Eric S. Krukonis

The Vibrio cholerae ToxR/TcpP/ToxT virulence cascade: distinct roles for two membrane-localized transcriptional activators on a single promoter

ToxR is required in Vibrio cholerae for transcriptional activation of the toxT gene, the protein product of which activates numerous genes involved in virulence. Although ToxR cannot activate the toxT promoter in Escherichia coli, the products of the tcpPH operon are shown here to activate the toxT promoter, and co‐expression with ToxRS enhances activation. An identical pattern was seen in a ΔtcpPΔtoxR strain of V. cholerae when TcpPH or ToxRS was expressed from plasmids. Although overexpression of the TcpP/H proteins in V. cholerae partially complemented both a ΔtoxR strain and a ΔtcpPΔtoxR double mutant for toxin production and toxT–lacZ activation, the presence of ToxR greatly increased their expression. Analysis of a toxT–lacZ promoter deletion series demonstrated that TcpP was able to interact functionally with the toxT promoter downstream of the ToxR binding site. This was confirmed using electrophoretic mobility shift assays of this toxT promoter deletion series and DNase I footprinting analysis, which showed that TcpP interacts with the promoter region from −51 to −32, whereas ToxR protected a region from −100 to −69. In addition, membranes containing endogenous levels of ToxR bound more readily to the toxT promoter than did membranes containing only TcpP. Characterization of a number of tcpP substitution mutants revealed one derivative (TcpP‐H93L) that, when overexpressed, was markedly defective for toxT activation, cholera toxin and TcpA (toxin co‐regulated pilus) production and DNA binding; however, toxT activation by TcpP‐H93L was restored in the presence of ToxR, suggesting that ToxR can provide the promoter recognition function for toxT activation. Two additional mutant derivatives, TcpP‐W68L and TcpP‐R86A, failed to activate toxT or direct toxin and TcpA production in the presence or absence of ToxR. Both TcpP‐W68L and TcpP‐R86A, like TcpP‐H93L, were defective for DNA binding. Finally, a ToxR mutant derivative, ToxR‐G80S, served to separate the different roles of ToxR on different promoters. Although ToxR‐G80S was inefficient at activating the ompU promoter in V. cholerae (ompU encodes an outer membrane porin regulated by ToxR), it was fully capable of activating the toxT promoter. These data suggest that ToxR is not a direct activator in the toxT expression system but, instead, enhances the activity of TcpP, perhaps by recruiting it to the toxT promoter under conditions in which expression levels of TcpP are too low for it to activate toxT efficiently on its own.

From motility to virulence: Sensing and responding to environmental signals in Vibrio cholerae.

Sensing its changing environment is key for Vibrio cholerae when making the transition from an aquatic lifestyle to one more suited to a human host. An inverse correlation between motility and virulence gene expression has been reported, with the NADH : ubiquinone oxidoreductase system which powers motility by generating a sodium-motive force, playing a pivotal role. Recent studies have demonstrated that bile inhibits activity of the transcription factor ToxT, a protein responsible for direct activation of numerous virulence gene promoters. In addition, recent technological advances have allowed for the analysis of in-vivo-induced genes and assessment of their timing of expression. Use of recombinase-based in vivo expression technology has revealed that the toxin-coregulated pilus (a colonization factor) is expressed before cholera toxin. Components of an acid-tolerance response system have also been found using this method as well as signature-tagged mutagenesis. Finally, a role for quorum sensing in regulation of virulence gene expression has recently been established.

The Yersinia pestis Ail Protein Mediates Binding and Yop Delivery to Host Cells Required for Plague Virulence

ABSTRACT Although adhesion to host cells is a critical step in the delivery of cytotoxic Yop proteins by Yersinia pestis, the mechanism has not been defined. To identify adhesins critical for Yop delivery, we initiated two transposon mutagenesis screens using the mariner transposon. To avoid redundant cell binding activities, we initiated the screen with a strain deleted for two known adhesins, pH 6 antigen and the autotransporter, YapC, as well as the Caf1 capsule, which is known to obscure some adhesins. The mutants that emerged contained insertions within the ail (attachment and invasion locus) gene of Y. pestis. A reconstructed mutant with a single deletion in the ail locus (y1324) was severely defective for delivery of Yops to HEp-2 human epithelial cells and significantly defective for delivery of Yops to THP-1 human monocytes. Specifically, the Yop delivery defect was apparent when cell rounding and translocation of an ELK-tagged YopE derivative into host cells were monitored. Although the ail mutant showed only a modest decrease in cell binding capacity in vitro, the KIM5 Δail mutant exhibited a >3,000-fold-increased 50% lethal dose in mice. Mice infected with the Δail mutant also had 1,000-fold fewer bacteria in their spleens, livers, and lungs 3 days after infection than did those infected with the parental strain, KIM5. Thus, the Ail protein is critical for both Y. pestis type III secretion in vitro and infection in mice.

TcpH Influences Virulence Gene Expression in Vibrio cholerae by Inhibiting Degradation of the Transcription Activator TcpP

Expression of toxT, the transcription activator of cholera toxin and pilus production in Vibrio cholerae, is the consequence of a complex cascade of regulatory events that culminates in activation of the toxT promoter by TcpP and ToxR, two membrane-localized transcription factors. Both are encoded in operons with genes whose products, TcpH and ToxS, which are also membrane localized, are hypothesized to control their activity. In this study we analyzed the role of TcpH in controlling TcpP function. We show that a mutant of V. cholerae lacking TcpH expressed virtually undetectable levels of TcpP, although tcpP mRNA levels remain unaffected. A time course experiment showed that levels of TcpP, expressed from a plasmid, are dramatically reduced over time without co-overexpression of TcpH. By contrast, deletion of toxS did not affect ToxR protein levels. A fusion protein in which the TcpP periplasmic domain is replaced with that of ToxR remains stable, suggesting that the periplasmic domain of TcpP is the target for degradation of the protein. Placement of the periplasmic domain of TcpP on ToxR, an otherwise stable protein, results in instability, providing further evidence for the hypothesis that the periplasmic domain of TcpP is a target for degradation. Consistent with this interpretation is our finding that derivatives of TcpP lacking a periplasmic domain are more stable in V. cholerae than are derivatives in which the periplasmic domain has been truncated. This work identifies at least one role for the periplasmic domain of TcpP, i.e., to act as a target for a protein degradation pathway that regulates TcpP levels. It also provides a rationale for why the V. cholerae tcpH mutant strain is avirulent. We hypothesize that regulator degradation may be an important mechanism for regulating virulence gene expression in V. cholerae.

Membrane localization of the ToxR winged-helix domain is required for TcpP-mediated virulence gene activation in Vibrio cholerae

ToxR is a bitopic membrane protein that controls virulence gene expression in Vibrio cholerae. Its cytoplasmic domain is homologous to the winged helix–turn–helix (‘winged helix’) DNA‐binding/transcription activation domain found in a variety of prokaryotic and eukaryotic regulators, whereas its periplasmic domain is of ill‐defined function. Several genes in V. cholerae are regulated by ToxR, but by apparently different mechanisms. Whereas ToxR directly controls the transcription of genes encoding two outer membrane proteins, OmpU and OmpT, it co‐operates with a second membrane‐localized transcription factor called TcpP to activate transcription of the gene encoding ToxT, which regulates transcription of cholera toxin (ctxAB) and the toxin‐co‐regulated pilus (tcp). To determine the requirements for gene activation by ToxR, different domains of the protein were analysed for their ability to control expression of toxT, ompU and ompT. Soluble forms of the cytoplasmic winged‐helix domain regulated ompU and ompT gene expression properly but did not activate toxT transcription. Membrane localization of the winged helix was sufficient for both omp gene regulation and TcpP‐dependent toxT transcription, irrespective of the type of periplasmic domain or even the presence of a periplasmic domain. These results suggest that (i) the major function for membrane localization of ToxR is for its winged‐helix domain to co‐operate with TcpP to activate transcription; (ii) the periplasmic domain of ToxR is not required for TcpP‐dependent activation of toxT transcription; and (iii) membrane localization is not a strict requirement for DNA binding and transcription activation by ToxR.

Structural insights into Ail-mediated adhesion in Yersinia pestis

Ail is an outer membrane protein from Yersinia pestis that is highly expressed in a rodent model of bubonic plague, making it a good candidate for vaccine development. Ail is important for attaching to host cells and evading host immune responses, facilitating rapid progression of a plague infection. Binding to host cells is important for injection of cytotoxic Yersinia outer proteins. To learn more about how Ail mediates adhesion, we solved two high-resolution crystal structures of Ail, with no ligand bound and in complex with a heparin analog called sucrose octasulfate. We identified multiple adhesion targets, including laminin and heparin, and showed that a 40 kDa domain of laminin called LG4-5 specifically binds to Ail. We also evaluated the contribution of laminin to delivery of Yops to HEp-2 cells. This work constitutes a structural description of how a bacterial outer membrane protein uses a multivalent approach to bind host cells.

The Yersinia pestis autotransporter YapC mediates host cell binding, autoaggregation and biofilm formation

YapC, a putative Yersinia pestis autotransporter protein, shows strong homology to the enterotoxigenic Escherichia coli adhesin TibA. As a potentially important surface protein of Y. pestis, we analysed YapC for several activities. When expressed in the non-pathogenic Fim(-) E. coli strain AAEC185, YapC mediated attachment to both murine-derived macrophage-like cells (RAW264.7) and human-derived epithelial-like cells (HEp-2). In addition, expression of YapC on the surface of E. coli led to autoaggregation in DMEM tissue culture medium, a phenomenon associated with virulence in Yersinia species. YapC also mediated formation of biofilm-like deposits by E. coli AAEC185. Deletion of yapC in Y. pestis strain KIM5 resulted in no change in adhesion to either RAW264.7 or HEp-2 cells, or in biofilm formation. Lack of a phenotype for the Y. pestis DeltayapC mutant may reflect the relatively low level of yapC expression in vitro, as assessed by RT-PCR, and/or redundant functions expressed in vitro. These data demonstrate several activities for YapC that may function during Y. pestis infection.

DNA Binding and ToxR Responsiveness by the Wing Domain of TcpP, an Activator of Virulence Gene Expression in Vibrio cholerae

Virulence in Vibrio cholerae requires activation of toxT by two membrane-localized activators, TcpP and ToxR. We isolated 12 tcpP activation mutants that fell into two classes: class I mutants were inactive irrespective of the presence of ToxR, and class II mutants exhibited near wild-type activity when coexpressed with ToxR. Most class I mutants had lesions in the wing domain predicted by homology with the winged helix-turn-helix family of activators. Class I mutants bound promoter DNA poorly and were largely unable to interact with ToxR in a crosslinking assay, whereas class II mutants retained physical interaction with ToxR. One mutant constructed in vitro bound DNA poorly but nevertheless responded to ToxR by activating toxT and also maintained ToxR interaction. We propose that ToxR interaction, but not DNA binding, is essential for TcpP function and that the wing domain of TcpP enables contact with ToxR required for productive TcpP-RNA polymerase association.

ToxR Recognizes a Direct Repeat Element in the toxT, ompU, ompT, and ctxA Promoters of Vibrio cholerae To Regulate Transcription

ABSTRACT ToxR facilitates TcpP-mediated activation of the toxT promoter in Vibrio cholerae, initiating a regulatory cascade that culminates in cholera toxin secretion and toxin coregulated pilus expression. ToxR binds a region from −104 to −68 of the toxT promoter, from which ToxR recruits TcpP to the TcpP-binding site from −53 to −38. To precisely define the ToxR-binding site within the toxT promoter, promoter derivatives with single-base-pair transversions spanning the ToxR-footprinted region were tested for transcription activation and DNA binding. Nine transversions between −96 to −83 reduced toxT promoter activity 3-fold or greater, and all nine reduced the relative affinity of the toxT promoter for ToxR at least 2-fold, indicating that activation defects were due largely to reduced binding of ToxR to the toxT promoter. Nucleotides important for ToxR-dependent toxT activation revealed a consensus sequence of TNAAA-N5-TNAAA extending from −96 to −83, also present in other ToxR-regulated promoters. When these consensus nucleotides were mutated in the ompU, ompT, or ctxA promoters, ToxR-mediated regulation was disrupted. Thus, we have defined the core ToxR-binding site present in numerous ToxR-dependent promoters and we have precisely mapped the binding site for ToxR to a position three helical turns upstream of TcpP in the toxT promoter.

Ail Proteins of Yersinia pestis and Y. pseudotuberculosis Have Different Cell Binding and Invasion Activities

The Yersinia pestis adhesin Ail mediates host cell binding and facilitates delivery of cytotoxic Yop proteins. Ail from Y. pestis and Y. pseudotuberculosis is identical except for one or two amino acids at positions 43 and 126 depending on the Y. pseudotuberculosis strain. Ail from Y. pseudotuberculosis strain YPIII has been reported to lack host cell binding ability, thus we sought to determine which amino acid difference(s) are responsible for the difference in cell adhesion. Y. pseudotuberculosis YPIII Ail expressed in Escherichia coli bound host cells, albeit at ∼50% the capacity of Y. pestis Ail. Y. pestis Ail single mutants, Ail-E43D and Ail-F126V, both have decreased adhesion and invasion in E. coli when compared to wild-type Y. pestis Ail. Y. pseudotuberculosis YPIII Ail also had decreased binding to the Ail substrate fibronectin, relative to Y. pestis Ail in E. coli. When expressed in Y. pestis, there was a 30–50% decrease in adhesion and invasion depending on the substitution. Ail-mediated Yop delivery by both Y. pestis Ail and Y. pseudotuberculosis Ail were similar when expressed in Y. pestis, with only Ail-F126V giving a statistically significant reduction in Yop delivery of 25%. In contrast to results in E. coli and Y. pestis, expression of Ail in Y. pseudotuberculosis led to no measurable adhesion or invasion, suggesting the longer LPS of Y. pseudotuberculosis interferes with Ail cell-binding activity. Thus, host context affects the binding activities of Ail and both Y. pestis and Y. pseudotuberculosis Ail can mediate cell binding, cell invasion and facilitate Yop delivery.