determination of the survival rate after intravenous (i.v.) bacterial inoculation of OF1 mice clearly showed that inactivation of ypmA, encoding YPMa, reduced the virulence of Y. pseudotuberculosis. Mice infected i.v. with 10(4) and 10(5) wild-type bacteria died within 9 days, whereas mice infected with the ypmA mutant survived 12 and 3 days longer, respectively. This decreased virulence of the ypmA mutant strain was not due to an impaired colonization of the spleen, liver, or lungs. In contrast to i.v. challenge, bacterial inoculation by the intragastric (i.g.) route did not reveal any difference in virulence between wild-type Y. pseudotuberculosis and the ypmA mutant since the 50% lethal doses were identical for both strains. Moreover, inactivation of ypmA gene did not affect the bacterial growth of Y. pseudotuberculosis in Peyer's patches, mesenteric lymph nodes (MLNs), and spleen after oral infection. Histological studies of spleen, liver, lungs, heart, Peyer's patches, and MLNs after i.v. or i.g. challenge with the wild type or the ypmA mutant did not reveal any feature that can be specifically related to YPMa. Our data show that the superantigenic toxin YPMa contributes to the virulence of Y. pseudotuberculosis in systemic infection in mice.

Yersiniae other than Y. enterocolitica, Y. pseudotuberculosis, and Y. pestis: the ignored species

The genus Yersinia is composed of 11 species, of which three (Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica) have been exhaustively characterized. The remaining eight species (Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. bercovieri, Y. mollaretii, Y. rohdei, Y. ruckeri, and Y. aldovae) have not been studied extensively and, because of the absence of classical Yersinia virulence markers, are generally considered to be nonpathogenic. However, recent data suggest that some of these eight species may cause disease by virtue of their having virulence factors distinct from those of Y. enterocolitica. These data raise intriguing questions about the mechanisms by which these species interact with their host cells and elicit human disease.

Two novel proteins, PopB, which has functional nuclear localization signals, and PopC, which has a large leucine-rich repeat domain, are secreted through the hrp-secretion apparatus of Ralstonia solanacearum

The Ralstonia solanacearum hrp gene cluster codes for components of a type III secretion pathway necessary for the secretion of PopA1, a hypersensitive response-like elicitor protein. In the present study, we show that several other Hrp-secreted proteins can be detected by growing wild-type bacteria in minimal medium in the presence of Congo red. Two of these proteins, PopB and PopC, are encoded by genes located downstream of popA and constitute an operon with popA. popABC mutants retain the wild-type ability to cause disease in hosts and to elicit the hypersensitive response on non-hosts. Expression of the popABC operon is controlled by the hrpB regulatory gene and is induced upon co-culture with Arabidopsis cell suspensions. This plant cell-specific induction depends on PrhA, a putative receptor for plant specific signal(s). The transcription of the popABC operon is not modified by the addition of Congo red to the growth medium and the intracellular pools of PopB and PopC are very similar in the absence or presence of Congo red. Preliminary data suggest that Congo red stabilizes secreted proteins in the extracellular medium. PopB is a 173-amino-acid-basic protein that contains a functional bipartite nuclear localization signal. PopC is a 1024-amino-acid protein that carries 22 tandem leucine-rich repeats (LRR). The LRR domain of this protein forms a consensus that perfectly matches the predicted eukaryotic cytoplasmic LRR consensus. We propose that PopB and PopC may be translocated into plant cells via the Hrp pathway.

The Yersinia pestis YscY protein directly binds YscX, a secreted component of the type III secretion machinery

Human pathogenic yersiniae organisms export and translocate the Yop virulence proteins and V antigen upon contact with a eukaryotic cell. Yersinia pestis mutants defective for production of YscX or YscY were unable to export the Yops and V antigen. YscX and YscY were both present in the Y. pestis cell pellet fraction; however, YscX was also found in the culture supernatant. YscY showed structural and amino acid sequence similarities to the Syc family of proteins. YscY specifically recognized and bound to a region of YscX that included a predicted coiled-coil region. These data suggest that YscY may function as a chaperone for YscX in Y. pestis.

Modulation of host immune responses, induction of apoptosis and inhibition of NF-kappaB activation by the Bordetella type III secretion system

Bordetella bronchiseptica establishes respiratory tract infections in laboratory animals with high efficiency. Colonization persists for the life of the animal and infection is usually asymptomatic in immunocompetent hosts. We hypothesize that this reflects a balance between immunostimulatory events associated with infection and immunomodulatory events mediated by the bacteria. We have identified 15 loci that are part of a type III secretion apparatus in B. bronchiseptica and three secreted proteins. The functions of the type III secretion system were investigated by comparing the phenotypes of wild-type bacteria with two strains that are defective in type III secretion using in vivo and in vitro infection models. Type III secretion mutants were defective in long-term colonization of the trachea in immunocompetent mice. The mutants also elicited higher titres of anti-Bordetella antibodies upon infection compared with wild-type bacteria. Type III secretion mutants also showed increased lethal virulence in immunodeficient SCID-beige mice. These observations suggest that type III-secreted products of B. bronchiseptica interact with components of both innate and adaptive immune systems of the host. B. bronchiseptica induced apoptosis in macrophages in vitro and inflammatory cells in vivo and type III secretion was required for this process. Infection of an epithelial cell line with high numbers of wild type, but not type III deficient B. bronchiseptica resulted in rapid aggregation of NF-kappaB into large complexes in the cytoplasm. NF-kappaB aggregation was dependent on type III secretion and aggregated NF-kappaB did not respond to TNFalpha activation, suggesting B. bronchiseptica may modulate host immunity by inactivating NF-kappaB. Based on these in vivo and in vitro results, we hypothesize that the Bordetella type III secretion system functions to modulate host immune responses during infection.

Interferon consensus sequence binding protein confers resistance against Yersinia enterocolitica

Interferon consensus sequence binding protein (ICSBP)-deficient mice display enhanced susceptibility to intracellular pathogens. At least two distinct immunoregulatory defects are responsible for this phenotype. First, diminished production of reactive oxygen intermediates in macrophages results in impaired intracellular killing of microorganisms. Second, defective early interleukin-12 (IL-12) production upon microbial challenge leads to a failure in gamma interferon (IFN-gamma) induction and subsequently in T helper 1 immune responses. Here, we investigated the role of ICSBP in resistance against the extracellular bacterium Yersinia enterocolitica. ICSBP(-/-) mice failed to produce IL-12 and IFN-gamma, but also IL-4, after Yersinia challenge. In addition, granuloma formation was highly disturbed in infected ICSBP(-/-) mice, leading to multiple necrotic abscesses in affected organs. Consequently, ICSBP(-/-) mice rapidly succumbed to acute Yersinia infection. In vitro treatment of spleen cells from ICSBP(-/-) mice with recombinant IL-12 (rIL-12) or rIL-18 in combination with a second stimulus resulted in IFN-gamma induction. In experimental therapy of infected ICSBP(-/-) mice, we observed that administration of rIL-12 induced IFN-gamma production which was associated with improved resistance to Yersinia. In contrast, treatment with rIL-18 failed to enhance endogenous IFN-gamma production but nevertheless reduced bacterial burden in ICSBP(-/-) mice. Although cytokine therapy with rIL-12 or rIL-18 ameliorated the course of Yersinia infection in ICSBP(-/-) mice, both cytokines failed to completely restore impaired immunity. Taken together, the results indicate that the transcription factor ICSBP is essential for efficient host immune defense against Yersinia. These results are important for understanding the complex host immune responses in bacterial infections.

A chromosomally encoded regulator is required for expression of the Yersinia enterocolitica inv gene and for virulence

The primary invasion factor of Yersinia enterocolitica, invasin, is encoded by inv. inv expression is regulated in response to pH, growth phase and temperature. In vitro, inv is maximally expressed at 26 degrees C, pH 8.0, or 37 degrees C, pH 5.5, in early stationary phase. At 37 degrees C, pH 8.0, inv is weakly expressed. To identify which gene(s) are required for inv regulation, we screened for transposon insertions that decreased expression of an inv-'phoA chromosomal reporter at 26 degrees C. Of 30 000 mutants screened, two were identified that had negligible inv expression in all conditions tested. Both of these independent mutants had an insertion into the same gene, designated rovA (regulator of virulence). RovA has 77% amino acid identity to the Salmonella typhimurium transcriptional regulator SlyA. Complementation with the wild-type rovA allele restores wild-type inv expression as monitored by Western blot analysis, tissue culture invasion assay and alkaline phosphatase assay. There is also a significant decrease in invasin levels in bacteria recovered from mice infected with the rovA mutant; therefore, RovA regulates inv expression in vivo as well as in vitro. In the mouse infection model, an inv mutant has a wild-type LD50, even though the kinetics of infection is changed. In contrast, the rovA mutant has altered kinetics, as well as a 70-fold increase in the LD50 compared with wild type. Furthermore, because the rovA mutant is attenuated in the mouse model, this suggests that RovA regulates other virulence factors in addition to inv. Analysis of other proposed virulence factors such as Ail, YadA and the Yop proteins shows no regulatory role for RovA. The more severe animal phenotype combined with the lack of impact on known virulence genes aside from inv suggests RovA regulates potentially novel virulence genes of Y. enterocolitica during infection.

Temperature-regulated expression of bacterial virulence genes

Virulence gene expression in most bacteria is a highly regulated phenomenon, affected by a variety of parameters including osmolarity, pH, ion concentration, iron levels, growth phase, and population density. Virulence genes are also regulated by temperature, which acts as an 'on-off' switch in a manner distinct from the more general heat-shock response. Here, we review temperature-responsive expression of virulence genes in four diverse pathogens.

Phosphatases and kinases delivered to the host cell by bacterial pathogens

The gram-negative type III secretion pathway translocates bacterial proteins directly into eukaryotic host cells, thus allowing a pathogen to interfere directly with host signalling pathways. Protein and inositol phosphatases and protein kinases have been identified as delivered effectors in three bacterial pathogens, Salmonella, Shigella and Yersinia, and it is expected that several more such type III effectors will be found.

The Bpel locus encodes type III secretion machinery in Bordetella pertussis

Type III secretory genes(Bscl, J, K, L, N and O) have recently been identified in Bordetella bronchiseptica and shown to be under the control of the BvgAS locus. We examined a 35 616 byte DNA sequence amplified from Bordetella pertussis Tohama I for homology with known type III secretory genes in Yersinia spp. and Pseudomonas sppand a total of 20 homologous open reading frames were detected. Putative type III secretion proteins in B. pertussis were designated according to their homology with type III secretion proteins in B. bronchiseptica, Yersinia and Pseudomonas. These ORFs were arranged in two putative operons, which together we have designated as the BpeI locus. The first spans nucleotides 23385-7888 and encodes the putative proteins LcrH1, BopD, BopB, LcfH2, BscI, BscJ, BscK, BscL, BscN, BscO, BscQ, BscR, BscS, BscT, BscU, and BscC, in this order. The second spans nucleotides 23580-29863 and encodes the putative proteins LcrE, LcrD, BscD and BscF, in this order. The homology of these proteins to type III secretory proteins was B. bronchiseptica (73-99%), Yersinia spp. (17-65%), Pseudomonas spp. (18-64%). The B. pertussis proteins were similar to their homologues in B. bronchiseptica, Yersinia and Pseudomonas in terms of length, molecular weight and isoelectric point. Coiled-coil domains were detected in putative translocation proteins, BopB and BopD. BopB and BopD were similar to each other, to the RTX toxin family and to cyaA, cyaB, cyaD and cyaE. The percentage G+C content of the sequence analysed was 66.16%, which is similar to the published percentage G+C (67-70%) for the B. pertussis chromosome.

The Agrobacterium tumefaciens chaperone-like protein, VirE1, interacts with VirE2 at domains required for single-stranded DNA binding and cooperative interaction

Agrobacterium tumefaciens transfers single-stranded DNA (ssDNA) into plants. Efficient tumorigenesis requires VirE1-dependent export of ssDNA-binding (SSB) protein VirE2. VirE1 binds VirE2 domains involved in SSB and self-association, and VirE1 may facilitate VirE2 export by preventing VirE2 aggregation and the premature binding of VirE2 to ssDNA.

Suppression of T and B lymphocyte activation by a Yersinia pseudotuberculosis virulence factor, yopH

The acquired immune responses are crucial to the survival of Yersinia-infected animals. Mice lacking T cells are sensitive to Yersinia infection, and a humoral response to Yersinia can be protective. Diverse mechanisms for Yersinia to impair and evade the host innate immune defense have been suggested, but the effects of Yersinia on lymphocytes are not known. Here, we demonstrate that after a transient exposure to Y. pseudotuberculosis, T and B cells are impaired in their ability to be activated through their antigen receptors. T cells are inhibited in their ability to produce cytokines, and B cells are unable to upregulate surface expression of the costimulatory molecule, B7.2, in response to antigenic stimulation. The block of lymphocyte activation results from the inhibition of early phosphorylation events of the antigen receptor signaling complex. Through the use of Y. pseudotuberculosis mutants, we show that the inhibitory effect in both T cells and B cells is dependent on the production of Yersinia outermembrane protein (Yop) H, a tyrosine phosphatase. Our results suggest a mechanism by which the pathogenic bacteria may modulate a wide range of T and B cell-mediated immune responses.

Reciprocal secretion of proteins by the bacterial type III machines of plant and animal pathogens suggests universal recognition of mRNA targeting signals

Bacterial pathogens of both animals and plants use type III secretion machines to inject virulence proteins into host cells. Although many components of the secretion machinery are conserved among different bacterial species, the substrates for their type III pathways are not. The Yersinia type III machinery recognizes some secretion substrates via a signal that is encoded within the first 15 codons of yop mRNA. These signals can be altered by frameshift mutations without affecting secretion of the encoded polypeptides, suggesting a mechanism whereby translation of yop mRNA is coupled to the translocation of newly synthesized polypeptide. We report that the type III machinery of Erwinia chrysanthemi cloned in Escherichia coli recognizes the secretion signals of yopE and yopQ. Pseudomonas syringae AvrB and AvrPto, two proteins exported by the recombinant Erwinia machine, can also be secreted by the Yersinia type III pathway. Mapping AvrPto sequences sufficient for the secretion of reporter fusions in Yersinia revealed the presence of an mRNA secretion signal. We propose that 11 conserved components of type III secretion machines may recognize signals that couple mRNA translation to polypeptide secretion.

The cytotoxin YopT of Yersinia enterocolitica induces modification and cellular redistribution of the small GTP-binding protein RhoA

Pathogenic Yersinia enterocolitica produces two virulence plasmid-encoded cytotoxins, YopE and YopT, that are translocated into target cells where they disrupt the actin cytoskeleton. Here we show that infection of cells with wild type Y. enterocolitica and a yopE mutant, but not with a yopT mutant, induces an increase in the electrophoretic mobility of the small GTPase RhoA. As tested by isoelectric focusing, YopT-dependent modification resulted in an acidic shift of RhoA. Furthermore, RhoA modification induced by YopT was accompanied by redistribution of membrane-bound RhoA toward the cytosol. Finally, a yopE mutant of Y. enterocolitica expressing the cytotoxic activity of YopT specifically disrupted RhoA-controlled actin stress fibers. These findings provide evidence for inactivation of RhoA by the translocated Y. enterocolitica cytotoxin YopT and suggest a novel inhibitory modification of RhoA by a bacterial virulence factor.

Virulence role of V antigen of Yersinia pestis at the bacterial surface

Yersinia pestis, the etiologic agent of plague, secretes a set of environmentally regulated, plasmid pCD1-encoded virulence proteins termed Yops and V antigen (LcrV) by a type III secretion mechanism (Ysc). LcrV is a multifunctional protein that has been shown to act at the level of secretion control by binding the Ysc inner-gate protein LcrG and to modulate the host immune response by altering cytokine production. LcrV also is essential for the unidirectional targeting of Yops to the cytosol of infected eukaryotic cells. In this study, we constructed an in-frame deletion within lcrG (DeltalcrG3) to further analyze the requirement of LcrV in Yop targeting. We confirmed the essentiality of LcrV and found that LcrG may have a facilitative role, perhaps by promoting efficient secretion of LcrV. We also constructed mutants of lcrV expressing LcrV truncated at the N or C terminus. Both the N and C termini of LcrV were required for the secretion of LcrV into the medium and targeting of Yops. LcrV was detected in punctate zones on the surface of fixed Y. pestis by laser-scanning confocal microscopy, and this localization required a functional Ysc. However, the truncated LcrV proteins were not found on the bacterial surface. Finally, we tested the ability of LcrV-specific Fab antibody fragments or full-length antibody to interfere with Yop targeting and found no interference, even though this antibody protects mice against plague. These results indicate that LcrV may function in Yop targeting at the extracellular surface of yersiniae and that the protective efficacy of LcrV-specific antibodies can be manifested without blocking Yop targeting.

Inhibition of the mitogen-activated protein kinase kinase superfamily by a Yersinia effector

The bacterial pathogen Yersinia uses a type III secretion system to inject several virulence factors into target cells. One of the Yersinia virulence factors, YopJ, was shown to bind directly to the superfamily of MAPK (mitogen-activated protein kinase) kinases (MKKs) blocking both phosphorylation and subsequent activation of the MKKs. These results explain the diverse activities of YopJ in inhibiting the extracellular signal-regulated kinase, c-Jun amino-terminal kinase, p38, and nuclear factor kappa B signaling pathways, preventing cytokine synthesis and promoting apoptosis. YopJ-related proteins that are found in a number of bacterial pathogens of animals and plants may function to block MKKs so that host signaling responses can be modulated upon infection.

Visualization of harpin secretion in planta during infection of apple seedlings by Erwinia amylovora

Erwinia amylovora is a Gram-negative pathogenic bacterium that infects pear and apple trees as well as other plants from the Rosaceae family. E. amylovora pathogenicity is dependent on a functional Hrp type III secretion system. Harpin, a protein playing a major role in virulence, has been shown to be exported in vitro via the type III secretion apparatus. The data presented here focus on harpin detection in planta after infection of apple seedlings with the wild-type strain CFPB 1430. Using a specific harpin antiserum, harpin was not detected inside the host plant cells, but was found associated with the bacteria and secreted. The extracellular localization of harpin is in agreement with the physiological effects induced by purified harpin when applied as an exogenous elicitor. Harpin was not found associated with the host plant cell wall, a result that weakens its postulated role in cell wall loosening. A differential labelling was observed at the bacterial level: for some bacteria, harpin was exclusively cytoplasmic, whereas in others, it appeared as small patches over the bacterial outer membrane or associated with extracellular linear structures. All the bacteria present within the same area were similarly labelled, suggesting co-ordination in the secretion process. All observations suggest that harpin is synthesized in the bacterial cytoplasm and that secretion occurs from this cytoplasmic pool upon sensing of a plant or bacterial signal.

A homologue of the Agrobacterium tumefaciens VirB and Bordetella pertussis Ptl type IV secretion systems is essential for intracellular survival of Brucella suis

Analysis of a TnblaM mutant of Brucella suis 1330, identified as being unable to multiply in Hela cells, allowed us to identify a 11 860 bp region of the B. suis genome encoding a type IV secretion system, homologous to the VirB system of Agrobacterium tumefaciens and the Ptl system of Bordetella pertussis. DNA sequence revealed 12 open reading frames (ORFs) encoding homologues of the 11 VirB proteins present in the pTi plasmid of Agrobacterium with a similar genetic organization, and a twelfth ORF encoding a putative lipoprotein, homologous to a protein involved in mating pair formation during bacterial conjugation and to adhesins used by Pseudomonas species to bind to plant roots. Phylogenetic trees based on the sequences of VirB4 and VirB9 protein homologues suggest that evolution of the systems from DNA transfer towards protein secretion did not stem from a single event but that the protein secretion systems have evolved independently. Four independent mutants in virB5, virB9 or virB10 were highly attenuated in an in vitro infection model with human macrophages. The virulence was restored by complementation with a plasmid containing the full virB region. The virB region appears to be essential for the intracellular survival and multiplication of B. suis.

LcrV of Yersinia pestis enters infected eukaryotic cells by a virulence plasmid-independent mechanism

Yersinia pestis is the causative agent of bubonic plague and possesses a set of plasmid-encoded, secretable virulence proteins termed LcrV and Yops which are essential for survival in mammalian hosts. Yops and LcrV are secreted by a type III mechanism (Ysc), and Yops are unidirectionally targeted into the cytosol of associated eukaryotic cells in a tissue culture infection model. LcrV is required for Yops targeting, and recent findings have revealed that it can localize to the bacterial surface; however, its fate in this infection model has not been investigated in detail. In this study, we compared the localization of LcrV to that of the targeted proteins YopE and YopM by immunoblot analysis of fractions of Yersinia-infected HeLa cultures or by laser-scanning confocal microscopy of infected monolayers. Both LcrV and YopE were secreted by contact-activated, extracellularly localized yersiniae and were targeted to the HeLa cell cytosol. Although a significant amount of LcrV partitioned to the culture medium (unlike YopE), this extracellular pool of LcrV was not the source of the LcrV that entered HeLa cells. Unlike targeting of YopE and YopM, targeting of LcrV occurred in the absence of a functional Ysc apparatus and other virulence plasmid (pCD1)-expressed proteins. However, the Ysc is necessary for LcrV to be released into the medium, and our recent work has shown that localization of LcrV on the bacterial surface requires the Ysc. These results indicate that two mechanisms exist for the secretion of LcrV by Y. pestis, both of which are activated by contact with eukaryotic cells. LcrV secreted by the Ysc reaches the bacterial surface and the surrounding medium, whereas the second is a novel, Ysc-independent pathway which results in localization of LcrV in the cytosol of infected cells but not the surrounding medium.

Identification of CesT, a chaperone for the type III secretion of Tir in enteropathogenic Escherichia coli

The locus of enterocyte effacement of enteropathogenic Escherichia coli encodes a type III secretion system, an outer membrane protein adhesin (intimin, the product of eae ) and Tir, a translocated protein that becomes a host cell receptor for intimin. Many type III secreted proteins require chaperones, which function to stabilize proteins, prevent inappropriate protein-protein interactions and aid in secretion. An open reading frame located between tir and eae, previously named orfU, was predicted to encode a protein with partial similarity to the Yersinia SycH chaperone. We examined the potential of the orfU gene product to serve as a chaperone for Tir. The orfU gene encoded a 15 kDa cytoplasmic protein that specifically interacted with Tir as demonstrated by the yeast two-hybrid assay, column binding and coimmunoprecipitation experiments. An orfU mutant was defective in attaching-effacing lesion formation and Tir secretion, but was unaffected in expression of other virulence factors. OrfU appeared to stabilize Tir levels in the cytoplasm, but was not absolutely necessary for secretion of Tir. Based upon the physical similarities, phenotypic characteristics and the demonstrated interaction with Tir, orfU is redesignated as cesT for the chaperone for E. coli secretion of T ir.

Purification and characterization of an extracellular protease from the fish pathogen Yersinia ruckeri and effect of culture conditions on production

A novel protease, hydrolyzing azocasein, was identified, purified, and characterized from the culture supernatant of the fish pathogen Yersinia ruckeri. Exoprotease production was detected at the end of the exponential growth phase and was temperature dependent. Activity was detected in peptone but not in Casamino Acid medium. Its synthesis appeared to be under catabolite repression and ammonium control. The protease was purified in a simple two-step procedure involving ammonium sulfate precipitation and ion-exchange chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the purified protein indicated an estimated molecular mass of 47 kDa. The protease had characteristics of a cold-adapted protein, i.e., it was more active in the range of 25 to 42 degrees C and had an optimum activity at 37 degrees C. The activation energy for the hydrolysis of azocasein was determined to be 15.53 kcal/mol, and the enzyme showed a rapid decrease in activity at 42 degrees C. The enzyme had an optimum pH of around 8. Characterization of the protease showed that it required certain cations such as Mg(2+) or Ca(2+) for maximal activity and was inhibited by EDTA, 1,10-phenanthroline, and EGTA but not by phenylmethylsulfonyl fluoride. Two N-methyl-N-nitro-N-nitrosoguanidine mutants were isolated and analyzed; one did not show caseinolytic activity and lacked the 47-kDa protein, while the other was hyperproteolytic and produced increased amounts of the 47-kDa protein. Azocasein activity, SDS-PAGE, immunoblotting by using polyclonal anti-47-kDa-protease serum, and zymogram analyses showed that protease activity was present in 8 of 14 strains tested and that two Y. ruckeri groups could be established based on the presence or absence of the 47-kDa protease.

Insertion of a Yop translocation pore into the macrophage plasma membrane by Yersinia enterocolitica: requirement for translocators YopB and YopD, but not LcrG

The Yersinia survival strategy is based on its ability to inject effector Yops into the cytosol of host cells. Translocation of these effectors across the eukaryotic cell membrane requires YopB, YopD and LcrG, but the mechanism is unclear. An effector polymutant of Y. pseudotuberculosis has a YopB-dependent contact haemolytic activity, indicating that YopB participates in the formation of a pore in the cell membrane. Here, we have investigated the formation of such a pore in the plasma membrane of macrophages. Infection of PU5-1.8 macrophages with an effector polymutant Y. enterocolitica led to complete flattening of the cells, similar to treatment with the pore-forming streptolysin O from Streptococcus pyogenes. Upon infection, cells released the low-molecular-weight marker BCECF (623 Da) but not the high-molecular-weight lactate dehydrogenase, indicating that there was no membrane lysis but, rather, insertion of a pore of small size into the macrophage plasma membrane. Permeation to lucifer yellow CH (443 Da) but not to Texas red-X phalloidin (1490 Da) supported this hypothesis. All these events were found to be dependent not only on translocator YopB as expected but also on YopD, which was required equally. In contrast, LcrG was not necessary. Consistently, lysis of sheep erythrocytes was also dependent on YopB and YopD, but not on LcrG.

Role of the Hrp type III protein secretion system in growth of Pseudomonas syringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought to better understand the role that hrp genes play in interactions of Pss with its host as they occur naturally under field conditions. Population sizes of hrp mutants with defects in genes that encode components of the Hrp secretion system (DeltahrcC::nptII and hrpJ:: OmegaSpc) and a protein secreted via the system (DeltahrpZ::nptII) were similar to B728a on germinating seeds. However, phyllosphere (i.e., leaf) population sizes of the hrcC and hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduced relative to B728a. Thus, the Hrp type III secretion system, but not HrpZ, plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere. The hrcC and hrpJ mutants caused brown spot lesions on primary leaves at a low frequency when they were inoculated onto seeds at the time of planting. Pathogenic reactions also were found when the hrp secretion mutants were co-infiltrated into bean leaves with a non-lesion-forming gacS mutant of B728a. In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be largely mediated by its requirement for growth of Pss in the phyllosphere. Without growth, disease does not occur.

DsbA is required for stable expression of outer membrane protein YscC and for efficient Yop secretion in Yersinia pestis

The role of the periplasmic disulfide oxidoreductase DsbA in Yop secretion was investigated in Yersinia pestis. A Y. pestis dsbA mutant secreted reduced amounts of the V antigen and Yops and expressed reduced amounts of the full-sized YscC protein. Site-directed mutagenesis of the four cysteine residues present in the YscC protein resulted in defects similar to those found in the dsbA mutant. These results suggest that YscC contains at least one disulfide bond that is essential for the function of this protein in Yop secretion.

Yersinia enterocolitica type III secretion. On the role of SycE in targeting YopE into HeLa cells

Yersinia enterocolitica inject toxic proteins (effector Yops) into the cytosol of eukaryotic cells by a mechanism requiring the type III machinery. Previous work mapped a signal sufficient for the targeting of fused reporter proteins to amino acids 1-100 of YopE. Targeting requires the binding of SycE to YopE residues 15-100 in the bacterial cytoplasm. We asked whether SycE functions only to stabilize YopE in the bacterial cytoplasm, or whether the secretion chaperone itself contributes to substrate recognition by the type III machinery. Fusions of glutathione S-transferase to either the N or C terminus of SycE resulted in hybrid proteins that bound YopE but prevented targeting of the export substrate into HeLa cells. As compared with wild-type SycE, glutathione S-transferase-SycE bound and stabilized YopE in the bacterial cytoplasm but failed to release the polypeptide for export by the type III machinery. Thus, it appears that SycE functions to deliver YopE to the type III secretion machinery. A model is presented that accounts for substrate recognition of effector Yops, a group of proteins that do not share amino acid sequence or physical similarities.

The tranquilizing injection of Yersinia proteins: a pathogen's strategy to resist host defense

Pathogenic bacteria of the genus Yersinia possess a type III secretion apparatus by which they can inject up to six effector proteins into host cells. These so-called effector Yops (Yersinia outer proteins) disrupt cellular immune defense functions such as TNF-alpha release, O2-production or phagocytosis and thereby allow Yersinia to grow extracellularly. Recent findings indicate that the effector Yops are highly active proteins that engage in crucial eukaryotic signaling mechanisms. For instance, the translocated tyrosine phosphatase YopH dephosphorylates the focal adhesion proteins paxillin and p130Cas within target cells. Furthermore, the Yersinia effector YopP is able to induce apoptosis in macrophages presumably by blocking MAP kinase and NFKB mediated signaling events. Here we discuss recent advances concerning the intracellular targets and biochemical signaling mechanisms regulated by the translocated Yersinia effectors.

Type III secretion machines: bacterial devices for protein delivery into host cells

Several Gram-negative pathogenic bacteria have evolved a complex protein secretion system termed type III to deliver bacterial effector proteins into host cells that then modulate host cellular functions. These bacterial devices are present in both plant and animal pathogenic bacteria and are evolutionarily related to the flagellar apparatus. Although type III secretion systems are substantially conserved, the effector molecules they deliver are unique for each bacterial species. Understanding the biology of these devices may allow the development of novel prevention and therapeutic approaches for several infectious diseases.

YscP of Yersinia pestis is a secreted component of the Yop secretion system

The Yersinia pestis low-Ca2+ response stimulon is responsible for the environmentally regulated expression and secretion of antihost proteins (V antigen and Yops). We have previously shown that yscO encodes a secreted core component of the Yop secretion (Ysc) mechanism. In this study, we constructed and characterized in-frame deletions in the adjacent gene, yscP, in the yscN-yscU operon. The DeltaP1 mutation, which removed amino acids 246 to 333 of YscP, had no effect on Yop expression or secretion, and the mutant protein, YscP1, was secreted, as was YscP in the parent. In contrast, the DeltaP2 strain expressed and secreted less of each Yop than did the parent under the inductive conditions of 37 degrees C and the absence of Ca2+, with an exception being YopE, which was only minimally affected by the mutation. The YscP2 protein, missing amino acids 57 to 324 of YscP, was expressed but not secreted by the DeltaP2 mutant. The effect of the DeltaP2 mutation was at the level of Yop secretion because YopM and V antigen still showed limited secretion when overproduced in trans. Excess YscP also affected secretion: overexpression of YscP in the parent, in either yscP mutant, or in an lcrG mutant effectively shut off secretion. However, co-overexpression of YscO and YscP had no effect on secretion, and YscP overexpression in an lcrE mutant had little effect on Yop secretion, suggesting that YscP acts, in conjunction with YscO, at the level of secretion control of LcrE at the bacterial surface. These findings place YscP among the growing family of mobile Ysc components that both affect secretion and themselves are secreted by the Ysc.