Interaction of Ler at the LEE5 (tir) operon of enteropathogenic Escherichia coli

Ler of pathogenic Escherichia coli forms toroidal protein-DNA complexes

Enteropathogenic and enterohaemorrhagic Escherichia coli are related pathotypes of bacteria that cause acute watery diarrhoea and haemorrhagic colitis, respectively, and enterohaemorrhagic E. coli can lead to a serious complication known as haemolytic uraemic syndrome. In both bacteria the global regulatory protein Ler controls virulence. The ler gene is found within the locus of enterocyte effacement, or LEE, encoding a type III secretion system necessary for injecting effector proteins into intestinal epithelial cells and causing net secretory diarrhoea. The nucleoid-associated protein H-NS silences, whereas Ler serves as an anti-silencer of, multiple LEE operons. Although Ler has a higher affinity for DNA than does H-NS, the precise molecular mechanism by which Ler increases LEE transcription remains to be determined. In this report we investigate the oligomerization activity of Ler. In solution, Ler forms dimers and soluble aggregates of up to 5000 kDa molecular mass, and appears to oligomerize more readily than the related protein H-NS. An insertional mutation into the Ler linker region diminished oligomerization activity. Despite being proteins of similar mass and having homologous DNA-binding domains, Ler and H-NS complexed to DNA migrated to distinct locations, as determined by an electrophoretic mobility shift assay, implying that the related proteins form different 3D shapes in the presence of DNA. Lastly, we present electron microscopy images of toroidal Ler-DNA structures that are predicted to be involved in stimulating gene expression.

Eavesdropping by bacteria: the role of SdiA in Escherichia coli and Salmonella enterica serovar Typhimurium quorum sensing

Many gram-negative bacteria utilize N-acyl-L-homoserine lactones (AHLs) to bind to transcriptional regulators leading to activation or repression of target genes. Escherichia coli and Salmonella enterica do not synthesize AHLs but do contain the AHL receptor, SdiA. Studies reveal that SdiA can bind AHLs produced by other bacterial species and thereby allow E. coli and S. enterica to regulate gene transcription. The Salmonella sdiA gene regulates the rck gene, which mediates Salmonella adhesion and invasion of epithelial cells and the resistance of the organism to complement. In E. coli, there is some evidence that SdiA may regulate genes associated with acid resistance, virulence, motility, biofilm formation, and autoinducer-2 transport and processing. However, there is a lack of information concerning the role of SdiA in regulating growth and survival of E. coli and Salmonella in food environments, and therefore studies in this area are needed.

Molecular characterization of GrlA, a specific positive regulator of ler expression in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) infections are characterized by the formation of attaching and effacing (A/E) lesions on the surfaces of infected epithelial cells. The genes required for the formation of A/E lesions are located within the locus of enterocyte effacement (LEE). Ler is the key regulatory factor controlling the expression of LEE genes. Expression of the ler gene is positively regulated by GrlA, which is encoded by the LEE. Here, we analyze the mechanism by which GrlA positively regulates ler expression and show that in the absence of H-NS, GrlA is no longer essential for ler activation, further confirming that GrlA acts in part as an H-NS antagonist on the ler promoter. Single-amino-acid mutants were constructed to test the functional significance of the putative helix-turn-helix (HTH) DNA binding motif found in the N-terminal half of GrlA, as well as at the C-terminal domain of the protein. Several mutations within the HTH motif, but not all, completely abolished GrlA activity, as well as specific binding to its target sequence downstream from position -54 in the ler regulatory region. Some of these mutants, albeit inactive, were still able to interact with the negative regulator GrlR, indicating that loss of activity was not a consequence of protein misfolding. Additional residues in the vicinity of the HTH domain, as well as at the end of the protein, were also shown to be important for GrlA activity as a transcriptional regulator, but not for its interaction with GrlR. In summary, GrlA consists of at least two functional domains, one involved in transcriptional activation and DNA binding and the other in heterodimerization with GrlR.

The LysR-type regulator QseA regulates both characterized and putative virulence genes in enterohaemorrhagic Escherichia coli O157:H7

Enterohaemorrhagic Escherichia coli (EHEC) colonizes the large intestine, causing attaching and effacing (AE) lesions. Most of the genes involved in AE lesion formation are encoded within a chromosomal pathogenicity island termed the locus of enterocyte effacement (LEE). The LysR-type transcriptional factor QseA regulates the LEE by binding to the regulatory region of ler. We performed transcriptome analyses comparing wild-type (WT) EHEC and the qseA mutant to elucidate QseA's role in gene regulation. During both growth phases, several genes carried in O-islands were activated by QseA, whereas genes involved in cell metabolism were repressed. During late-logarithmic growth, QseA activated expression of the LEE genes as well as non-LEE-encoded effector proteins. We also performed electrophoretic mobility shift assays, competition experiments and DNase I footprints. The results demonstrated that QseA directly binds both the ler proximal and distal promoters, its own promoter, as well as promoters of genes encoded in EHEC-specific O-islands. Additionally, we mapped the transcriptional start site of qseA, leading to the identification of two promoter sequences. Taken together, these results indicate that QseA acts as a global regulator in EHEC, co-ordinating expression of virulence genes.

The LysR-type transcriptional regulator QseD alters type three secretion in enterohemorrhagic Escherichia coli and motility in K-12 Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 responds to the host-produced epinephrine and norepinephrine, and bacterially produced autoinducer 3 (AI-3), through two-component systems. Further integration of multiple regulatory signaling networks, involving regulators such as the LysR-type transcriptional regulator (LTTR) QseA, promotes effective regulation of virulence factors. These include the production of flagella, a phage-encoded Shiga toxin, and genes within the locus of enterocyte effacement (LEE) responsible for attaching and effacing (AE) lesion formation. Here, we describe a new member of this signaling cascade, an LTTR heretofore renamed QseD (quorum-sensing E. coli regulator D). QseD is present in all enterobacteria but exists almost exclusively in O157:H7 isolates as a helix-turn-helix (HTH) truncated isoform. This "short" isoform (sQseD) is still able to regulate gene expression through a different mechanism than the full-length K-12 E. coli "long" QseD isoform (lQseD). The EHEC Delta qseD mutant exhibits increased expression of all LEE operons and deregulation of AE lesion formation. The loss of qseD in EHEC does not affect motility, but the K-12 Delta qseD mutant is hypermotile. While the lQseD directly binds to the ler promoter, encoding the LEE master regulator, to repress LEE transcription, the sQseD isoform does not. LTTRs bind to DNA as tetramers, and these data suggest that sQseD regulates ler by forming heterotetramers with another LTTR. The LTTRs known to regulate LEE transcription, QseA and LrhA, do not interact with sQseD, suggesting that sQseD acts as a dominant-negative partner with a yet-unidentified LTTR.

Transcriptional analysis of the grlRA virulence operon from Citrobacter rodentium

The locus for enterocyte effacement (LEE) is the virulence hallmark of the attaching-and-effacing (A/E) intestinal pathogens, namely, enteropathogenic Escherichia coli, enterohemorrhagic E. coli, and Citrobacter rodentium. The LEE carries more than 40 genes that are arranged in several operons, e.g., LEE1 to LEE5. Expression of the various transcriptional units is subject to xenogeneic silencing by the histone-like protein H-NS. The LEE1-encoded regulator, Ler, plays a key role in relieving this repression at several major LEE promoters, including LEE2 to LEE5. To achieve appropriate intracellular concentrations of Ler in different environments, A/E pathogens have evolved a sophisticated regulatory network to control ler expression. For example, the LEE-encoded GrlA and GrlR proteins work as activator and antiactivator, respectively, of ler transcription. Thus, control of the transcriptional activities of the LEE1 (ler) promoter and the grlRA operon determines the rate of transcription of all of the LEE-encoded virulence factors. To date, only a single promoter has been identified for the grlRA operon. In this study, we showed that the non-LEE-encoded AraC-like regulatory protein RegA of C. rodentium directly stimulates transcription of the grlRA promoter by binding to an upstream region in the presence of bicarbonate ions. In addition, in vivo and in vitro transcription assays revealed a sigma(70) promoter that is specifically responsible for transcription of grlA. Expression from this promoter was strongly repressed by H-NS and its paralog StpA but was activated by Ler. DNase I footprinting demonstrated that Ler binds to a region upstream of the grlA promoter, whereas H-NS interacts specifically with a region extending from the grlA core promoter into its coding sequence. Together, these findings provide new insights into the environmental regulation and differential expressions of the grlR and grlA genes of C. rodentium.

Increased adherence and expression of virulence genes in a lineage of Escherichia coli O157:H7 commonly associated with human infections

BACKGROUND

Enterohemorrhagic Escherichia coli (EHEC) O157:H7, a food and waterborne pathogen, can be classified into nine phylogenetically distinct lineages, as determined by single nucleotide polymorphism genotyping. One lineage (clade 8) was found to be associated with hemolytic uremic syndrome (HUS), which can lead to kidney failure and death in some cases, particularly young children. Another lineage (clade 2) differs considerably in gene content and is phylogenetically distinct from clade 8, but caused significantly fewer cases of HUS in a prior study. Little is known, however, about how these two lineages vary with regard to phenotypic traits important for disease pathogenesis and in the expression of shared virulence genes.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we quantified the level of adherence to and invasion of MAC-T bovine epithelial cells, and examined the transcriptomes of 24 EHEC O157:H7 strains with varying Shiga toxin profiles from two common lineages. Adherence to epithelial cells was >2-fold higher for EHEC O157:H7 strains belonging to clade 8 versus clade 2, while no difference in invasiveness was observed between the two lineages. Whole-genome 70-mer oligo microarrays, which probe for 6088 genes from O157:H7 Sakai, O157:H7 EDL 933, pO157, and K12 MG1655, detected significant differential expression between clades in 604 genes following co-incubation with epithelial cells for 30 min; 186 of the 604 genes had a >1.5 fold change difference. Relative to clade 2, clade 8 strains showed upregulation of major virulence genes, including 29 of the 41 locus of enterocyte effacement (LEE) pathogenicity island genes, which are critical for adherence, as well as Shiga toxin genes and pO157 plasmid-encoded virulence genes. Differences in expression of 16 genes that encode colonization factors, toxins, and regulators were confirmed by qRT-PCR, which revealed a greater magnitude of change than microarrays.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrate that the EHEC O157:H7 lineage associated with HUS expresses higher levels of virulence genes and has an enhanced ability to attach to epithelial cells relative to another common lineage.

Biofilm formation by and multicellular behavior of Escherichia coli O55:H7, an atypical enteropathogenic strain

Enteropathogenic Escherichia coli (EPEC) is an important causal agent of diarrheal illness throughout the world. Nevertheless, researchers have only recently begun to explore its capacity to form biofilms. Strain O55:H7 (DMS9) is a clinical isolate belonging to the atypical EPEC (aEPEC) group, which displays a high degree of genetic relatedness to enterohemorrhagic E. coli. Strain DMS9 formed a robust biofilm on an abiotic surface at 26 degrees C, but not at 37 degrees C. It also formed a dense pellicle at the air-liquid interface and developed a red, rough, and dry (RDAR) morphotype on Congo red agar. Unlike a previously described E. coli O157:H7 strain, the aEPEC strain seems to express cellulose. Transposon mutagenesis was used to identify biofilm-deficient mutants. One of the mutants was inactivated in the csgFG genes, required for assembly and secretion of curli fimbriae, while a second mutant had a mutation in crl, a thermosensitive global regulator that modulates sigma(S) activity and downstream expression of curli and cellulose. The two mutants were deficient in their biofilm formation capabilities and did not form a pellicle at the air-liquid interface. Unlike in Salmonella, the csgFG mutant in aEPEC completely lost the RDAR phenotype, while the crl mutant displayed a unique RDAR "pizza"-like morphotype. Genetic complementation of the two mutants resulted in restoration of the wild-type phenotype. This report is the first to describe and analyze a multicellular behavior in aEPEC and support a major role for curli and the crl regulator in biofilm development at low temperatures corresponding to the nonmammalian host environment.

Role of RpoS in virulence of pathogens

Understanding mechanisms of bacterial pathogenesis is critical for infectious disease control and treatment. Infection is a sophisticated process that requires the participation of global regulators to coordinate expression of not only genes coding for virulence factors but also those involved in other physiological processes, such as stress response and metabolic flux, to adapt to host environments. RpoS is a key response regulator to stress conditions in Escherichia coli and many other proteobacteria. In contrast to its conserved well-understood role in stress response, effects of RpoS on pathogenesis are highly variable and dependent on species. RpoS contributes to virulence through either enhancing survival against host defense systems or directly regulating expression of virulence factors in some pathogens, while RpoS is dispensable, or even inhibitory, to virulence in others. In this review, we focus on the distinct and niche-dependent role of RpoS in virulence by surveying recent findings in many pathogens.

Differences in adherence and virulence gene expression between two outbreak strains of enterohaemorrhagic Escherichia coli O157 : H7

The Escherichia coli O157 : H7 TW14359 strain was implicated in a multi-state outbreak in North America in 2006, which resulted in high rates of severe disease. Similarly, the O157 : H7 RIMD0509952 (Sakai) strain caused the largest O157 : H7 outbreak to date. Both strains were shown to represent divergent phylogenetic lineages. Here we compared global gene expression patterns before and after epithelial cell exposure, as well as the ability to adhere to and invade epithelial cells, between the two outbreak strains. Epithelial cell assays demonstrated a 2.5-fold greater adherence of the TW14359 strain relative to Sakai, while whole-genome microarrays detected significant differential expression of 914 genes, 206 of which had a fold change >/=1.5. Interestingly, most locus of enterocyte effacement (LEE) genes were upregulated in TW14359, whereas flagellar and chemotaxis genes were primarily upregulated in Sakai, suggesting discordant expression of these genes between the two strains. The Shiga toxin 2 genes were also upregulated in the TW14359 strain, as were several pO157-encoded genes that promote adherence, including type II secretion genes and their effectors stcE and adfO. Quantitative RT-PCR confirmed the expression differences detected in the microarray analysis, and expression levels were lower for a subset of LEE genes before versus after exposure to epithelial cells. In all, this study demonstrated the upregulation of major and ancillary virulence genes in TW14359 and of flagellar and chemotaxis genes in Sakai, under conditions that precede intimate bacterial attachment to epithelial cells. Differences in the level of adherence to epithelial cells were also observed, implying that these two phylogenetically divergent O157 : H7 outbreak strains vary in their ability to colonize, or initiate the disease process.

Cell-to-Cell Signaling in Escherichia coli and Salmonella

Bacteria must be able to respond rapidly to changes in the environment in order to survive. One means of coordinating gene expression relies on tightly regulated and complex signaling systems. One of the first signaling systems that was described in detail is quorum sensing (QS). During QS, a bacterial cell produces and secretes a signaling molecule called an autoinducer (AI). As the density of the bacterial population increases, so does the concentration of secreted AI molecules, thereby allowing a bacterial species to coordinate gene expression based on population density. Subsequent studies have demonstrated that bacteria are also able to detect signal molecules produced by other species of bacteria as well as hormones produced by their mammalian hosts. These types of signaling interactions have been termed cell-to-cell signaling because the interaction does not rely on a threshold concentration of bacterial cells. This review discusses the three main types of cell-to-cell signaling mechanisms used by E. coli and Salmonella, including the LuxR process, in which E. coli and Salmonella detect signals produced by other species of bacteria; the LuxS/AI-2 system, in which E. coli and Salmonella participate in intra- and interspecies signaling; and the AI-3/ epinephrine/norepinephrine system, in which E. coli and Salmonella recognize self-produced AI, signal produced by other microbes, and/or the human stress hormones epinephrine or norepinephrine.

The RNA binding protein CsrA is a pleiotropic regulator of the locus of enterocyte effacement pathogenicity island of enteropathogenic Escherichia coli

The attaching and effacing (A/E) pathogen enteropathogenic Escherichia coli (EPEC) forms characteristic actin-filled membranous protrusions upon infection of host cells termed pedestals. Here we examine the role of the RNA binding protein CsrA in the expression of virulence genes and proteins that are necessary for pedestal formation. The csrA mutant was defective in forming actin pedestals on epithelial cells and in disrupting transepithelial resistance across polarized epithelial cells. Consistent with reduced pedestal formation, secretion of the translocators EspA, EspB, and EspD and the effector Tir was substantially reduced in the csrA mutant. Purified CsrA specifically bound to the sepL espADB mRNA leader, and the corresponding transcript levels were reduced in the csrA mutant. In contrast, Tir synthesis was unaffected in the csrA mutant. Reduced secretion of Tir appeared to be in part due to decreased synthesis of EscD, an inner membrane architectural protein of the type III secretion system (TTSS) and EscF, a protein that forms the protruding needle complex of the TTSS. These effects were not mediated through the locus of enterocyte effacement (LEE) transcriptional regulator GrlA or Ler. In contrast to the csrA mutant, multicopy expression of csrA repressed transcription from LEE1, grlRA, LEE2, LEE5, escD, and LEE4, an effect mediated by GrlA and Ler. Consistent with its role in other organisms, CsrA also regulated flagellar motility and glycogen levels. Our findings suggest that CsrA governs virulence factor expression in an A/E pathogen by regulating mRNAs encoding translocators, effectors, or transcription factors.

Hfq affects the expression of the LEE pathogenicity island in enterohaemorrhagic Escherichia coli

Colonization of the intestinal epithelium by enterohaemorrhagic Escherichia coli (EHEC) is characterized by an attaching and effacing (A/E) histopathology. The locus of enterocyte effacement (LEE) pathogenicity island encodes many genes required for the A/E phenotype including the global regulator of EHEC virulence gene expression, Ler. The LEE is subject to a complex regulatory network primarily targeting ler transcription. The RNA chaperone Hfq, implicated in post-transcriptional regulation, is an important virulence factor in many bacterial pathogens. Although post-transcriptional regulation of EHEC virulence genes is known to occur, a regulatory role of Hfq in EHEC virulence gene expression has yet to be defined. Here, we show that an hfq mutant expresses increased levels of LEE-encoded proteins prematurely, leading to earlier A/E lesion formation relative to wild type. Hfq indirectly affects LEE expression in exponential phase independent of Ler by negatively controlling levels of the regulators GrlA and GrlR through post-transcriptional regulation of the grlRA messenger. Moreover, Hfq negatively affects LEE expression in stationary phase independent of GrlA and GrlR. Altogether, Hfq plays an important role in co-ordinating the temporal expression of the LEE by controlling grlRA expression at the post-transcriptional level.

Virulence regulation in Citrobacter rodentium: the art of timing

The mouse enteric pathogen Citrobacter rodentium, like its human counterpart, enteropathogenic Escherichia coli, causes attaching and effacing lesions in the intestinal epithelium of its host. This phenotype requires virulence factors encoded by the locus for enterocyte effacement (LEE) pathogenicity island. For timely expression of these virulence determinants at the site of infection and for efficient delivery of some virulence factors into epithelial cells, C. rodentium utilizes a positive regulatory loop involving the LEE‐encoded regulatory proteins Ler, GrlA and GrlR to control LEE expression. Several transcription factors not encoded by LEE, some of which respond to specific environmental signals, also participate in this regulatory loop. Recently, we identified a non‐LEE encoded, AraC‐like regulatory protein, RegA, which plays a key role in the ability of C. rodentium to colonize the intestine. RegA functions by activating the transcription of a number of horizontally acquired operons encoding virulence‐associated factors, such as autotransporters, fimbriae, a dispersin‐like protein and its transporter. In addition, RegA represses transcription of a number of housekeeping genes. Importantly, RegA requires a gut‐specific environmental signal, bicarbonate, to exert its effects on gene expression. In our proposed model, when C. rodentium senses bicarbonate ions in the gastrointestinal tract, RegA directs the bacterium to reduce the production of proteins involved in normal cellular functions, while enhancing the production of factors required for colonization and virulence.

Ler interdomain linker is essential for anti-silencing activity in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) expresses a type III secretion system (T3SS) required for pathogenesis. Regulation of the genes encoding the T3SS is complex; two major regulators control transcription, the silencer H-NS, and the related H-NS-like protein Ler. Our laboratory is interested in understanding the molecular differences that distinguish the anti-silencer Ler from H-NS, and how Ler differentially regulates EPEC virulence genes. Here, we demonstrate that mutated Ler proteins either containing H-NS alpha-helices 1 and 2, missing from Ler, or truncated for the 11 aa C-terminal extension compared with the related H-NS protein, did not appreciably alter Ler function. In contrast, mutating the proline at position 92 of Ler, in the conserved C-terminal DNA binding motif, eliminated Ler activity. Inserting 11 H-NS-specific amino acids, 11 alanines or 6 alanines into the Ler linker severely impaired the ability of Ler to increase LEE5 transcription. To extend our analysis, we constructed six chimeric proteins containing the N terminus, linker region or C terminus of Ler in different combinations with the complementary domains of H-NS, and monitored their in vivo activities. Replacing the Ler linker domain with that of H-NS, or replacing the Ler C-terminal, DNA binding domain with that of H-NS eliminated the ability of Ler to increase transcription at the LEE5 promoter. Thus, the linker and C-terminal domains of Ler and H-NS are not functionally equivalent. Conversely, replacing the H-NS linker region with that of Ler caused increased transcription at LEE5 in a strain deleted for hns. In summary, the interdomain linker specific to Ler is necessary for anti-silencing activity in EPEC.

TraR, a homolog of a RNAP secondary channel interactor, modulates transcription

Recent structural and biochemical studies have identified a novel control mechanism of gene expression mediated through the secondary channel of RNA Polymerase (RNAP) during transcription initiation. Specifically, the small nucleotide ppGpp, along with DksA, a RNAP secondary channel interacting factor, modifies the kinetics of transcription initiation, resulting in, among other events, down-regulation of ribosomal RNA synthesis and up-regulation of several amino acid biosynthetic and transport genes during nutritional stress. Until now, this mode of regulation of RNAP was primarily associated with ppGpp. Here, we identify TraR, a DksA homolog that mimics ppGpp/DksA effects on RNAP. First, expression of TraR compensates for dksA transcriptional repression and activation activities in vivo. Second, mutagenesis of a conserved amino acid of TraR known to be critical for DksA function abolishes its activity, implying both structural and functional similarity to DksA. Third, unlike DksA, TraR does not require ppGpp for repression of the rrnB P1 promoter in vivo and in vitro or activation of amino acid biosynthesis/transport genes in vivo. Implications for DksA/ppGpp mechanism and roles of TraR in horizontal gene transfer and virulence are discussed.

Control of gene expression is central for cell operation. Transcription regulation is a first step to control gene expression and is largely mediated by DNA-binding factors. These recruit or prevent RNA polymerase binding to promoters of specific genes. Recently, a novel way to control transcription has emerged from studying nutritional stress in bacteria. In this case, a small nucleotide effector, ppGpp, with the help of a protein DksA, interacts with the secondary channel of RNAP, affecting RNA polymerase kinetics at promoters without binding to specific DNA sequences. This interaction results in up-regulation and down-regulation of genes involved in responding to nutritional stress. This work describes TraR, a factor found on conjugative plasmids that can regulate gene expression similarly to DksA, but in the absence of any nucleotide effector, like ppGpp. Thus, regulation of transcription similarly to DksA/ppGpp may be a more general mechanism. The presence of TraR on conjugative plasmids suggests a role for TraR in pathogenicity, virulence, and antibiotic resistance. These observations should provide a basis for new studies designed to combat antibiotic resistance and virulence in emerging pathogens.

Transient shielding of intimin and the type III secretion system of enterohemorrhagic and enteropathogenic Escherichia coli by a group 4 capsule

Enterohemorrhagic and enteropathogenic Escherichia coli (EHEC and EPEC, respectively) strains represent a major global health problem. Their virulence is mediated by the concerted activity of an array of virulence factors including toxins, a type III protein secretion system (TTSS), pili, and others. We previously showed that EPEC O127 forms a group 4 capsule (G4C), and in this report we show that EHEC O157 also produces a G4C, whose assembly is dependent on the etp, etk, and wzy genes. We further show that at early time points postinfection, these G4Cs appear to mask surface structures including intimin and the TTSS. This masking inhibited the attachment of EPEC and EHEC to tissue-cultured epithelial cells, diminished their capacity to induce the formation of actin pedestals, and attenuated TTSS-mediated protein translocation into host cells. Importantly, we found that Ler, a positive regulator of intimin and TTSS genes, represses the expression of the capsule-related genes, including etp and etk. Thus, the expression of TTSS and G4C is conversely regulated and capsule production is diminished upon TTSS expression. Indeed, at later time points postinfection, the diminishing capsule no longer interferes with the activities of intimin and the TTSS. Notably, by using the rabbit infant model, we found that the EHEC G4C is required for efficient colonization of the rabbit large intestine. Taken together, our results suggest that temporal expression of the capsule, which is coordinated with that of the TTSS, is required for optimal EHEC colonization of the host intestine.

Regulation of expression and secretion of NleH, a new non-locus of enterocyte effacement-encoded effector in Citrobacter rodentium

Together with enterohemorrhagic Escherichia coli and enteropathogenic Escherichia coli, Citrobacter rodentium is a member of the attaching-and-effacing (A/E) family of bacterial pathogens. A/E pathogens use a type III secretion system (T3SS) to translocate an assortment of effector proteins, encoded both within and outside the locus of enterocyte effacement (LEE), into the colonized host cell, leading to the formation of A/E lesions and disease. Here we report the identification and characterization of a new non-LEE encoded effector, NleH, in C. rodentium. NleH is conserved among A/E pathogens and shares identity with OspG, a type III secreted effector protein in Shigella flexneri. Downstream of nleH, genes encoding homologues of the non-LEE-encoded effectors EspJ and NleG/NleI are found. NleH secretion and translocation into Caco-2 cells requires a functional T3SS and signals located at its amino-terminal domain. Transcription of nleH is not significantly reduced in mutants lacking the LEE-encoded regulators Ler and GrlA; however, NleH protein levels are highly reduced in these strains, as well as in escN and cesT mutants. Inactivation of Lon, but not of ClpP, protease restores NleH levels even in the absence of CesT. Our results indicate that the efficient engagement of NleH in active secretion is needed for its stability, thus establishing a posttranslational regulatory mechanism that coregulates NleH levels with the expression of LEE-encoded proteins. A C. rodentium nleH mutant shows a moderate defect during the colonization of C57BL/6 mice at early stages of infection.

Effect of zinc in enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) infection triggers the release of ATP from host intestinal cells, and the ATP is broken down to ADP, AMP, and adenosine in the lumen of the intestine. Ecto-5'-nucleotidase (CD73) is the main enzyme responsible for the conversion of 5'-AMP to adenosine, which triggers fluid secretion from host intestinal cells and also has growth-promoting effects on EPEC bacteria. In a recent study, we examined the role of the host enzyme CD73 in EPEC infection by testing the effect of ecto-5'-nucleotidase inhibitors. Zinc was a less potent inhibitor of ecto-5'-nucleotidase in vitro than the nucleotide analog alpha,beta-methylene-ADP, but in vivo, zinc was much more efficacious in preventing EPEC-induced fluid secretion in rabbit ileal loops than alpha,beta-methylene-ADP. This discrepancy between the in vitro and in vivo potencies of the two inhibitors prompted us to search for potential targets of zinc other than ecto-5'-nucleotidase. Zinc, at concentrations that produced little or no inhibition of EPEC growth, caused a decrease in the expression of EPEC protein virulence factors, such as bundle-forming pilus (BFP), EPEC secreted protein A, and other EPEC secreted proteins, and reduced EPEC adherence to cells in tissue culture. The effects of zinc were not mimicked by other transition metals, such as manganese, iron, copper, or nickel, and the effects were not reversed by an excess of iron. Quantitative real-time PCR showed that zinc reduced the abundance of the RNAs encoded by the bfp gene, by the plasmid-encoded regulator (per) gene, by the locus for the enterocyte effacement (LEE)-encoded regulator (ler) gene, and by several of the esp genes. In vivo, zinc reduced EPEC-induced fluid secretion into ligated rabbit ileal loops, decreased the adherence of EPEC to rabbit ileum, and reduced histopathological damage such as villus blunting. Some of the beneficial effects of zinc on EPEC infection appear to be due to the action of the metal on EPEC bacteria as well as on the host.

Global effects of the cell-to-cell signaling molecules autoinducer-2, autoinducer-3, and epinephrine in a luxS mutant of enterohemorrhagic Escherichia coli

Intrakingdom cell-to-cell communication and interkingdom cell-to-cell communication play essential roles in the virulence of enterohemorrhagic Escherichia coli (EHEC). Four signals, autoinducer 2 (AI-2), AI-3, and the human hormones epinephrine and norepinephrine, are important in this communication. The effect of these signaling compounds on the transcriptome of EHEC was examined in this study. We demonstrated that the luxS mutation affects primarily central metabolic genes in both pathogenic and nonpathogenic strains of E. coli and that addition of exogenous AI-2 does not fully restore the expression profile in a luxS-deficient strain lacking the ability to synthesize AI-2. Addition of AI-3 or epinephrine increased expression of the locus of enterocyte effacement regulon, which is known to play a pivotal role in EHEC virulence. Moreover, when epinephrine was added to the culture medium, the greatest number of gene alterations was observed. These alterations included a greater proportion of alterations in EHEC genes than in MG1655 genes, suggesting that epinephrine may be a global virulence signal. Detailed examination with real-time reverse transcriptase PCR (RT-PCR) confirmed the increases in virulence gene expression with addition of AI-3 and epinephrine. Additional studies with real-time RT-PCR examining the EHEC secreted effectors and putative fimbrial gene expression showed a variable expression profile, indicating that there is differential regulation of the secreted molecules. This study began to examine the global signaling networks in EHEC and revealed expression profiles that are signal and pathogen specific.

Ler and H-NS, regulators controlling expression of the long polar fimbriae of Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 colonizes the human intestine and is responsible for diarrheal outbreaks worldwide. Previously we showed that EHEC produces long polar fimbriae (LPF) and that maximum expression is observed during the exponential phase of growth at 37 degrees C and pH 6.5. In this study, we analyzed the roles of several regulators in the expression of LPF using the beta-galactosidase reporter system, and we found that H-NS functions as a transcriptional silencer while Ler functions as an antisilencer of LPF expression. Interestingly, deletion of the hns and ler genes in EHEC caused constitutive expression of the fusion reporter protein. Semiquantitative reverse transcription (RT)-PCR was also used to analyze LPF expression in the EHEC ler or hns mutant strain. The hns mutant exhibited an increase in lpf mRNA expression, while expression in the ler mutant was decreased, compared to that in the wild-type strain. Using primer extension analysis, we identified two potential transcriptional start sites within the regulatory region of lpf and located consensus hexamers of -10 (CAAGAT) and -35 (TTCAAA), which are commonly found in sigma(70)-dependent promoters. Further, we determined whether H-NS and Ler interact directly with the lpf promoter region by using purified His-tagged proteins and electrophoretic mobility shift assays. Our data are the first to show direct binding interactions between the H-NS and Ler proteins within the regulatory sequence of the lpf operon. Based on the electrophoretic mobility shift assay, RT-PCR, primer extension, and beta-galactosidase assay results, we concluded that the E. coli O157:H7 lpf operon possesses a promoter dependent on sigma(70), that H-NS binds to the regulatory sequence of lpfA and "silences" the transcription of lpf, and that Ler binds to the regulatory sequence and inhibits the action of the H-NS protein.

Silencing of xenogeneic DNA by H-NS--facilitation of lateral gene transfer in bacteria by a defense system that recognizes foreign DNA

Lateral gene transfer has played a prominent role in bacterial evolution, but the mechanisms allowing bacteria to tolerate the acquisition of foreign DNA have been incompletely defined. Recent studies show that H-NS, an abundant nucleoid-associated protein in enteric bacteria and related species, can recognize and selectively silence the expression of foreign DNA with higher adenine and thymine content relative to the resident genome, a property that has made this molecule an almost universal regulator of virulence determinants in enteric bacteria. These and other recent findings challenge the ideas that curvature is the primary determinant recognized by H-NS and that activation of H-NS-silenced genes in response to environmental conditions occurs through a change in the structure of H-NS itself. Derepression of H-NS-silenced genes can occur at specific promoters by several mechanisms including competition with sequence-specific DNA-binding proteins, thereby enabling the regulated expression of foreign genes. The possibility that microorganisms maintain and exploit their characteristic genomic GC ratios for the purpose of self/non-self-discrimination is discussed.

QseA and GrlR/GrlA regulation of the locus of enterocyte effacement genes in enterohemorrhagic Escherichia coli

Transcription of the locus of enterocyte effacement (LEE) genes in enterohemorrhagic Escherichia coli (EHEC) is regulated by the LEE-encoded Ler and GrlR/GrlA proteins as well as the non-LEE-encoded regulator QseA. This work demonstrates that GrlR/GrlA activate LEE2 transcription in a Ler-independent fashion, whereas transcription of grlRA is activated by QseA in both Ler-dependent and -independent manners.

QseA directly activates transcription of LEE1 in enterohemorrhagic Escherichia coli

Quorum sensing (QS) in enterohemorrhagic Escherichia coli (EHEC) regulates the expression of the locus of enterocyte effacement (LEE). The LEE contains five major operons named LEE1 through LEE5. QseA was previously shown to be activated through QS and to activate the transcription of LEE1. The LEE1 operon encodes Ler, the transcription activator of all other LEE genes, and has two promoters: a distal promoter (P1) and a proximal promoter (P2). We have previously reported that QseA acts on P1 and not P2. To identify the minimal region of LEE1 that is necessary for QseA-mediated activation, a series of nested-deletion constructs of the LEE1 promoter fused to a lacZ reporter were constructed in both the EHEC and E. coli K-12 backgrounds. In an EHEC background, QseA-dependent activation of LEE1 can be observed for the entire regulatory region (beginning at nucleotide -393 and ending at nucleotide -123). In contrast to what occurred in EHEC, in K-12 there was no QseA-dependent activation of LEE1 transcription between base pairs -393 and -343. These data indicate that a QseA-dependent EHEC-specific regulator is required for the activation of transcription in this region. We also observed QseA-dependent LEE1 activation from nucleotides -218 to -123 in K-12, similar to results of the nested-deletion analysis performed with EHEC. Electrophoretic mobility shift assays established that QseA directly binds to the region of LEE1 from bp -173 to -42 and not to the region from bp -393 to -343. These studies suggest that QseA activates the transcription of LEE1 by directly binding upstream of its P1 promoter region.

SOS regulation of the type III secretion system of enteropathogenic Escherichia coli

Genomes of bacterial pathogens contain and coordinately regulate virulence-associated genes in order to cause disease. Enteropathogenic Escherichia coli (EPEC), a major cause of watery diarrhea in infants and a model gram-negative pathogen, expresses a type III secretion system (TTSS) that is encoded by the locus of enterocyte effacement (LEE) and is necessary for causing attaching and effacing intestinal lesions. Effector proteins encoded by the LEE and in cryptic prophage are injected into the host cell cytoplasm by the TTTS apparatus, ultimately leading to diarrhea. The LEE is comprised of multiple polycistronic operons, most of which are controlled by the global, positive regulator Ler. Here we demonstrated that the LEE2 and LEE3 operons also responded to SOS signaling and that this regulation was LexA dependent. As determined by a DNase I protection assay, purified LexA protein bound in vitro to a predicted SOS box located in the divergent, overlapping LEE2/LEE3 promoters. Expression of the lexA1 allele, encoding an uncleavable LexA protein in EPEC, resulted in reduced secretion, particularly in the absence of the Ler regulator. Finally, we obtained evidence that the cryptic phage-located nleA gene encoding an effector molecule is SOS regulated. Thus, we demonstrated, for the first time to our knowledge, that genes encoding components of a TTSS are regulated by the SOS response, and our data might explain how a subset of EPEC effector proteins, encoded in cryptic prophages, are coordinately regulated with the LEE-encoded TTSS necessary for their translocation into host cells.

Autoinducer 3 and epinephrine signaling in the kinetics of locus of enterocyte effacement gene expression in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is responsible for causing outbreaks of bloody diarrhea and hemolytic-uremic syndrome throughout the world. The locus of enterocyte effacement (LEE) consists of five major operons and is required for the formation of attaching and effacing lesions that disrupt intestinal epithelial microvilli. We have previously reported that expression of EHEC LEE genes is regulated by the luxS quorum-sensing system. The luxS gene in EHEC affects the production of autoinducer 3 (AI-3), which activates the LEE. Epinephrine and norepinephrine also activate the LEE in a manner similar to that of AI-3. Previous studies of quorum-sensing regulation of LEE transcription have thus far been restricted to using reporter systems in an E. coli K-12 background. Here, we examined the kinetics of LEE gene transcription, protein expression, and function of the LEE type III secretion apparatus in wild-type (WT) EHEC and an isogenic luxS mutant. The results revealed that the luxS mutant had diminished transcription from the LEE promoters during the mid-exponential growth phase; decreased protein levels of EscJ, Tir, and EspA; and reduced secretion of EspA and EspB. The luxS mutation also caused a delay in the formation of attaching and effacing lesions on cultured epithelial cells compared to the wild type. Epinephrine enhanced LEE expression in both the WT and the luxS mutant, but the WT still exhibited greater LEE activation. The results suggest a possible synergistic relationship between AI-3 and epinephrine. The combined effects of these two signaling molecules may lead to greater LEE expression and a more efficient infection.

Identification and characterization of NleI, a new non-LEE-encoded effector of enteropathogenic Escherichia coli (EPEC)

Enteropathogenic Escherichia coli (EPEC), a major gastrointestinal pathogen, causes infantile diarrhea in many developing countries. EPEC is an attaching and effacing (A/E) pathogen that utilizes a LEE-encoded type III secretion system (TTSS) to deliver effector proteins into host cells. These effectors have been identified as potential virulence factors in A/E pathogens including EPEC, enterohemorrhagic E. coli (EHEC) and Citrobacter rodentium (CR). We used a proteomics approach to identify a new non-LEE-encoded effector, NleI, from the EPEC sepL and sepD mutants. The nleI gene, located in a prophage-associated island with nleBCD, is also present in EHEC and CR but not in E. coli K-12. In EPEC, the transcription of nleI was increased upon sepD inactivation but remained unaffected in ler and sepL mutants. We demonstrated that NleI is secreted and translocated into HeLa cells in a TTSS-dependent manner and that the CesT chaperone is required for efficient NleI translocation. Nevertheless, under overexpression conditions, the first 20 amino acids of NleI are sufficient to support both secretion and translocation. After translocation, NleI can be detected in the cytoplasmic and membrane of HeLa cells.

AI-3 synthesis is not dependent on luxS in Escherichia coli

The quorum-sensing (QS) signal autoinducer-2 (AI-2) has been proposed to promote interspecies signaling in a broad range of bacterial species. AI-2 is spontaneously derived from 4,5-dihydroxy-2,3-pentanedione that, along with homocysteine, is produced by cleavage of S-adenosylhomocysteine (SAH) and S-ribosylhomocysteine by the Pfs and LuxS enzymes. Numerous phenotypes have been attributed to AI-2 QS signaling using luxS mutants. We have previously reported that the luxS mutation also affects the synthesis of the AI-3 autoinducer that activates enterohemorrhagic Escherichia coli virulence genes. Here we show that several species of bacteria synthesize AI-3, suggesting a possible role in interspecies bacterial communication. The luxS mutation leaves the cell with only one pathway, involving oxaloacetate and l-glutamate, for de novo synthesis of homocysteine. The exclusive use of this pathway for homocysteine production appears to alter metabolism in the luxS mutant, leading to decreased levels of AI-3. The addition of aspartate and expression of an aromatic amino acid transporter, as well as a tyrosine-specific transporter, restored AI-3-dependent phenotypes in an luxS mutant. The defect in AI-3 production, but not in AI-2 production, in the luxS mutant was restored by expressing the Pseudomonas aeruginosa S-adenosylhomocysteine hydrolase that synthesizes homocysteine directly from SAH. Furthermore, phenotype microarrays revealed that the luxS mutation caused numerous metabolic deficiencies, while AI-3 signaling had little effect on metabolism. This study examines how AI-3 production is affected by the luxS mutation and explores the roles of the LuxS/AI-2 system in metabolism and QS.

Concert of regulators to switch on LEE expression in enterohemorrhagic Escherichia coli O157:H7: Interplay between Ler, GrlA, HNS and RpoS

Enterohemorrhagic (EHEC) and enteropathogenic (EPEC) Escherichia coli strains carry a pathogenicity island termed locus of enterocyte effacement (LEE) responsible for attaching and effacing lesions on epithelial cells. The expression of LEE varies among isolates and is dependent on environmental cues. In the EHEC O157:H7 Sakaï isolate (RIMD-0509952 strain), we found that the non-coding RNA, DsrA, activates the expression of the LEE. This activation requires RpoS, the stress sigma factor. The DsrA/RpoS regulatory pathway mediates its positive effect by stimulating the transcription of ler, a positive regulatory gene encoded by the LEE. A second regulatory pathway, repressed by HNS, is also able to activate the transcription of ler and requires GrlA, another LEE-encoded regulator. Both regulatory pathways, DsrA/RpoS and HNS/GrlA, affect the activity of the ler distal promoter and require the Ler protein to be functional. Our data demonstrate that the LEE expression can be turned on by at least two separate pathways acting on the transcription of ler.

Bundle-forming pili and EspA are involved in biofilm formation by enteropathogenic Escherichia coli

Microcolony formation is one of the initial steps in biofilm development, and in enteropathogenic Escherichia coli (EPEC) it is mediated by several adhesins, including the bundle-forming pilus (BFP) and the EspA filament. Here we report that EPEC forms biofilms on plastic under static conditions and a flowthrough continuous culture system. The abilities of several EPEC isogenic mutants to form biofilms were assessed. Adhesins such as BFP and EspA, important in microcolony formation on epithelial cells, are also involved in bacterial aggregation during biofilm formation on abiotic surfaces. Mutants that do not express BFP or EspA form more-diffuse biofilms than does the wild type. We also determined, using gfp transcriptional fusions, that, consistent with the role of these adhesins in biofilms, the genes encoding BFP and EspA are expressed during biofilm formation. Finally, expression of espA is controlled by a quorum-sensing (QS) regulatory mechanism, and the EPEC qseA QS mutant also forms altered biofilms, suggesting that this signaling mechanism plays an important role in EPEC biofilm development. Taken together, these studies allowed us to propose a model of EPEC biofilm formation.

Selective silencing of foreign DNA with low GC content by the H-NS protein in Salmonella

Horizontal gene transfer plays a major role in microbial evolution. However, newly acquired sequences can decrease fitness unless integrated into preexisting regulatory networks. We found that the histone-like nucleoid structuring protein (H-NS) selectively silences horizontally acquired genes by targeting sequences with GC content lower than the resident genome. Mutations in hns are lethal in Salmonella unless accompanied by compensatory mutations in other regulatory loci. Thus, H-NS provides a previously unrecognized mechanism of bacterial defense against foreign DNA, enabling the acquisition of DNA from exogenous sources while avoiding detrimental consequences from unregulated expression of newly acquired genes. Characteristic GC/AT ratios of bacterial genomes may facilitate discrimination between a cell's own DNA and foreign DNA.

Multiple functions of l0036 in the regulation of the pathogenicity island of enterohaemorrhagic Escherichia coli O157:H7

Diarrhoeagenic enterohaemorrhagic Escherichia coli and enteropathogenic E. coli attach to human intestinal epithelium and efface brush-border microvilli, forming an A/E (attaching and effacing) lesion. These human pathogens are phenotypically similar to the mouse pathogen Citrobacter rodentium. Genetically, they all have a homologous set of virulent genes involved in the A/E lesion, and these genes are organized on a LEE (locus of enterocyte effacement), a pathogenicity island. This island comprises 41 specific open reading frames, of which most are organized at five operons, LEE1, LEE2, LEE3, LEE4 and tir (LEE5). The expression of the LEE genes is regulated in a complicated manner, and current knowledge is that there are at least two positive regulators, Ler (LEE-encoded regulator) and GrlA (global regulator of LEE activator), and one negative regulator, called GrlR (global regulator of LEE repressor). In enterohaemorrhagic E. coli, GrlA is encoded by l0043, whereas GrlR is encoded by l0044. Here we report a fourth regulatory gene located in LEE3, namely l0036. Its expression is tightly controlled. When overexpressed, this factor, named Mpc (multiple point controller), interacts with Ler and suppresses the expression of the LEE proteins. When the translation is not initiated or terminated before maturation, the type III secretion of effectors is completely abolished. Therefore, together with the fact that several cis elements reside in the region that l0036 spans, l0036 appeared to have multiple functions in the regulation of LEE expression.

Transcriptional inhibitor of virulence factors in enteropathogenic Escherichia coli

The type III secretion system (TTSS) is a key virulence mechanism of many important gram-negative bacterial pathogens. The TTSS is conserved among different bacterial pathogens, and mutations and deletions to the system significantly decrease virulence, making the TTSS an important potential therapeutic target. We have developed a high-throughput assay to search for inhibitors of the TTSS. We screened a commercial library of 20,000 small molecules for their ability to inhibit type III secretion by enteropathogenic Escherichia coli (EPEC). After discarding compounds that had no effect on secretion, inhibited bacterial growth, and/or caused degradation of EPEC-secreted proteins, the search was focused on a class of compounds that, while not direct inhibitors of type III secretion, inhibit expression of TTSS-related genes and other genes involved in virulence. This class of compounds does not affect bacterial viability or motility, indicating that it is not significantly affecting the expression of essential genes and is specific to virulence-associated genes. Transcriptional fusion assays confirmed that virulence-associated promoters were more sensitive to inhibition by this class of compounds. Overall, we have identified a class of compounds that can be used as a tool to probe the mechanism(s) that regulates virulence gene expression in EPEC.

A positive regulatory loop controls expression of the locus of enterocyte effacement-encoded regulators Ler and GrlA

The formation of attaching and effacing (A/E) lesions on intestinal epithelial cells is an essential step in the pathogenesis of human enteropathogenic and enterohemorrhagic Escherichia coli and of the mouse pathogen Citrobacter rodentium. The genes required for the development of the A/E phenotype are located within a pathogenicity island known as the locus of enterocyte effacement (LEE). The LEE-encoded transcriptional regulators Ler, an H-NS-like protein, and GrlA, a member of a novel family of transcriptional activators, positively control the expression of the genes located in the LEE and their corresponding virulence. In this study, we used C. rodentium as a model to study the mechanisms controlling the expression of Ler and GrlA. By deletion analysis of the ler and grlRA regulatory regions and complementation experiments, negative and positive cis-acting regulatory motifs were identified that are essential for the regulation of both genes. This analysis confirmed that GrlA is required for the activation of ler, but it also showed that Ler is required for the expression of grlRA, revealing a novel regulatory loop controlling the optimal expression of virulence genes in A/E pathogens. Furthermore, our results indicate that Ler and GrlA induce the expression of each other by, at least in part, counteracting the repression mediated by H-NS. However, whereas GrlA is still required for the optimal expression of ler even in the absence of H-NS, Ler is not needed for the expression of grlRA in the absence of H-NS. This type of transcriptional positive regulatory loop represents a novel mechanism in pathogenic bacteria that is likely required to maintain an appropriate spatiotemporal transcriptional response during infection.

Operon structure and gene expression of the espJ--tccP locus of enterohaemorrhagic Escherichia coli O157:H7

In this study, we investigated the genetic organisation and expression of the espJ--tccP locus of enterohaemorrhagic Escherichia coli (EHEC O157:H7), which encodes the type III secretion system effector proteins EspJ and TccP. Using flow cytometry and fluorescence microscopy, we found that espJ and tccP constitute an operon, whose expression in vitro is affected by the composition of the medium and environmental signals such as temperature, pH, osmolarity and O(2) pressure. Moreover, expression of the espJ--tccP operon is not regulated by Ler, a LEE-encoded transcriptional regulator, but by an as yet unidentified regulatory system(s).

Direct and indirect transcriptional activation of virulence genes by an AraC-like protein, PerA from enteropathogenic Escherichia coli

The plasmid-encoded Per regulatory locus of enteropathogenic Escherichia coli (EPEC) is generally considered to consist of three genes, perA, perB and perC. PerA, a member of the AraC-like family of transcriptional regulators, is known to be an activator of its own promoter (autoactivation) as well as of the plasmid-located bfp operon encoding bundle-forming pili, but its role in activation of the chromosomal locus of enterocyte effacement (LEE) pathogenicity island, which confers the property of intimate adherence on EPEC, requires clarification. Here, we show that PerA is also required for activation of the master regulatory LEE operon, LEE1, but that this activation is indirect, being achieved via autoactivation of the per promoter which ensures sufficient production of the PerC protein to activate LEE1. In contrast, PerA-dependent activation of the per and bfp promoters is direct and does not require the other Per proteins, but is modulated by the nucleoid-associated protein H-NS. The closely related VirF regulator from Shigella flexneri cannot substitute for PerA to activate these promoters, despite being able to bind their upstream regions in vitro. PerA can bind the per and bfp promoter fragments to form multiple complexes, while VirF forms only a single complex. Site-directed mutagenesis of the PerA protein suggests that, like VirF, it may use both of its carboxy-terminal helix-turn-helix motifs for DNA interaction, and may also make direct contacts with RNA polymerase. In addition, we have isolated mutations in the poorly characterized amino-terminal domain of PerA which affect its ability to activate gene expression.

Towards a vaccine for attaching/effacing Escherichia coli: a LEE encoded regulator (ler) mutant of rabbit enteropathogenic Escherichia coli is attenuated, immunogenic, and protects rabbits from lethal challenge with the wild-type virulent strain

The ler (LEE encoded regulator) gene product is a central regulator for the genes encoded on the locus of enterocyte effacement (LEE) pathogenicity island of attaching/effacing (A/E) pathogens, including human enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) as well as animal isolates. Although an in vivo role for Ler in bacterial virulence has not been documented, we hypothesized that a Ler deletion mutant should be attenuated for virulence but might retain immunogenicity. The goals of this study were to genetically characterize ler of a rabbit EPEC (rEPEC) strain (O103:H2), to examine the effect of ler on in vivo virulence, and to determine if intragastric inoculation of an attenuated rEPEC ler mutant was immunogenic and could protect rabbits against subsequent challenge with the wild-type virulent parent strain. The predicted ler gene product of rEPEC strain O103:H2 shares high homology (over 95% amino acid identity) with the Lers of another rEPEC strain RDEC-1 (O15:H-) and human EPEC and EHEC. A defined internal ler deletion mutant of rEPEC O103:H2 showed reduced production of secreted proteins. Although orogastric inoculation of rabbits with the virulent parent O103:H2 strain induced severe diarrhea, significant weight loss and early mortality with adherent mucosal bacteria found at sacrifice, the isogeneic ler mutant strain was well tolerated. Animals gained weight and showed no clinical signs of disease. Examination of histological sections of intestinal segments revealed the absence of mucosal bacterial adherence. This result demonstrates an essential role for Ler in in vivo pathogenicity of A/E E. coli. Single dose orogastric immunization with the rEPEC ler mutant induced serum IgG antibody to whole bacteria (but not to intimin). Immunized animals were protected against enteric infection with the WT virulent parent strain exhibiting normal weight gain, absence of diarrhea and absence of mucosally adherent bacteria at sacrifice. Such attenuated ler mutant strains may have potential for use as oral vaccines, or as vaccine vectors for delivery of foreign antigens. It remains to be determined whether such regulatory mutants can protect against infection with A/E bacteria of differing serotypes affecting different hosts.

The H-NS protein represses transcription of the eltAB operon, which encodes heat-labile enterotoxin in enterotoxigenic Escherichia coli, by binding to regions downstream of the promoter

Heat-labile enterotoxin, a major virulence determinant of enterotoxigenic Escherichia coli, is encoded by the eltAB operon. To elucidate the molecular mechanism by which the heat-stable nucleoid-structural (H-NS) protein controls transcription of eltAB, the authors constructed an eltAB-lacZ transcriptional fusion and performed beta-galactosidase analysis. The results showed that H-NS protein exerts fivefold repression on transcription from the eltAB promoter at 37 degrees C and 10-fold repression at 22 degrees C. Two silencer regions that were required for H-NS-mediated repression of eltAB expression were identified, both of which were located downstream of the start site of transcription. One silencer was located between +31 and +110, the other between +460 and +556, relative to the start site of transcription, and they worked cooperatively in repression. DNA sequences containing the silencers were predicted to be curved by in silico analysis and bound H-NS protein directly in vitro. Repression of eltAB transcription by H-NS was independent of promoter strength, and the presence of H-NS protein did not affect promoter opening in vitro, indicating that repression was achieved by inhibiting promoter clearance or blocking transcription elongation, probably via DNA looping between the two silencers.

Identification of a negative regulator for the pathogenicity island of enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) forms histological lesions termed attaching and effacing lesions (A/E lesions) on infected large intestine tissue. The major virulence factors involved in A/E lesions reside on a locus of enterocyte effacement (LEE), a pathogenicity island. The LEE comprises 41 specific open reading frames, of which most are organized in 5 major operons, LEE1, LEE2, LEE3, LEE4, and tir(LEE5). The expression of LEE genes is regulated in a complicated manner by environmental factors such as temperature, osmolarity, and quorum sensing. Current knowledge is that regulation is hierarchical: a pivotal positive regulator, ler, is first stimulated, which in turn activates the expression of other operons. Herein, we report on the presence of a negative regulation protein located within the LEE. L0044 is 372 bp in length and is located outside of the 5 major operons. An isogenic L0044 deletion mutant displayed loss of the repression phenotype and increased synthesis of several LEE proteins when bacteria were cultured under repressive conditions that disfavor expression of LEE proteins. Reciprocally, trans expression of L0044 suppressed the expression of the LEE. Furthermore, mRNA of ler increased as a result of deleting L0044, and disrupting ler in a L0044-deleted background reversed the loss of the repression phenotype. Thus, L0044 plays a role in regulating the expression of virulence genes in EHEC by modulating the activation of ler.

Hyperadherence of an hha mutant of Escherichia coli O157:H7 is correlated with enhanced expression of LEE-encoded adherence genes

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 virulence factors, specifically those conferring intimate adherence to and formation of attaching and effacing lesions (A/E) on host cells, are encoded by a horizontally acquired locus of enterocyte effacement (LEE). Expression of several LEE-encoded genes, which are organized into operons LEE1 through LEE5, is under the positive regulation of ler, the first gene in the LEE1 operon. We have recently demonstrated that EHEC O157:H7 lacking hha exhibited greater than a 10-fold increase in ler expression and that the repression of ler results from the binding of Hha to the ler promoter. In this report, we show that an hha mutant of EHEC O157:H7 exhibited increased adherence to Hep-2 cells, had increased transcriptional activities of LEE1, LEE2, LEE3, and LEE5 as determined by reverse transcriptase-polymerase chain reaction assays, and expressed LEE5::lac transcriptional fusion at levels that were several-fold higher than that expressed by the parental hha+ strain. These results demonstrate that hha is an important regulatory component of the cascade that governs the expression of LEE operons and the resulting ability of EHEC O157:H7 to intimately adhere to host cells.

Antagonistic role of H-NS and GadX in the regulation of the glutamate decarboxylase-dependent acid resistance system in Escherichia coli

One of the most efficient systems of acid resistance in Escherichia coli, the gad system, is based on the coordinated action of two isoforms of glutamate decarboxylase (GadA and GadB) and of a specific glutamate/gamma-aminobutyrate antiporter (GadC). The gadA/BC genes, activated in response to acid stress and in stationary phase cells, are subjected to complex circuits of regulation involving sigma70, sigmaS, cAMP receptor protein, H-NS, EvgAS, TorRS, GadE, GadX, GadW, and YdeO. Herein, we provide evidence that the nucleoid-associated protein H-NS directly functions as repressor of gadA, one of the structural genes, and gadX, a regulatory gene encoding one of the primary activators of the gad system. Band shift and DNase I footprints reveal that H-NS indeed binds to specific sites in the promoter regions of gadA and gadX and represses the transcription of these genes both in an in vitro system and in vivo. Moreover, we show that a maltose-binding protein MalE-GadX fusion is able to stimulate the promoter activity of gadA/BC, thus indicating that GadX is by itself able to up-regulate the gad genes and that a functional competition between H-NS and GadX takes place at the gadA promoter. Altogether, our results indicate that H-NS directly inhibits gadA and gadX transcription and, by controlling the intracellular level of the activator GadX, indirectly affects the expression of the whole gad system.

Temporal expression of enteropathogenic Escherichia coli virulence genes in an in vitro model of infection

The hallmark of enteropathogenic Escherichia coli (EPEC) infection is the ability of EPEC to cause attaching and effacing (A/E) lesions on intestinal epithelium. This event is reproducible in in vitro tissue culture models of infection. We used real-time PCR to measure transcription from several locus of enterocyte effacement (LEE) operons (LEE1 to LEE5) and from bfp during a 5-h infection of HEp-2 cells with EPEC. We found that after the initial formation of A/E lesions, which occurs as early as 5 min postinfection, EPEC continues to increase transcription from LEE3 to LEE5 as well as from bfp. These levels are maximized by 3 h postinfection and remain constant throughout the course of infection. This increase in transcription from LEE3 to LEE5 occurs when LEE1 (ler) transcription is decreasing. EspA, EspB, intimin, Tir, and bundle-forming pilus expression is detectable during the entire 5-h infection. These results indicate that the EPEC genes involved in localized and intimate adherence are continually expressed after the initial stages of A/E lesion formation on host cells.

Bioinformatics analysis of the locus for enterocyte effacement provides novel insights into type-III secretion

Background

Like many other pathogens, enterohaemorrhagic and enteropathogenic strains of Escherichia coli employ a type-III secretion system to translocate bacterial effector proteins into host cells, where they then disrupt a range of cellular functions. This system is encoded by the locus for enterocyte effacement. Many of the genes within this locus have been assigned names and functions through homology with the better characterised Ysc-Yop system from Yersinia spp. However, the functions and homologies of many LEE genes remain obscure.

Results

We have performed a fresh bioinformatics analysis of the LEE. Using PSI-BLAST we have been able to identify several novel homologies between LEE-encoded and Ysc-Yop-associated proteins: Orf2/YscE, Orf5/YscL, rORF8/EscI, SepQ/YscQ, SepL/YopN-TyeA, CesD2/LcrR. In addition, we highlight homology between EspA and flagellin, and report many new homologues of the chaperone CesT.

Conclusion

We conclude that the vast majority of LEE-encoded proteins do indeed possess homologues and that homology data can be used in combination with experimental data to make fresh functional predictions.

Ler is a negative autoregulator of the LEE1 operon in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) causes severe diarrhea in young children. Essential for colonization of the host intestine is the LEE pathogenicity island, which comprises a cluster of operons encoding a type III secretion system and related proteins. The LEE1 operon encodes Ler, which positively regulates many EPEC virulence genes in the LEE region and elsewhere in the chromosome. We found that Ler acts as a specific autorepressor of LEE1 transcription. We further show that Ler specifically binds upstream of the LEE1 operon in vivo and in vitro. A comparison of the Ler affinities to different DNA regions suggests that the autoregulation mechanism limits the steady-state level of Ler to concentrations that are just sufficient for activation of the LEE2 and LEE3 promoters and probably other LEE promoters. This mechanism may reflect the need of EPEC to balance maximizing the colonization efficiency by increasing the expression of the virulence genes and minimizing the immune response of the host by limiting their expression. In addition, we found that the autoregulation mechanism reduces the cell-to-cell variability in the levels of LEE1 expression. Our findings point to a new negative regulatory circuit that suppresses the noise and optimizes the expression levels of ler and other LEE1 genes.

Regulators encoded in the Escherichia coli type III secretion system 2 gene cluster influence expression of genes within the locus for enterocyte effacement in enterohemorrhagic E. coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 subverts host cells through a type III secretion system encoded by the locus for enterocyte effacement (LEE). Genome sequencing of this pathotype revealed the existence of a gene cluster encoding components of a second cryptic type III secretion system, E. coli type III secretion system 2 (ETT2). Recently, we showed that the ETT2 gene cluster is present in whole or in part in the majority of E. coli strains but is unable to encode a functional secretion system in most strains, including EHEC O157:H7. However, here we show that mutational inhibition of two regulatory genes (ECs3720 or etrA and ECs3734 or eivF) from the ETT2 cluster in EHEC O157:H7 leads to greatly increased secretion of proteins encoded by the LEE and to increased adhesion to human intestinal cells. Studies in which transcriptional fusions and microarrays were used indicated that EtrA and EivF exert profound negative effects on gene transcription within the LEE. Consistent with these observations, expression of these regulators in an EHEC O26:H- strain led to suppression of protein secretion under LEE-inducing conditions. These findings provide fresh examples of the influence of mobile genetic elements on regulation of the LEE and of cross talk between type III secretion system gene clusters. In addition, they provide a cautionary tale because they show that the effects of regulatory genes can outlive widespread decay of other genes in a functionally coherent gene cluster, a phenomenon that we have named the "Cheshire cat effect." It also seems likely that variations in the ETT2 regulator repertoire might account for strain-to-strain variation in secretion of LEE-encoded proteins.

A truncated H-NS-like protein from enteropathogenic Escherichia coli acts as an H-NS antagonist

The H-NS nucleoid-associated protein of Escherichia coli is the founder member of a widespread family of gene regulatory proteins which have a bipartite structure, consisting of an N-terminal coiled-coil oligomerization domain and a C-terminal DNA-binding domain. Here we characterize a family of naturally occurring truncated H-NS derivatives lacking the DNA-binding domain, which we term the H-NST family. H-NST proteins are found in large genomic islands in pathogenic E. coli strains, which are absent from the corresponding positions in the E. coli K-12 genome. Detailed analysis of the H-NST proteins from enteropathogenic E. coli (EPEC) and uropathogenic E. coli (UPEC) shows that the EPEC protein (H-NST(EPEC)) has a potent anti-H-NS function at the classical H-NS-repressed operon proU. This correlates with the ability of H-NST(EPEC) to co-purify with H-NS in vitro, and can be abolished by a mutation of leucine 30 to proline which is predicted to prevent the N-terminal region from forming a coiled-coil structure. In contrast, despite being 90% identical to H-NST(EPEC) at the protein level, the UPEC homologue (H-NST(UPEC)) has only a weak anti-H-NS activity, correlating with a much-reduced ability to interact with H-NS during column chromatography. A single amino acid difference at residue 16 appears to account for these different properties. The hnsT(EPEC) gene is transcribed monocistronically and expressed throughout the exponential growth phase in DMEM medium. Our data suggest that a truncated derivative of H-NS encoded by an ancestral mobile DNA element can interact with the endogenous H-NS regulatory network of a bacterial pathogen.

Co-ordinate single-cell expression of LEE4- and LEE5-encoded proteins of Escherichia coli O157:H7

Escherichia coli O157:H7 is a zoonotic pathogen that can express a type III secretion system (TTSS) considered important for colonization and persistence in ruminants. E. coli O157:H7 strains have been shown to vary markedly in levels of protein secreted using the TTSS and this study has confirmed that a high secretion phenotype is more prevalent among isolates associated with human disease than isolates shed by healthy cattle. The variation in secretion levels is a consequence of heterogeneous expression, being dependent on the proportion of bacteria in a population that are actively engaged in protein secretion. This was demonstrated by indirect immunofluorescence and eGFP fusions that examined the expression of locus of enterocyte effacement (LEE)-encoded factors in individual bacteria. In liquid media, the expression of EspA, tir::egfp, intimin, but not map::egfp were co-ordinated in a subpopulation of bacteria. In contrast to E. coli O157:H7, expression of tir::egfp in EPEC E2348/69 was equivalent in all bacteria although the same fusion exhibited variable expression when transformed into an E. coli O157:H7 background. An E. coli O157:H7 strain deleted for the LEE demonstrated weak but variable expression of tir::egfp indicating that the elements controlling the heterogeneous expression lie outside the LEE. The research also demonstrated the rapid induction of tir::egfp and map::egfp on contact with bovine epithelial cells. This control in E. coli O157:H7 may be required to limit exposure of key surface antigens, EspA, Tir and intimin during colonization of cattle but allow their rapid production on contact with bovine gastrointestinal epithelium at the terminal rectum.