The type III secretion chaperone LcrH co-operates with YopD to establish a negative, regulatory loop for control of Yop synthesis in Yersinia pseudotuberculosis

Macrophage innate immune responses delineate between defective translocon assemblies produced by Yersinia pseudotuberculosis YopD mutants

Translocon pores formed in the eukaryotic cell membrane by a type III secretion system facilitate the translocation of immune-modulatory effector proteins into the host cell interior. The YopB and YopD proteins produced and secreted by pathogenic Yersinia spp. harboring a virulence plasmid-encoded type III secretion system perform this pore-forming translocator function. We had previously characterized in vitro T3SS function and in vivo pathogenicity of a number of strains encoding sited-directed point mutations in yopD. This resulted in the classification of mutants into three different classes based upon the severity of the phenotypic defects. To investigate the molecular and functional basis for these defects, we explored the effectiveness of RAW 264.7 cell line to respond to infection by representative YopD mutants of all three classes. Signature cytokine profiles could separate the different YopD mutants into distinct categories. The activation and suppression of certain cytokines that function as central innate immune response modulators correlated well with the ability of mutant bacteria to alter anti-phagocytosis and programmed cell death pathways. These analyses demonstrated that sub-optimal translocon pores impact the extent and magnitude of host cell responsiveness, and this limits the capacity of pathogenic Yersinia spp. to fortify against attack by both early and late arms of the host innate immune response.

Drug sensitivity and genome-wide analysis of two strains of Mycoplasma gallisepticum with different biofilm intensity

Mycoplasma gallisepticum (MG) is one of the major causative agents of chronic respiratory diseases in poultry. The biofilms of MG are highly correlated to its chronic infection. However data on genes involved in biofilm formation ability are still scarse. MG strains with distinct biofilm intensity were screened by crystal violet staining morphotyped and characterized for the drug sensitivity. Two MG strains NX-01 and NX-02 showed contrasted ability to biofilm formation. The biofilm formation ability of NX-01 strain was significantly higher than that of NX-02 strain (p < 0.01). The drug sensitivity test showed that the stronger the ability of MG stain to form biofilms, the weaker its sensitivity to 17 antibiotic drugs. Moreover, putative key genes related to biofilm formation were screened by genome-wide analysis. A total of 13 genes and proteins related to biofilm formation, including ManB, oppA, oppD, PDH, eno, RelA, msbA, deoA, gapA, rpoS, Adhesin P1 precursor, S-adenosine methionine synthetase, and methionyl tRNA synthetase were identified. There were five major discrepancies between the two isolated MG strains and the five NCBI-published MG strains. These findings provide potential targets for inhibiting the formation of biofilm of MG, and lay a foundation for treating chronic infection.

Direct interaction of a chaperone-bound type III secretion substrate with the export gate

Several gram-negative bacteria employ type III secretion systems (T3SS) to inject effector proteins into eukaryotic host cells directly from the bacterial cytoplasm. The export gate SctV (YscV in Yersinia) binds substrate:chaperone complexes such as YscX:YscY, which are essential for formation of a functional T3SS. Here, we present structures of the YscX:YscY complex alone and bound to nonameric YscV. YscX binds its chaperone YscY at two distinct sites, resembling the heterotrimeric complex of the T3SS needle subunit with its chaperone and co-chaperone. In the ternary complex the YscX N-terminus, which mediates YscX secretion, occupies a binding site within one YscV that is also used by flagellar chaperones, suggesting the interaction's importance for substrate recognition. The YscX C-terminus inserts between protomers of the YscV ring where the stalk protein binds to couple YscV to the T3SS ATPase. This primary YscV-YscX interaction is essential for the formation of a secretion-competent T3SS.

The Regulatory Circuit Underlying Downregulation of a Type III Secretion System in Yersinia enterocolitica by Transcription Factor OmpR

In a previous study, differential proteomic analysis was used to identify membrane proteins of the human enteropathogen Yersinia enterocolitica, whose levels are influenced by OmpR, the transcriptional regulator in the two-component EnvZ/OmpR system. Interestingly, this analysis demonstrated that at 37 °C, OmpR negatively affects the level of over a dozen Ysc-Yop proteins, which constitute a type III secretion system (T3SS) that is essential for the pathogenicity of Y. enterocolitica. Here, we focused our analysis on the role of OmpR in the expression and secretion of Yops (translocators and effectors). Western blotting with anti-Yops antiserum and specific anti-YopD, -YopE and -YopH antibodies, confirmed that the production of Yops is down-regulated by OmpR with the greatest negative effect on YopD. The RT-qPCR analysis demonstrated that, while OmpR had a negligible effect on the activity of regulatory genes virF and yscM1, it highly repressed the expression of yopD. OmpR was found to bind to the promoter of the lcrGVsycD-yopBD operon, suggesting a direct regulatory effect. In addition, we demonstrated that the negative regulatory influence of OmpR on the Ysc-Yop T3SS correlated with its positive role in the expression of flhDC, the master regulator of the flagellar-associated T3SS.

Type III secretion by Yersinia pseudotuberculosis is reliant upon an authentic N-terminal YscX secretor domain

YscX was discovered as an essential part of the Yersinia type III secretion system about 20 years ago. It is required for substrate secretion and is exported itself. Despite this central role, its precise function and mode of action remains unknown. In order to address this knowledge gap, this present study refocused attention on YscX to build on the recent advances in the understanding of YscX function. Our experiments identified a N-terminal secretion domain in YscX promoting its secretion, with the first five codons constituting a minimal signal capable of promoting secretion of the signalless β-lactamase reporter. Replacing the extreme YscX N-terminus with known secretion signals of other Ysc-Yop substrates revealed that the YscX N-terminal segment contains non-redundant information needed for YscX function. Further, both in cis deletion of the YscX N-terminus in the virulence plasmid and ectopic expression of epitope tagged YscX variants again lead to stable YscX production but not type III secretion of Yop effector proteins. Mislocalisation of the needle components, SctI and SctF, accompanied this general defect in Yops secretion. Hence, a coupling exists between YscX secretion permissiveness and the assembly of an operational secretion system.

LcrQ Coordinates with the YopD-LcrH Complex To Repress lcrF Expression and Control Type III Secretion by Yersinia pseudotuberculosis

Human-pathogenic Yersinia species employ a plasmid-encoded type III secretion system (T3SS) to negate immune cell function during infection. A critical element in this process is the coordinated regulation of T3SS gene expression, which involves both transcriptional and posttranscriptional mechanisms. LcrQ is one of the earliest identified negative regulators of Yersinia T3SS, but its regulatory mechanism is still unclear. In a previous study, we showed that LcrQ antagonizes the activation role played by the master transcriptional regulator LcrF. In this study, we confirm that LcrQ directly interacts with LcrH, the chaperone of YopD, to facilitate the negative regulatory role of the YopD-LcrH complex in repressing lcrF expression at the posttranscriptional level. Negative regulation is strictly dependent on the YopD-LcrH complex, more so than on LcrQ. The YopD-LcrH complex helps to retain cytoplasmic levels of LcrQ to facilitate the negative regulatory effect. Interestingly, RNase E and its associated protein RhlB participate in this negative regulatory loop through a direct interaction with LcrH and LcrQ. Hence, we present a negative regulatory loop that physically connects LcrQ to the posttranscriptional regulation of LcrF, and this mechanism incorporates RNase E involved in mRNA decay.

Molecular Targets and Strategies for Inhibition of the Bacterial Type III Secretion System (T3SS); Inhibitors Directly Binding to T3SS Components

The type III secretion system (T3SS) is a virulence apparatus used by many Gram-negative pathogenic bacteria to cause infections. Pathogens utilizing a T3SS are responsible for millions of infections yearly. Since many T3SS knockout strains are incapable of causing systemic infection, the T3SS has emerged as an attractive anti-virulence target for therapeutic design. The T3SS is a multiprotein molecular syringe that enables pathogens to inject effector proteins into host cells. These effectors modify host cell mechanisms in a variety of ways beneficial to the pathogen. Due to the T3SS’s complex nature, there are numerous ways in which it can be targeted. This review will be focused on the direct targeting of components of the T3SS, including the needle, translocon, basal body, sorting platform, and effector proteins. Inhibitors will be considered a direct inhibitor if they have a binding partner that is a T3SS component, regardless of the inhibitory effect being structural or functional.

Backbone Interactions Between Transcriptional Activator ExsA and Anti-Activator ExsD Facilitate Regulation of the Type III Secretion System in Pseudomonas aeruginosa

The type III secretion system (T3SS) is a pivotal virulence mechanism of many Gram-negative bacteria. During infection, the syringe-like T3SS injects cytotoxic proteins directly into the eukaryotic host cell cytoplasm. In Pseudomonas aeruginosa, expression of the T3SS is regulated by a signaling cascade involving the proteins ExsA, ExsC, ExsD, and ExsE. The AraC-type transcription factor ExsA activates transcription of all T3SS-associated genes. Prior to host cell contact, ExsA is inhibited through direct binding of the anti-activator protein ExsD. Host cell contact triggers secretion of ExsE and sequestration of ExsD by ExsC to cause the release of ExsA. ExsA does not bind ExsD through the canonical ligand binding pocket of AraC-type proteins. Using site-directed mutagenesis and a specific in vitro transcription assay, we have now discovered that backbone interactions between the amino terminus of ExsD and the ExsA beta barrel constitute a pivotal part of the ExsD-ExsA interface. Follow-up bacterial two-hybrid experiments suggest additional contacts create an even larger protein–protein interface. The discovered role of the amino terminus of ExsD in ExsA binding explains how ExsC might relieve the ExsD-mediated inhibition of T3SS gene expression, because the same region of ExsD interacts with ExsC following host cell contact.

A bacterial secreted translocator hijacks riboregulators to control type III secretion in response to host cell contact

Numerous Gram-negative pathogens use a Type III Secretion System (T3SS) to promote virulence by injecting effector proteins into targeted host cells, which subvert host cell processes. Expression of T3SS and the effectors is triggered upon host cell contact, but the underlying mechanism is poorly understood. Here, we report a novel strategy of Yersinia pseudotuberculosis in which this pathogen uses a secreted T3SS translocator protein (YopD) to control global RNA regulators. Secretion of the YopD translocator upon host cell contact increases the ratio of post-transcriptional regulator CsrA to its antagonistic small RNAs CsrB and CsrC and reduces the degradosome components PNPase and RNase E levels. This substantially elevates the amount of the common transcriptional activator (LcrF) of T3SS/Yop effector genes and triggers the synthesis of associated virulence-relevant traits. The observed hijacking of global riboregulators allows the pathogen to coordinate virulence factor expression and also readjusts its physiological response upon host cell contact.

YopN Is Required for Efficient Effector Translocation and Virulence in Yersinia pseudotuberculosis

Type III secretion systems (T3SSs) are used by various Gram-negative pathogens to subvert the host defense by a host cell contact-dependent mechanism to secrete and translocate virulence effectors. While the effectors differ between pathogens and determine the pathogenic life style, the overall mechanism of secretion and translocation is conserved. T3SSs are regulated at multiple levels, and some secreted substrates have also been shown to function in regulation. In Yersinia, one of the substrates, YopN, has long been known to function in the host cell contact-dependent regulation of the T3SS. Prior to contact, through its interaction with TyeA, YopN blocks secretion. Upon cell contact, TyeA dissociates from YopN, which is secreted by the T3SS, resulting in the induction of the system. YopN has also been shown to be translocated into target cells by a T3SS-dependent mechanism. However, no intracellular function has yet been assigned to YopN. The regulatory role of YopN involves the N-terminal and C-terminal parts, while less is known about the role of the central region of YopN. Here, we constructed different in-frame deletion mutants within the central region. The deletion of amino acids 76 to 181 resulted in an unaltered regulation of Yop expression and secretion but triggered reduced YopE and YopH translocation within the first 30 min after infection. As a consequence, this deletion mutant lost its ability to block phagocytosis by macrophages. In conclusion, we were able to differentiate the function of YopN in translocation and virulence from its function in regulation.

Effects of Psidium guajava leaf extract on secretion systems of Gram-negative enteropathogenic bacteria

We screened a total of 672 plant-tissue extracts to search for phytochemicals that inhibit the function of the type III secretion system (T3SS) of enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC). Among candidates examined, we found that an extract from the leaves of Psidium guajava (guava) inhibited the secretion of the EspB protein from EPEC and EHEC without affecting bacterial growth. The guava extract (GE) also inhibited EPEC and EHEC from adhering to and injecting EspB protein into HEp-2 cells. GE seemed to block the translocation of EspB from the bacterial cells to the culture medium. In addition to EPEC and EHEC, GE also inhibited the T3SS of Yersinia pseudotuberculosis and Salmonella enterica serovar Typhimurium. After exposure to GE, Y. pseudotuberculosis stopped the secretion of Yop proteins and lost its ability to induce the apoptosis of mouse bone marrow-derived macrophages. S. Typhimurium exposed to GE ceased the secretion of Sip proteins and lost its ability to invade HEp-2 cells. GE inhibited EspC secretion, the type V secretion protein of EPEC, but not Shiga toxin2 from EHEC. Thus, our results suggest that guava leaves contain a novel type of antimicrobial compound that could be used for the therapeutic treatment and prevention of gram-negative enteropathogenic bacterial infections.

Heterologous Complementation Studies With the YscX and YscY Protein Families Reveals a Specificity for Yersinia pseudotuberculosis Type III Secretion

Type III secretion systems harbored by several Gram-negative bacteria are often used to deliver host-modulating effectors into infected eukaryotic cells. About 20 core proteins are needed for assembly of a secretion apparatus. Several of these proteins are genetically and functionally conserved in type III secretion systems of bacteria associated with invertebrate or vertebrate hosts. In the Ysc family of type III secretion systems are two poorly characterized protein families, the YscX family and the YscY family. In the plasmid-encoded Ysc-Yop type III secretion system of human pathogenic Yersinia species, YscX is a secreted substrate while YscY is its non-secreted cognate chaperone. Critically, neither an yscX nor yscY null mutant of Yersinia is capable of type III secretion. In this study, we show that the genetic equivalents of these proteins produced as components of other type III secretion systems of Pseudomonas aeruginosa (PscX and PscY), Aeromonas species (AscX and AscY), Vibrio species (VscX and VscY), and Photorhabdus luminescens (SctX and SctY) all possess an ability to interact with its native cognate partner and also establish cross-reciprocal binding to non-cognate partners as judged by a yeast two-hybrid assay. Moreover, a yeast three-hybrid assay also revealed that these heterodimeric complexes could maintain an interaction with YscV family members, a core membrane component of all type III secretion systems. Despite maintaining these molecular interactions, only expression of the native yscX in the near full-length yscX deletion and native yscY in the near full-length yscY deletion were able to complement for their general substrate secretion defects. Hence, YscX and YscY must have co-evolved to confer an important function specifically critical for Yersinia type III secretion.

Site-Directed Mutagenesis and Its Application in Studying the Interactions of T3S Components

Type III secretion systems are a prolific virulence determinant among Gram-negative bacteria. They are used to paralyze the host cell, which enables bacterial pathogens to establish often fatal infections-unless an effective therapeutic intervention is available. However, as a result of a catastrophic rise in infectious bacteria resistant to conventional antibiotics, these bacteria are again a leading cause of worldwide mortality. Hence, this report describes a pDM4-based site-directed mutagenesis strategy that is assisting in our foremost objective to better understand the fundamental workings of the T3SS, using Yersinia as a model pathogenic bacterium. Examples are given that clearly document how pDM4-mediated site-directed mutagenesis has been used to establish clean point mutations and in-frame deletion mutations that have been instrumental in identifying and understanding the molecular interactions between components of the Yersinia type III secretion system.

Type III secretion systems: the bacterial flagellum and the injectisome

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

The Burkholderia pseudomallei Proteins BapA and BapC Are Secreted TTSS3 Effectors and BapB Levels Modulate Expression of BopE

Many Gram-negative pathogens use a type III secretion system (TTSS) for the injection of bacterial effector proteins into host cells. The injected effector proteins play direct roles in modulation of host cell pathways for bacterial benefit. Burkholderia pseudomallei, the causative agent of melioidosis, expresses three different TTSSs. One of these systems, the TTSS3, is essential for escape from host endosomes and therefore intracellular survival and replication. Here we have characterized three putative TTSS3 proteins; namely BapA, BapB and BapC. By employing a tetracysteine (TC)-FlAsH^™ labelling technique to monitor the secretion of TC-tagged fusion proteins, BapA and BapC were shown to be secreted during in vitro growth in a TTSS3-dependant manner, suggesting a role as TTSS3 effectors. Furthermore, we constructed B. pseudomallei bapA, bapB and bapC mutants and used the well-characterized TTSS3 effector BopE as a marker of secretion to show that BapA, BapB and BapC are not essential for the secretion process. However, BopE transcription and secretion were significantly increased in the bapB mutant, suggesting that BapB levels modulate BopE expression. In a BALB/c mouse model of acute melioidosis, the bapA, bapB and bapC mutants showed a minor reduction of in vivo fitness. Thus, this study defines BapA and BapC as novel TTSS3 effectors, BapB as a regulator of BopE production, and all three as necessary for full B. pseudomallei in vivo fitness.

Random mutagenesis identifies a C-terminal region of YopD important for Yersinia type III secretion function

A common virulence mechanism among bacterial pathogens is the use of specialized secretion systems that deliver virulence proteins through a translocation channel inserted in the host cell membrane. During Yersinia infection, the host recognizes the type III secretion system mounting a pro-inflammatory response. However, soon after they are translocated, the effectors efficiently counteract that response. In this study we sought to identify YopD residues responsible for type III secretion system function. Through random mutagenesis, we identified eight Y. pseudotuberculosis yopD mutants with single amino acid changes affecting various type III secretion functions. Three severely defective mutants had substitutions in residues encompassing a 35 amino acid region (residues 168-203) located between the transmembrane domain and the C-terminal putative coiled-coil region of YopD. These mutations did not affect regulation of the low calcium response or YopB-YopD interaction but markedly inhibited MAPK and NFκB. [corrected] activation. When some of these mutations were introduced into the native yopD gene, defects in effector translocation and pore formation were also observed. We conclude that this newly identified region is important for YopD translocon function. The role of this domain in vivo remains elusive, as amino acid substitutions in that region did not significantly affect virulence of Y. pseudotuberculosis in orogastrically-infected mice.

RcsB positively regulates the Yersinia Ysc-Yop type III secretion system by activating expression of the master transcriptional regulator LcrF

The Rcs phosphorelay is a complex signaling pathway used by the family Enterobacteriaceae to sense, respond and adapt to environmental changes during free-living or host-associated lifestyles. In this study, we show that the Rcs phosphorelay pathway positively regulates the virulence plasmid encoded Ysc-Yop type III secretion system (T3SS) in the enteropathogen Yesinia pseudotuberculosis. Both the overexpression of the wild-type Rcs regulator RcsB or the constitutive active RscB(D56E) variant triggered more abundant Ysc-Yop synthesis and secretion, whereas the non-phosphorylatable mutant RcsB(D56Q) negated this. Congruently, enhanced Yops expression and secretion occurred in an in cis rscB(D56E) mutant but not in an isogenic rscB(D56Q) mutant. Screening for regulatory targets of RcsB identified the virG-lcrF operon that encodes for LcrF, the Ysc-Yop T3SS master regulator. Protein-DNA binding assays confirmed that RcsB directly bound to this operon promoter, which subsequently caused stimulated lcrF transcription. Moreover, active RcsB enhanced the ability of bacteria to deliver Yop effectors into immune cells during cell contact, and this promoted an increase in bacterial viability. Taken together, our study demonstrates the role of the Rcs system in regulating the Ysc-Yop T3SS in Yersinia and reports on RcsB being the first transcriptional activator known to directly control lcrF transcription.

The Chlamydia trachomatis type III secretion chaperone Slc1 engages multiple early effectors, including TepP, a tyrosine-phosphorylated protein required for the recruitment of CrkI-II to nascent inclusions and innate immune signaling

Chlamydia trachomatis, the causative agent of trachoma and sexually transmitted infections, employs a type III secretion (T3S) system to deliver effector proteins into host epithelial cells to establish a replicative vacuole. Aside from the phosphoprotein TARP, a Chlamydia effector that promotes actin re-arrangements, very few factors mediating bacterial entry and early inclusion establishment have been characterized. Like many T3S effectors, TARP requires a chaperone (Slc1) for efficient translocation into host cells. In this study, we defined proteins that associate with Slc1 in invasive C. trachomatis elementary bodies (EB) by immunoprecipitation coupled with mass spectrometry. We identified Ct875, a new Slc1 client protein and T3S effector, which we renamed TepP (Translocated early phosphoprotein). We provide evidence that T3S effectors form large molecular weight complexes with Scl1 in vitro and that Slc1 enhances their T3S-dependent secretion in a heterologous Yersinia T3S system. We demonstrate that TepP is translocated early during bacterial entry into epithelial cells and is phosphorylated at tyrosine residues by host kinases. However, TepP phosphorylation occurs later than TARP, which together with the finding that Slc1 preferentially engages TARP in EBs leads us to postulate that these effectors are translocated into the host cell at different stages during C. trachomatis invasion. TepP co-immunoprecipitated with the scaffolding proteins CrkI-II during infection and Crk was recruited to EBs at entry sites where it remained associated with nascent inclusions. Importantly, C. trachomatis mutants lacking TepP failed to recruit CrkI-II to inclusions, providing genetic confirmation of a direct role for this effector in the recruitment of a host factor. Finally, endocervical epithelial cells infected with a tepP mutant showed altered expression of a subset of genes associated with innate immune responses. We propose a model wherein TepP acts downstream of TARP to recruit scaffolding proteins at entry sites to initiate and amplify signaling cascades important for the regulation of innate immune responses to Chlamydia.

Yersinia Ysc-Yop type III secretion feedback inhibition is relieved through YscV-dependent recognition and secretion of LcrQ

Human pathogenic Yersinia species share a virulence plasmid encoding the Ysc-Yop type III secretion system (T3SS). A plasmid-encoded anti-activator, LcrQ, negatively regulates the expression of this secretion system. Under inducible conditions, LcrQ is secreted outside of bacterial cells and this activates the T3SS, but the mechanism of targeting LcrQ for type III secretion remains largely unknown. In this study, we characterized the regulatory role of the export apparatus component YscV. Depletion or overexpression of YscV compromised Yop synthesis and this primarily prevented secretion of LcrQ. It followed that a lcrQ deletion reversed the repressive effects of excessive YscV. Further characterization demonstrated that the YscV residues 493-511 located within the C-terminal soluble cytoplasmic domain directly bound with LcrQ. Critically, YscV-LcrQ complex formation was a requirement for LcrQ secretion, since YscVΔ493-511 failed to secrete LcrQ. This forced a cytoplasmic accumulation of LcrQ, which predictably caused the feedback inhibition of Yops synthesis. Based on these observations, we proposed a model for the YscV-dependent secretion of LcrQ and its role in regulating Yop synthesis in Yersinia.

A type III secretion system inhibitor targets YopD while revealing differential regulation of secretion in calcium-blind mutants of Yersinia pestis

Numerous Gram-negative pathogens rely upon type III secretion (T3S) systems to cause disease. Several small-molecule inhibitors of the type III secretion systems have been identified; however, few targets of these inhibitors have been elucidated. Here we report that 2,2'-thiobis-(4-methylphenol) (compound D), inhibits type III secretion in Yersinia pestis, Yersinia pseudotuberculosis, and Pseudomonas aeruginosa. YopD, a protein involved in the formation of the translocon and regulatory processes of the type III secretion system, appears to play a role in the inhibition of secretion by compound D. The use of compound D in T3S regulatory mutants demonstrated a difference in secretion inhibition in the presence and absence of calcium. Interestingly, compound D was effective only under conditions without calcium, indicating that a secretion-active needle structure may be necessary for compound D to inhibit secretion.

Translational regulation of Yersinia enterocolitica mRNA encoding a type III secretion substrate

Yersinia enterocolitica type III secretion machines transport YopQ and other Yop effectors into host immune cells. YopD and its chaperone LcrH are essential components of the Yersinia type III pathway, enabling effector translocation into host cells. YopD, LcrH, and YscM1 also regulate yop expression post-transcriptionally in response to environmental signals; however, the molecular mechanisms for this regulation and Yop secretion are unknown. We show here that YopD associates with 30 S ribosomal particles in a manner requiring LcrH. When added to ribosomes, YopD, LcrH, and YscM1 block the translation of yopQ mRNA. We propose a model whereby LcrH-dependent association of YopD with 30 S ribosomal particles enables YscM1 to block yopQ translation unless type III machines are induced to secrete the effector.

The SseC translocon component in Salmonella enterica serovar Typhimurium is chaperoned by SscA

BACKGROUND

Salmonella enterica is a causative agent of foodborne gastroenteritis and the systemic disease known as typhoid fever. This bacterium uses two type three secretion systems (T3SSs) to translocate protein effectors into host cells to manipulate cellular function. Salmonella pathogenicity island (SPI)-2 encodes a T3SS required for intracellular survival of the pathogen. Genes in SPI-2 include apparatus components, secreted effectors and chaperones that bind to secreted cargo to coordinate their release from the bacterial cell. Although the effector repertoire secreted by the SPI-2 T3SS is large, only three virulence-associated chaperones have been characterized.

RESULTS

Here we report that SscA is the chaperone for the SseC translocon component. We show that SscA and SseC interact in bacterial cells and that deletion of sscA results in a loss of SseC secretion, which compromises intracellular replication and leads to a loss of competitive fitness in mice.

CONCLUSIONS

This work completes the characterization of the chaperone complement within SPI-2 and identifies SscA as the chaperone for the SseC translocon.

Type III secretion translocon assemblies that attenuate Yersinia virulence

Type III secretion enables bacteria to intoxicate eukaryotic cells with anti-host effectors. A class of secreted cargo are the two hydrophobic translocators that form a translocon pore in the host cell plasma membrane through which the translocated effectors may gain cellular entry. In pathogenic Yersinia, YopB and YopD shape this translocon pore. Here, four in cis yopD mutations were constructed to disrupt a predicted α-helix motif at the C-terminus. Mutants YopD(I262P) and YopD(K267P) poorly localized Yop effectors into target eukaryotic cells and failed to resist uptake and killing by immune cells. These defects were due to deficiencies in host-membrane insertion of the YopD-YopB translocon. Mutants YopDA(263P) and YopD(A270P) had no measurable in vitro translocation defect, even though they formed smaller translocon pores in erythrocyte membranes. Despite this, all four mutants were attenuated in a mouse infection model. Hence, YopD variants have been generated that can spawn translocons capable of targeting effectors in vitro, yet were bereft of any lethal effect in vivo. Therefore, Yop translocators may possess other in vivo functions that extend beyond being a portal for effector delivery into host cells.

Tetratricopeptide repeat motifs in the world of bacterial pathogens: role in virulence mechanisms

The tetratricopeptide repeat (TPR) structural motif is known to occur in a wide variety of proteins present in prokaryotic and eukaryotic organisms. The TPR motif represents an elegant module for the assembly of various multiprotein complexes, and thus, TPR-containing proteins often play roles in vital cell processes. As the TPR profile is well defined, the complete TPR protein repertoire of a bacterium with a known genomic sequence can be predicted. This provides a tremendous opportunity for investigators to identify new TPR-containing proteins and study them in detail. In the past decade, TPR-containing proteins of bacterial pathogens have been reported to be directly related to virulence-associated functions. In this minireview, we summarize the current knowledge of the TPR-containing proteins involved in virulence mechanisms of bacterial pathogens while highlighting the importance of TPR motifs for the proper functioning of class II chaperones of a type III secretion system in the pathogenesis of Yersinia, Pseudomonas, and Shigella.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Crystal structure of the Yersinia enterocolitica type III secretion chaperone SycD in complex with a peptide of the minor translocator YopD

Background

Type III secretion systems are used by Gram-negative bacteria as “macromolecular syringes” to inject effector proteins into eukaryotic cells. Two hydrophobic proteins called translocators form the necessary pore in the host cell membrane. Both translocators depend on binding to a single chaperone in the bacterial cytoplasm to ensure their stability and efficient transport through the secretion needle. It was suggested that the conserved chaperones bind the more divergent translocators via a hexapeptide motif that is found in both translocators and conserved between species.

Results

We crystallized a synthetic decapeptide from the Yersinia enterocolitica minor type III secretion translocator YopD bound to its cognate chaperone SycD and determined the complex structure at 2.5 Å resolution. The structure of peptide-bound SycD is almost identical to that of apo SycD with an all helical fold consisting of three tetratricopeptide repeats (TPRs) and an additional C-terminal helix. Peptide-bound SycD formed a kinked head-to-head dimer that had previously been observed for the apo form of SycD. The homodimer interface comprises both helices of the first tetratricopeptide repeat. The YopD peptide bound in extended conformation into a mainly hydrophobic groove on the concave side of SycD. TPRs 1 and 2 of SycD form three hydrophobic pockets that accommodated the conserved hydrophobic residues at position 1, 3 and 6 of the translocator hexapeptide sequence. Two tyrosines that are highly conserved among translocator chaperones contribute to the hydrophobic patches but also form hydrogen bonds to the peptide backbone.

Conclusions

The interaction between SycD and YopD is very similar to the binding of the Pseudomonas minor translocator PopD to its chaperone PcrH and the Shigella major translocator IpaB to its chaperone IpgC. This confirms the prediction made by Kolbe and co-workers that a hexapeptide with hydrophobic residues at three positions is a conserved chaperone binding motif. Because the hydrophobic groove on the concave side of translocator chaperones is involved in binding of the major and the minor translocator, simultaneous binding of both translocators to a single type III secretion class II chaperone appears unlikely.

Impact of the N-terminal secretor domain on YopD translocator function in Yersinia pseudotuberculosis type III secretion

Type III secretion systems (T3SSs) secrete needle components, pore-forming translocators, and the translocated effectors. In part, effector recognition by a T3SS involves their N-terminal amino acids and their 5' mRNA. To investigate whether similar molecular constraints influence translocator secretion, we scrutinized this region within YopD from Yersinia pseudotuberculosis. Mutations in the 5' end of yopD that resulted in specific disruption of the mRNA sequence did not affect YopD secretion. On the other hand, a few mutations affecting the protein sequence reduced secretion. Translational reporter fusions identified the first five codons as a minimal N-terminal secretion signal and also indicated that the YopD N terminus might be important for yopD translation control. Hybrid proteins in which the N terminus of YopD was exchanged with the equivalent region of the YopE effector or the YopB translocator were also constructed. While the in vitro secretion profile was unaltered, these modified bacteria were all compromised with respect to T3SS activity in the presence of immune cells. Thus, the YopD N terminus does harbor a secretion signal that may also incorporate mechanisms of yopD translation control. This signal tolerates a high degree of variation while still maintaining secretion competence suggestive of inherent structural peculiarities that make it distinct from secretion signals of other T3SS substrates.

BopC is a type III secreted effector protein of Burkholderia pseudomallei

Burkholderia pseudomallei, the causative agent of melioidosis, exploits the Bsa type III secretion system (T3SS) to deliver effector proteins into host cells. These effectors manipulate host cell functions; thus, contributing to the ability of the bacteria to evade the immune response and cause disease. Only two Bsa-secreted effectors have been conclusively identified to date. Here, we report the identification of the third B. pseudomallei type III secreted effector protein, designated BopC. BopC is encoded by the bpss1516 gene abutting bpss1517, which encodes its putative chaperone. The genes are located in the close proximity to the bsa T3SS gene cluster of B. pseudomallei K96243 (Fig. 1). BopC was secreted into culture supernatant by the wild-type B. pseudomallei strain, but its secretion was abolished in the bsaZ T3SS mutant. Using pull down and co-purification assays, we confirmed that BopC interacts with its putative chaperone, BPSS1517, in vitro. Furthermore, the first 20 N-terminal amino acids of BopC were found to be sufficient to mediate the T3SS-dependent translocation of a reporter protein from a heterologous enteropathogenic Escherichia coli host into mammalian cells. Finally, bopC mutant was found to be less invasive than the wild-type strain in the epithelial cells.

Scc1 (CP0432) and Scc4 (CP0033) function as a type III secretion chaperone for CopN of Chlamydia pneumoniae

The Chlamydia pneumoniae CopN protein is a member of the YopN/TyeA/InvE/MxiC family of secreted proteins that function to regulate the secretion of type III secretion system (T3SS) translocator and effector proteins. In this study, the Scc1 (CP0432) and Scc4 (CP0033) proteins of C. pneumoniae AR-39 were demonstrated to function together as a type III secretion chaperone that binds to an N-terminal region of CopN. The Scc1/Scc4 chaperone promoted the efficient secretion of CopN via a heterologous T3SS, whereas, the Scc3 chaperone, which binds to a C-terminal region of CopN, reduced CopN secretion.

YopD self-assembly and binding to LcrV facilitate type III secretion activity by Yersinia pseudotuberculosis

YopD-like translocator proteins encoded by several Gram-negative bacteria are important for type III secretion-dependent delivery of anti-host effectors into eukaryotic cells. This probably depends on their ability to form pores in the infected cell plasma membrane, through which effectors may gain access to the cell interior. In addition, Yersinia YopD is a negative regulator essential for the control of effector synthesis and secretion. As a prerequisite for this functional duality, YopD may need to establish molecular interactions with other key T3S components. A putative coiled-coil domain and an alpha-helical amphipathic domain, both situated in the YopD C terminus, may represent key protein-protein interaction domains. Therefore, residues within the YopD C terminus were systematically mutagenized. All 68 mutant bacteria were first screened in a variety of assays designed to identify individual residues essential for YopD function, possibly by providing the interaction interface for the docking of other T3S proteins. Mirroring the effect of a full-length yopD gene deletion, five mutant bacteria were defective for both yop regulatory control and effector delivery. Interestingly, all mutations clustered to hydrophobic amino acids of the amphipathic domain. Also situated within this domain, two additional mutants rendered YopD primarily defective in the control of Yop synthesis and secretion. Significantly, protein-protein interaction studies revealed that functionally compromised YopD variants were also defective in self-oligomerization and in the ability to engage another translocator protein, LcrV. Thus, the YopD amphipathic domain facilitates the formation of YopD/YopD and YopD/LcrV interactions, two critical events in the type III secretion process.

Electrostatic interactions play a minor role in the binding of ExoS to 14-3-3 proteins

14-3-3 proteins belong to a family of conserved molecules expressed in all eukaryotic cells that play an important role in a multitude of signalling pathways. 14-3-3 proteins bind either to phosphoserine/phosphothreonine residues or to sequence-specific non-phosphorylated motifs in more than 200 interaction partners [Pozuelo Rubio, Geraghty, Wong, Wood, Campbell, Morrice and Mackintosh (2004) Biochem. J. 379, 395-408]. These interactions result in cell-cycle regulation, apoptosis, stress responses, cell metabolism and malignant transformation. One example of a phosphorylation-independent interaction is the binding of 14-3-3 to ExoS (exoenzyme S), a bacterial ADP-ribosyltransferase toxin of Pseudomonas aeruginosa. In the present study, we have utilized additional biochemical and infection analyses to define further the structural basis of the interaction between ExoS and 14-3-3. An ExoS leucine-substitution mutant dramatically reduced the interaction potential with 14-3-3 suggesting that Leu422, Leu423, Leu426 and Leu428 of ExoS are important for its interaction with 14-3-3, its enzymatic activity and cytotoxicity. However, ExoS substitution mutants of residues that interact with 14-3-3 through an electrostatic interaction, such as Ser416, His418, Asp424 and Asp427, showed no reduction in their interaction potential with 14-3-3. These ExoS substitution mutants were also as aggressive as wild-type ExoS at inducing cell death and to modify endogenous ExoS target within the cell. In conclusion, electrostatic interaction between ExoS and 14-3-3 via polar residues (Ser416, His418, Asp424 and Asp427) appears to be of secondary importance. Thus the interaction between the 'roof' of the groove of 14-3-3 and ExoS relies more on hydrophobic interaction forces, which probably contributes to induce cell death after ExoS infection and activation.

A dominant-negative needle mutant blocks type III secretion of early but not late substrates in Yersinia

Yersinia pseudotuberculosis uses a type III secretion system (T3SS) to deliver effectors into host cells. A key component of the T3SS is the needle, which is a hollow tube on the bacterial surface through which effectors are secreted, composed of the YscF protein. To study needle assembly, we performed a screen for dominant-negative yscF alleles that prevented effector secretion in the presence of wild-type (WT) YscF. One allele, yscF-L54V, prevents WT YscF secretion and needle assembly, although purified YscF-L54V polymerizes in vitro. YscF-L54V binds to its chaperones YscE and YscG, and the YscF-L54V-EG complex targets to the T3SS ATPase, YscN. We propose that YscF-L54V stalls at a binding site in the needle assembly pathway following its release from the chaperones, which blocks the secretion of WT YscF and other early substrates required for building a needle. Interestingly, YscF-L54V does not affect the activity of pre-assembled actively secreting machines, indicating that a factor and/or binding site required for YscF secretion is absent from T3SS machines already engaged in effector secretion. Thus, substrate switching may involve the removal of an early substrate-specific binding site as a mechanism to exclude early substrates from Yop-secreting machines.

Mapping of the chaperone AcrH binding regions of translocators AopB and AopD and characterization of oligomeric and metastable AcrH-AopB-AopD complexes in the type III secretion system of Aeromonas hydrophila

In the type III secretion system (T3SS) of Aeromonas hydrophila, AcrH acts as a chaperone for translocators AopB and AopD. AcrH forms a stable 1:1 monomeric complex with AopD, whereas the 1:1 AcrH-AopB complex exists mainly as a metastable oligomeric form and only in minor amounts as a stable monomeric form. Limited protease digestion shows that these complexes contain highly exposed regions, thus allowing mapping of intact functional chaperone binding regions of AopB and AopD. AopD uses the transmembrane domain (DF1, residues 16-147) and the C-terminal amphipathic helical domain (DF2, residues 242-296) whereas AopB uses a discrete region containing the transmembrane domain and the putative N-terminal coiled coil domain (BF1, residues 33-264). Oligomerization of the AcrH-AopB complex is mainly through the C-terminal coiled coil domain of AopB, which is dispensable for chaperone binding. The three proteins, AcrH, AopB, and AopD, can be coexpressed to form an oligomeric and metastable complex. These three proteins are also oligomerized mainly through the C-terminal domain of AopB. Formation of such an oligomeric and metastable complex may be important for the proper formation of translocon of correct topology and stoichiometry on the host membrane.

Functional characterization of SsaE, a novel chaperone protein of the type III secretion system encoded by Salmonella pathogenicity island 2

The type III secretion system (T3SS) encoded by Salmonella pathogenicity island 2 (SPI-2) is involved in systemic infection and intracellular replication of Salmonella enterica serovar Typhimurium. In this study, we investigated the function of SsaE, a small cytoplasmic protein encoded within the SPI-2 locus, which shows structural similarity to the T3SS class V chaperones. An S. enterica serovar Typhimurium ssaE mutant failed to secrete SPI-2 translocator SseB and SPI-2-dependent effector PipB proteins. Coimmunoprecipitation and mass spectrometry analyses using an SsaE-FLAG fusion protein indicated that SsaE interacts with SseB and a putative T3SS-associated ATPase, SsaN. A series of deleted and point-mutated SsaE-FLAG fusion proteins revealed that the C-terminal coiled-coil domain of SsaE is critical for protein-protein interactions. Although SseA was reported to be a chaperone for SseB and to be required for its secretion and stability in the bacterial cytoplasm, an sseA deletion mutant was able to secrete the SseB in vitro when plasmid-derived SseB was overexpressed. In contrast, ssaE mutant strains could not transport SseB extracellularly under the same assay conditions. In addition, an ssaE(I55G) point-mutated strain that expresses the SsaE derivative lacking the ability to form a C-terminal coiled-coil structure showed attenuated virulence comparable to that of an SPI-2 T3SS null mutant, suggesting that the coiled-coil interaction of SsaE is absolutely essential for the functional SPI-2 T3SS and for Salmonella virulence. Based on these findings, we propose that SsaE recognizes translocator SseB and controls its secretion via SPI-2 type III secretion machinery.

Growth of calcium-blind mutants of Yersinia pestis at 37 degrees C in permissive Ca2+-deficient environments

Cells of wild-type Yersinia pestis exhibit a low-calcium response (LCR) defined as bacteriostasis with expression of a pCD-encoded type III secretion system (T3SS) during cultivation at 37 degrees C without added Ca(2+) versus vegetative growth with downregulation of the T3SS with Ca(2+) (>or=2.5 mM). Bacteriostasis is known to reflect cumulative toxicity of Na(+), l-glutamic acid and culture pH; control of these variables enables full-scale growth ('rescue') in the absence of Ca(2+). Several T3SS regulatory proteins modulate the LCR, because their absence promotes a Ca(2+)-blind phenotype in which growth at 37 degrees C ceases and the T3SS is constitutive even with added Ca(2+). This study analysed the connection between the LCR and Ca(2+) by determining the response of selected Ca(2+)-blind mutants grown in Ca(2+)-deficient rescue media containing Na(+) plus l-glutamate (pH 5.5), where the T3SS is not expressed, l-glutamate alone (pH 6.5), where l-aspartate is fully catabolized, and Na(+) alone (pH 9.0), where the electrogenic sodium pump NADH : ubiquinone oxidoreductase becomes activated. All three conditions supported essentially full-scale Ca(2+)-independent growth at 37 degrees C of wild-type Y. pestis as well as lcrG and yopN mutants (possessing a complete but dysregulated T3SS), indicating that bacteriostasis reflects a Na(+)-dependent lesion in bioenergetics. In contrast, mutants lacking the negative regulator YopD or the YopD chaperone (LcrH) failed to grow in any rescue medium and are therefore truly temperature-sensitive. The Ca(2+)-blind yopD phenotype was fully suppressed in a Ca(2+)-independent background lacking the injectisome-associated inner-membrane component YscV but not peripheral YscK, suggesting that the core translocon energizes YopD.

Type III secretion: a secretory pathway serving both motility and virulence (Review)

'Type III secretion' (T3S) refers to a secretion pathway that is common to the flagellae of eubacteria and the injectisomes of some gram-negative bacteria. Flagellae are rotary nanomachines allowing motility but they contain a built-in secretion apparatus that exports their own distal components to the distal end of the growing structure where they polymerize. In some cases they have been shown to export non-flagellar proteins. Injectisomes are transkingdom communication apparatuses allowing bacteria docked at the surface of a eukaryotic cell membrane to inject effector proteins across the two bacterial membranes and the eukaryotic cell membrane. Both nanomachines share a similar basal body embedded in the two bacterial membranes, topped either by a hook and a filament or by a stiff short needle. Both appear to be assembled in the same fashion. They recognize their substrate by a loose N-terminal peptide signal and the help of individual chaperones of a new type.

The type III secretion system tip complex and translocon

The type III secretion machinery of Gram-negative bacteria, also known as the injectisome or needle complex, is composed of a basal body spanning both bacterial membranes and the periplasm, and an external needle protruding from the bacterial surface. A set of three proteins, two hydrophobic and one hydrophilic, are required to allow translocation of proteins from the bacterium to the host cell cytoplasm. These proteins are involved in the formation of a translocation pore, the translocon, in the host cell membrane. Exciting progress has recently been made on the interaction between the translocators and the injectisome needle and the assembly of the translocon in the host cell membrane. As expected, the two hydrophobic translocators insert into the target cell membrane. Unexpectedly, the third, hydrophilic translocator, forms a complex on the distal end of the injectisome needle, the tip complex, and serves as an assembly platform for the two hydrophobic translocators.

Activation of the Cpx envelope stress response down-regulates expression of several locus of enterocyte effacement-encoded genes in enteropathogenic Escherichia coli

The Cpx two-component system regulates an extracytoplasmic stress response that functions to rid the envelope of misfolded and mislocalized proteins that may interfere with normal cellular processes. The Cpx pathway is also involved in pathogenesis. This study investigated the role of the Cpx response in enteropathogenic Escherichia coli (EPEC) type III secretion (T3S). It was determined that a functional Cpx pathway is not required for T3S but that pathway activation inhibits secretion by reducing the cellular pools of T3S substrates. The EPEC T3S system structural components, as well as a number of its substrates, are encoded on the locus of enterocyte effacement (LEE) pathogenicity island. Transcriptional fusions to the five major operons of the LEE were constructed and examined under Cpx pathway-activating conditions. Induction of the Cpx response caused a decrease in the transcription of several LEE operons, with the most pronounced effect on LEE4 and LEE5. Collectively, these two operons encode components of the T3S translocation apparatus, the bacterial adhesin intimin, and the translocated bacterial receptor Tir. These data show for the first time that activation of the Cpx envelope stress response in EPEC inhibits T3S of both translocators and effectors, likely through down regulation of LEE transcription. Coupled with recent findings, our results suggest that Cpx-mediated down regulation of virulence is a conserved theme in a number of bacterial pathogens.

Structural characterization of the Yersinia pestis type III secretion system needle protein YscF in complex with its heterodimeric chaperone YscE/YscG

The plague-causing bacterium Yersinia pestis utilizes a type III secretion system to deliver effector proteins into mammalian cells where they interfere with signal transduction pathways that mediate phagocytosis and the inflammatory response. Effector proteins are injected through a hollow needle structure composed of the protein YscF. YscG and YscE act as "chaperones" to prevent premature polymerization of YscF in the cytosol of the bacterium prior to assembly of the needle. Here, we report the crystal structure of the YscEFG protein complex at 1.8 A resolution. Overall, the structure is similar to that of the analogous PscEFG complex from the Pseudomonas aeruginosa type III secretion system, but there are noteworthy differences. The structure confirms that, like PscG, YscG is a member of the tetratricopeptide repeat family of proteins. YscG binds tightly to the C-terminal half of YscF, implying that it is this region of YscF that controls its polymerization into the needle structure. YscE interacts with the N-terminal tetratricopeptide repeat motif of YscG but makes very little direct contact with YscF. Its function may be to stabilize the structure of YscG and/or to participate in recruiting the complex to the secretion apparatus. No electron density could be observed for the 49 N-terminal residues of YscF. This and additional evidence suggest that the N-terminus of YscF is disordered in the complex with YscE and YscG. As expected, conserved residues in the C-terminal half of YscF mediate important intra- and intermolecular interactions in the complex. Moreover, the phenotypes of some previously characterized mutations in the C-terminal half of YscF can be rationalized in terms of the structure of the heterotrimeric YscEFG complex.

Polynucleotide phosphorylase and the T3SS

Low temperatures as well as encounters with host phagocytes are two stresses that have been relatively well studied in many species of bacteria. The exoribonuclease polynucleotide phosphorylase (PNPase) has previously been shown to be required by several species of bacteria, including Yersinia, for low-temperature growth. We have shown that PNPase also enhances the ability of Yersinia to withstand the killing activities of murine macrophages. We have gone on to show that PNPase is required for the optimal functioning of Yersinia's type three secretion system (T3SS), an organelle that injects effector proteins directly into host cells. Surprisingly, the PNPase-mediated effect on T3SS activity is independent of PNPase's ribonuclease activity and instead requires only its S1 RNA-binding domain. In stark contrast, the catalytic activity of PNPase is strictly required for enhanced growth at low temperature. Preliminary experiments suggest that the RNA-binding interface of the S1 domain is critical for its T3SS-enhancing activity. Our findings indicate that PNPase plays versatile roles in promoting Yersinia's survival in response to stressful conditions.

The response regulator SsrB activates expression of diverse Salmonella pathogenicity island 2 promoters and counters silencing by the nucleoid-associated protein H-NS

The two-component system SsrA-SsrB activates expression of a type III secretion system required for replication in macrophages and systemic infection in mice. Here we characterize the SsrB-dependent regulation of genes within Salmonella pathogenicity island 2 (SPI-2). Primer extension and DNase I footprinting identified multiple SsrB-regulated promoters within SPI-2 located upstream of ssaB, sseA, ssaG and ssaM. We previously demonstrated that ssrA and ssrB transcription is uncoupled. Overexpression of SsrB in the absence of its cognate kinase, SsrA, is sufficient to activate SPI-2 transcription. Because SsrB requires phosphorylation to relieve inhibitory contacts that occlude its DNA-binding domain, additional components must phosphorylate SsrB. SPI-2 promoters examined in single copy were highly SsrB-dependent, activated during growth in macrophages and induced by acidic pH. The nucleoid structuring protein H-NS represses horizontally acquired genes; we confirmed that H-NS is a negative regulator of SPI-2 gene expression. In the absence of H-NS, the requirement for SsrB in activating SPI-2 genes is substantially reduced, suggesting a role for SsrB in countering H-NS silencing. SsrB activates transcription of multiple operons within SPI-2 by binding to degenerate DNA targets at diversely organized promoters. SsrB appears to possess dual activities to promote SPI-2 gene expression: activation of transcription and relief of H-NS-mediated repression.

Diminished LcrV secretion attenuates Yersinia pseudotuberculosis virulence

Many gram-negative bacterial pathogenicity factors that function beyond the outer membrane are secreted via a contact-dependent type III secretion system. Two types of substrates are predestined for this mode of secretion, namely, antihost effectors that are translocated directly into target cells and the translocators required for targeting of the effectors across the host cell membrane. N-terminal secretion signals are important for recognition of the protein cargo by the type III secretion machinery. Even though such signals are known for several effectors, a consensus signal sequence is not obvious. One of the translocators, LcrV, has been attributed other functions in addition to its role in translocation. These functions include regulation, presumably via interaction with LcrG inside bacteria, and immunomodulation via interaction with Toll-like receptor 2. Here we wanted to address the significance of the specific targeting of LcrV to the exterior for its function in regulation, effector targeting, and virulence. The results, highlighting key N-terminal amino acids important for LcrV secretion, allowed us to dissect the role of LcrV in regulation from that in effector targeting/virulence. While only low levels of exported LcrV were required for in vitro effector translocation, as deduced by a cell infection assay, fully functional export of LcrV was found to be a prerequisite for its role in virulence in the systemic murine infection model.

Induction of the Yersinia type 3 secretion system as an all-or-none phenomenon

Pathogenic Yersinia spp. possess a protein secretion system, designated as type 3, that plays a clear role in promoting their survival vis-à-vis the macrophage. Inductive expression of the Yersinia type 3 secretion system (T3SS), triggered either by host cell contact, or, in the absence of host cells, by a reduction in extracellular calcium ion levels, is accompanied by a withdrawal from the bacterial division cycle. Here, we analyzed Ca(2+)-dependent induction of the T3SS at the single-cell level to understand how Yersinia coordinates pro-survival and growth-related activities. We utilized a novel high-throughput quantitative microscopy approach as well as flow cytometry to determine how Ca(2+) levels, T3SS expression, and cellular division are interrelated. Our analysis showed that there is a high degree of homogeneity in terms of T3SS expression levels among a population of Y. pseudotuberculosis cells following the removal of Ca(2+), and that T3SS expression appears to be independent of the cellular division cycle. Unexpectedly, our analysis showed that Ca(2+) levels are inversely related to the initiation of inductive T3SS expression, and not to the intensity of activation once initiated, thus providing a basis for the seemingly graded response of T3SS activation observed in bulk-level analyses. The properties of the system described here display both similarities to and differences from that of the lac operon first described 50 years ago by Novick and Weiner.

Tetratricopeptide repeats are essential for PcrH chaperone function in Pseudomonas aeruginosa type III secretion

The type III secretion system (T3SS) is a specialized apparatus evolved by Gram-negative bacteria to deliver effector proteins into host cells, thus facilitating the establishment of an infection. Effector translocation across the target cell plasma membrane is believed to occur via pores formed by at least two secreted translocator proteins, the functions of which are dependent upon customized class II T3SS chaperones. Recently, three internal tetratricopeptide repeats (TPRs) were identified in this class of chaperones. Here, defined mutagenesis of the class II chaperone PcrH of Pseudomonas aeruginosa revealed these TPRs to be essential for chaperone activity towards the translocator proteins PopB and PopD and subsequently for the translocation of exoenzymes into host cells.

Extracytoplasmic-stress-responsive pathways modulate type III secretion in Yersinia pseudotuberculosis

Three signal transduction pathways, the two-component systems CpxRA and BaeSR and the alternative sigma factor sigma(E), respond to extracytoplasmic stress that facilitates bacterial adaptation to changing environments. At least the CpxRA and sigma(E) pathways control the production of protein-folding and degradation factors that counter the effects of protein misfolding in the periplasm. This function also influences the biogenesis of multicomponent extracellular appendages that span the bacterial envelope, such as various forms of pili. Herein, we investigated whether any of these regulatory pathways in the enteropathogen Yersinia pseudotuberculosis affect the functionality of the Ysc-Yop type III secretion system. This is a multicomponent molecular syringe spanning the bacterial envelope used to inject effector proteins directly into eukaryotic cells. Disruption of individual components revealed that the Cpx and sigma(E) pathways are important for Y. pseudotuberculosis type III secretion of Yops (Yersinia outer proteins). In particular, a loss of CpxA, a sensor kinase, reduced levels of structural Ysc (Yersinia secretion) components in bacterial membranes, suggesting that these mutant bacteria are less able to assemble a functional secretion apparatus. Moreover, these bacteria were no longer capable of localizing Yops into the eukaryotic cell interior. In addition, a cpxA lcrQ double mutant engineered to overproduce and secrete Yops was still impaired in intoxicating cells. Thus, the Cpx pathway might mediate multiple influences on bacterium-target cell contact that modulate Yersinia type III secretion-dependent host cell cytotoxicity.

Polynucleotide phosphorylase independently controls virulence factor expression levels and export in Yersinia spp

Previously, it was shown that optimal functioning of the Yersinia type III secretion system (T3SS) in cell culture infection assays requires the exoribonuclease polynucleotide phosphorylase (PNPase) and that normal T3SS activity could be restored in the Deltapnp strains by expressing just the approximately 70-aa S1 RNA-binding domain of PNPase. Here, it is shown that the Yersinia Deltapnp strain is less virulent in the mouse compared with the isogenic wild-type strain. To begin to understand what could be limiting T3SS activity in the absence of PNPase, T3SS-encoding transcripts and proteins in the YersiniaDeltapnp strains were analyzed. Surprisingly, it was found that the Deltapnp Yersinia strains possessed enhanced levels of T3SS-encoding transcripts and proteins compared with the wild-type strains. We then found that an S1 variant containing a disruption in its RNA-binding subdomain was inactive in terms of restoring normal T3SS activity. However, T3SS expression levels did not differ between Deltapnp strains expressing active and inactive S1 proteins, further showing that T3SS activity and expression levels, at least as related to PNPase and its S1 domain, are not linked. The results suggest that PNPase affects the expression and activity of the T3SS by distinct mechanisms and that the S1-dependent effect on T3SS activity involves an RNA intermediate.