Differential response of the bvg virulence regulon of Bordetella pertussis to MgSO4 modulation

Four single-basepair mutations in the ptx promoter of Bordetella bronchiseptica are sufficient to activate the expression of pertussis toxin

Secretion of pertussis toxin (PT) is the preeminent virulence trait of the human pathogen Bordetella pertussis, causing whooping cough. Bordetella bronchiseptica, although it harbors an intact 12-kb ptx-ptl operon, does not express PT due to an inactive ptx promoter (Pptx), which contains 18 SNPs (single nucleotide polymorphisms) relative to B. pertussis Pptx. A systematic analysis of these SNPs was undertaken to define the degree of mutational divergence necessary to activate B. bronchiseptica Pptx. A single change (C^-13T), which created a better - 10 element, was capable of activating B. bronchiseptica Pptx sufficiently to allow secretion of low but measureable levels of active PT. Three additional changes in the BvgA-binding region, only in the context of C^-13T mutant, raised the expression of PT to B. pertussis levels. These results illuminate a logical evolutionary pathway for acquisition of this key virulence trait in the evolution of B. pertussis from a B. bronchiseptica-like common ancestor.

Regulatory Networks of the T4SS Control: From Host Cell Sensing to the Biogenesis and the Activity during the Infection

Delivery of effectors, DNA or proteins, that hijack host cell processes to the benefit of bacteria is a mechanism widely used by bacterial pathogens. It is achieved by complex effector injection devices, the secretion systems, among which Type 4 Secretion Systems (T4SSs) play a key role in bacterial virulence of numerous animal and plant pathogens. Considerable progress has recently been made in the structure-function analyses of T4SSs. Nevertheless, the signals and processes that trigger machine assembly and activity during infection, as well as those involved in substrate recognition and transfer, are complex and still poorly understood. In this review, we aim at summarizing the last updates of the knowledge on signaling pathways that regulate the biogenesis and the activity of T4SSs in important bacterial pathogens.

Regulated, sequential processing by multiple proteases is required for proper maturation and release of Bordetella filamentous hemagglutinin

Filamentous hemagglutinin (FHA) is a critically important virulence factor produced by Bordetella species that cause respiratory infections in humans and other animals. It is also a prototypical member of the widespread two partner secretion (TPS) pathway family of proteins. First synthesized as a ~370 kDa protein called FhaB, its C-terminal ~1,200 amino acid 'prodomain' is removed during translocation to the cell surface via the outer membrane channel FhaC. Here, we identify CtpA as a periplasmic protease that is responsible for the regulated degradation of the prodomain and for creation of an intermediate polypeptide that is cleaved by the autotransporter protease SphB1 to generate FHA. We show that the central prodomain region is required to initiate degradation of the prodomain and that CtpA degrades the prodomain after a third, unidentified protease (P3) first removes the extreme C-terminus of the prodomain. Stepwise proteolysis by P3, CtpA and SphB1 is required for maturation of FhaB, release of FHA into the extracellular milieu, and full function in vivo. These data support a substantially updated model for the mechanism of secretion, maturation and function of this model TPS protein.

Blood or Serum Exposure Induce Global Transcriptional Changes, Altered Antigenic Profile, and Increased Cytotoxicity by Classical Bordetellae

The classical bordetellae sense and respond to a variety of environments outside and within their mammalian hosts. By causing inflammation and tissue damage, we reasoned that bordetellae are likely to encounter components of blood and/or serum during the course of a respiratory infection, and that detecting and responding to these would be advantageous. Therefore, we hypothesized that classical bordetellae have the ability to sense and respond to blood or serum. Blood or serum exposure resulted in substantial transcriptional changes in Bordetella bronchiseptica, including enhanced expression of many virulence-associated genes. Exposure to blood or serum additionally elicited production of multiple antigens not otherwise detectable, and led to increased bacterial cytotoxicity against macrophages. Transcriptional responses to blood/serum were observed in a Bvg^- phase-locked mutant, indicating that the response is not solely dependent on a functional BvgAS system. Similar transcriptional responses to blood/serum were observed for the other classical bordetellae, Bordetella pertussis and Bordetella parapertussis. These data suggest the classical bordetellae respond to signals present in blood and serum by changing their behavior in ways that likely contribute to their remarkable success, via effects on pathogenesis, persistence and/or transmission between hosts.

Identification and characterization of a brilliant yellow pigment produced by Bordetella pertussis

Culture supernatants of Bordetella pertussis are a brilliant yellow; however, the structure and biological role of the responsible pigment have not been investigated. In this study, a brilliant yellow-colored fraction was extracted from culture supernatants of B. pertussis and analyzed by HPLC. UV-visible spectral analysis and mass spectrometry identified the brilliant yellow pigment as riboflavin. Riboflavin production was high in lag and early log phases and riboflavin was found to enhance growth of B. pertussis in low-density cultures. Riboflavin production is not regulated by the BvgAS system. In addition, it was found that other Bordetella species, such as B. parapertussis, B. holmesii and B. bronchiseptica, also release riboflavin into their culture supernatants. This is the first report that B. pertussis secrets riboflavin to the extracellular space and that riboflavin may promote its growth. The mechanism may be associated with pathogenesis of B. pertussis.

Evidence for phenotypic bistability resulting from transcriptional interference of bvgAS in Bordetella bronchiseptica

Bordetella species cause respiratory infections in mammals. Their master regulatory system BvgAS controls expression of at least three distinct phenotypic phases in response to environmental cues. The Bvg⁺ phase is necessary and sufficient for respiratory infection while the Bvg⁻ phase is required for survival ex vivo. We obtained large colony variants (LCVs) from the lungs of mice infected with B. bronchiseptica strain RBX9, which contains an in-frame deletion mutation in fhaB, encoding filamentous haemagglutinin. RBX9 also yielded LCVs when switched from Bvg⁻ phase conditions to Bvg⁺ phase conditions in vitro. We determined that LCVs are composed of both Bvg⁺ and Bvg⁻ phase bacteria and that they result from defective bvgAS positive autoregulation. The LCV phenotype was linked to the presence of a divergent promoter 5' to bvgAS, suggesting a previously undescribed mechanism of transcriptional interference that, in this case, leads to feedback-based bistability (FBM). Our results also indicate that a small proportion of RBX9 bacteria modulates to the Bvg⁻ phase in vivo. In addition to providing insight into transcriptional interference and FBM, our data provide an example of an in-frame deletion mutation exerting a 'polar' effect on nearby genes.

Different requirements for σ Region 4 in BvgA activation of the Bordetella pertussis promoters P(fim3) and P(fhaB)

Bordetella pertussis BvgA is a global response regulator that activates virulence genes, including adhesin-encoding fim3 and fhaB. At the fhaB promoter, P(fhaB), a BvgA binding site lies immediately upstream of the -35 promoter element recognized by Region 4 of the σ subunit of RNA polymerase (RNAP). We demonstrate that σ Region 4 is required for BvgA activation of P(fhaB), a hallmark of Class II activation. In contrast, the promoter-proximal BvgA binding site at P(fim3) includes the -35 region, which is composed of a tract of cytosines that lacks specific sequence information. We demonstrate that σ Region 4 is not required for BvgA activation at P(fim3). Nonetheless, Region 4 mutations that impair its typical interactions with core and with the -35 DNA affect P(fim3) transcription. Hydroxyl radical cleavage using RNAP with σD581C-FeBABE positions Region 4 near the -35 region of P(fim3); cleavage using RNAP with α276C-FeBABE or α302C-FeBABE also positions an α subunit C-terminal domain within the -35 region, on a different helical face from the promoter-proximal BvgA~P dimer. Our results suggest that the -35 region of P(fim3) accommodates a BvgA~P dimer, an α subunit C-terminal domain, and σ Region 4. Molecular modeling suggests how BvgA, σ Region 4, and α might coexist within this DNA in a conformation that suggests a novel mechanism of activation.

Detection of small RNAs in Bordetella pertussis and identification of a novel repeated genetic element

Background

Small bacterial RNAs (sRNAs) have been shown to participate in the regulation of gene expression and have been identified in numerous prokaryotic species. Some of them are involved in the regulation of virulence in pathogenic bacteria. So far, little is known about sRNAs in Bordetella, and only very few sRNAs have been identified in the genome of Bordetella pertussis, the causative agent of whooping cough.

Results

An in silico approach was used to predict sRNAs genes in intergenic regions of the B. pertussis genome. The genome sequences of B. pertussis, Bordetella parapertussis, Bordetella bronchiseptica and Bordetella avium were compared using a Blast, and significant hits were analyzed using RNAz. Twenty-three candidate regions were obtained, including regions encoding the already documented 6S RNA, and the GCVT and FMN riboswitches. The existence of sRNAs was verified by Northern blot analyses, and transcripts were detected for 13 out of the 20 additional candidates. These new sRNAs were named Bordetella pertussis RNAs, bpr. The expression of 4 of them differed between the early, exponential and late growth phases, and one of them, bprJ2, was found to be under the control of BvgA/BvgS two-component regulatory system of Bordetella virulence. A phylogenetic study of the bprJ sequence revealed a novel, so far undocumented repeat of ~90 bp, found in numerous copies in the Bordetella genomes and in that of other Betaproteobacteria. This repeat exhibits certain features of mobile elements.

Conclusion

We shown here that B. pertussis, like other pathogens, expresses sRNAs, and that the expression of one of them is controlled by the BvgA/BvgS system, similarly to most virulence genes, suggesting that it is involved in virulence of B. pertussis.

Osmolarity affects Bvg-mediated virulence regulation by Bordetella pertussis

Bordetella pertussis dramatically alters its phenotype by sensing its environment via the BvgAS regulatory system. Increased concentrations of specific chemicals are used in vitro to induce modulation of the bacterium from the Bvg(+) virulent phenotype to a fully Bvg(-) phenotype. Varied expression of sets of Bvg(-)regulated molecules depends on the modulating capacity of the environment. We examined the effect of a number of chemicals on the modulating capacity of B. pertussis growth media, both alone and in combination with known modulators. It was demonstrated that under certain conditions the Bvg(-)intermediate protein, BipA, is coexpressed with the Bvg(-) antigen, VraA. This demonstrates that the patterns of molecules expressed in the different phenotypes of B. pertussis are more fluid than has previously been demonstrated. The in vitro modulator, sulfate, was found to be a relatively inefficient modulator of our Tohama I-derived B. pertussis strain. However, addition of nicotinic acid, MgCl2, or sucrose in combination with relatively low sulfate concentrations resulted in effective modulation. This suggests that multiple signals may affect modulation through the BvgAS system or possibly through other regulatory networks. In addition, the cooperative modulating effect of sucrose implicates osmolarity as an environmental stimulus that affects phenotypic modulation.

Autoregulation is essential for precise temporal and steady-state regulation by the Bordetella BvgAS phosphorelay

The Bordetella BvgAS virulence control system is prototypical of phosphorelays that use a polydomain sensor and a response regulator to control gene expression in response to environmental cues. BvgAS controls the expression of at least three distinct phenotypic phases (Bvg(-), Bvg(i), and Bvg(+)) by differentially regulating the expression of at least four classes of genes. Among the loci regulated by BvgAS is bvgAS itself. We investigated the role of autoregulation in the ability of BvgAS to control multiple gene expression patterns in a temporal and steady-state manner by constructing Bordetella bronchiseptica strains in which the bvgAS promoter was replaced with constitutively active promoters. Our results show that positive autoregulation of bvgAS transcription is required for the temporal expression of multiple phenotypic phases that occurs in response to a shift from Bvg(-)-phase conditions to Bvg(+)-phase conditions. Autoregulation was also shown to contribute to steady-state regulation; it influences the sensitivity of the system in response to subtle differences in signal intensity. In addition, considered in relation to BvgA and BvgS activities demonstrated in vitro, our results provide insight into how BvgA and BvgS function mechanistically.

Role of BvgA phosphorylation and DNA binding affinity in control of Bvg-mediated phenotypic phase transition in Bordetella pertussis

To investigate the mechanism by which the Bordetella BvgAS phosphorelay controls expression of at least three distinct phenotypic phases, we isolated and characterized two B. pertussis mutants that were able to express Bvg- and Bvg(i) phase phenotypes but not Bvg+ phase phenotypes. In both cases, the mutant phenotype was due to a single nucleotide change in bvgA resulting in a single amino acid substitution in BvgA. In vitro phosphorylation assays showed that BvgA containing the T194M substitution was significantly impaired in its ability to use either BvgS or acetyl phosphate as a substrate for phosphorylation. Binding studies indicated that this mutant protein was able to bind an oligonucleotide containing a high-affinity BvgA binding site in a manner similar to wild-type BvgA, but was defective for binding the fhaB promoter in the absence of RNA polymerase (RNAP). By contrast, BvgA containing the R152H substitution had wild-type phosphorylation properties but was severely defective in its ability to bind either the high-affinity BvgA binding site-containing oligonucleotide or the fhaB promoter by itself. Both mutant BvgA proteins were able to bind the fhaB promoter in the presence of RNAP however, demonstrating the profound effect that RNAP has on stabilizing the ternary complexes between promoter DNA, BvgA and RNAP. Our results are consistent with the hypothesis that BvgAS controls expression of multiple phenotypic phases by adjusting the intracellular concentration of BvgA-P and they demonstrate the additive nature of BvgA binding site affinity and protein-protein interactions at different Bvg-regulated promoters.

Bordetella pertussis risA, but not risS, is required for maximal expression of Bvg-repressed genes

Expression of virulence determinants by Bordetella pertussis, the primary etiological agent of whooping cough, is regulated by the BvgAS two-component regulatory system. The role of a second two-component regulatory system, encoded by risAS, in this process is not defined. Here, we show that mutation of B. pertussis risA does not affect Bvg-activated genes or proteins. However, mutation of risA resulted in greatly diminished expression of Bvg-repressed antigens and decreased transcription of Bvg-repressed genes. In contrast, mutation of risS had no effect on the expression of Bvg-regulated molecules. Mutation of risA also resulted in decreased bacterial invasion in a HeLa cell model. However, decreased invasion could not be attributed to the decreased expression of Bvg-repressed products, suggesting that mutation of risA may affect the expression of a variety of genes. Unlike the risAS operons in B. parapertussis and B. bronchiseptica, B. pertussis risS is a pseudogene that encodes a truncated RisS sensor. Deletion of the intact part of the B. pertussis risS gene does not affect the expression of risA-dependent, Bvg-repressed genes. These observations suggest that RisA activation occurs through cross-regulation by a heterologous system.

The EAL domain protein VieA is a cyclic diguanylate phosphodiesterase

The newly recognized bacterial second messenger 3',5'-cyclic diguanylic acid (cyclic diguanylate (c-di-GMP)) has been shown to regulate a wide variety of bacterial behaviors and traits. Biosynthesis and degradation of c-di-GMP have been attributed to the GGDEF and EAL protein domains, respectively, based primarily on genetic evidence. Whereas the GGDEF domain was demonstrated to possess diguanylate cyclase activity in vitro, the EAL domain has not been tested directly for c-di-GMP phosphodiesterase activity. This study describes the analysis of c-di-GMP hydrolysis by an EAL domain protein in a purified system. The Vibrio cholerae EAL domain protein VieA has been shown to inversely regulate biofilm-specific genes (vps) and virulence genes (ctxA), presumably by decreasing the cellular pool of c-di-GMP. VieA was maximally active at neutral pH, physiological ionic strength, and ambient temperatures and demonstrated c-di-GMP hydrolytic activity with a Km of 0.06 microM. VieA was unable to hydrolyze cGMP. The putative metal coordination site of the EAL domain, Glu170, was demonstrated to be necessary for VieA activity. Furthermore, the divalent cations Mg2+ and Mn2+ were necessary for VieA activity; conversely, Ca2+ and Zn2+ were potent inhibitors of the VieA phosphodiesterase. Calcium inhibition of the VieA EAL domain provides a potential mechanism for regulation of c-di-GMP degradation.

Functional characterization of the BvgAS two-component system of Bordetella holmesii

The BvgAS two-component system is the master regulator of virulence gene expression in the mammalian pathogens Bordetella pertussis, Bordetella parapertussis and Bordetella bronchiseptica. This paper reports the partial cloning and characterization of the bvgAS loci of the 'new' Bordetella species Bordetella holmesii, Bordetella trematum and Bordetella hinzii, which are increasingly recognized as opportunistic pathogens in humans. It is demonstrated that the cytoplasmic signalling domains of the BvgS histidine kinases of B. pertussis and B. holmesii are functionally interchangeable, while signal perception by the two sensor proteins seems to be different. Furthermore, it is shown that, despite the high similarity of the BvgA proteins of B. pertussis and B. holmesii, promoter recognition by the response regulator proteins differs substantially in these organisms.

The BvgAS signal transduction system regulates biofilm development in Bordetella

The majority of Bordetella sp. virulence determinants are regulated by the BvgAS signal transduction system. BvgAS mediates the control of multiple phenotypic phases and a spectrum of gene expression profiles specific to each phase in response to incremental changes in the concentrations of environmental signals. Studies highlighting the critical role of this signaling circuitry in the Bordetella infectious cycle have focused on planktonically growing bacterial cells. It is becoming increasingly clear that the major mode of bacterial existence in the environment and within the body is a surface-attached state known as a biofilm. Biofilms are defined as consortia of sessile microorganisms that are embedded in a matrix. During routine growth of Bordetella under agitating conditions, we noticed the formation of a bacterial ring at the air-liquid interface of the culture tubes. We show here that this surface adherence property reflects the ability of these organisms to form biofilms. Our data demonstrate that the BvgAS locus regulates biofilm development in Bordetella. The results reported in this study suggest that the Bvg-mediated control in biofilm development is exerted at later time points after the initial attachment of bacteria to the different surfaces. Additionally, we show that these biofilms are highly tolerant of a number of antimicrobials, including the ones that are currently recommended for treatment of veterinary and human infections caused by Bordetella spp. Finally, we discuss the significance of the biofilm lifestyle mode as a potential contributor to persistent infections.

Environmental sensing mechanisms in Bordetella

The success of a bacterial pathogen may depend on its ability to sense and respond to different environments. This is particularly true of those pathogens whose survival depends on adaptation to different niches both within and outside the host. Members of the genus Bordetella cause infections in humans, other animals and birds. Two closely related species, B. pertussis and B. bronchiseptica, cause respiratory disease and express a similar range of virulence factors during infection, but exhibit different host ranges and responses to environmental change. B. pertussis has no known reservoir other than humans and is assumed to be transmitted directly via aerosol droplets between hosts. B. bronchiseptica, on the other hand, has the potential to survive and grow in the natural environment. Comparison of the manner in which these two organisms respond to external signals has provided important insights into the co-ordinate regulation of gene expression as a response to a changing environment. During infection, both species produce a range of virulence factors whose expression is co-ordinated by two members of the two-component family of signal transduction proteins, the bvg (bordetella virulence gene) and ris (regulator of intracellular stress response) loci. When active, the bvg locus directs the activity of a number of virulence determinants in both species whose products, such as adhesins and toxins, establish colonization of the host by the bacteria, although each organism has evolved a slightly different strategy during pathogenesis. B. pertussis, the causative agent of whooping cough, promotes an acute disease and tends to be more virulent than B. bronchiseptica which generally causes chronic and persistent asymptomatic colonization of the respiratory tract. The recently identified ris locus appears to control the expression of factors important for intracellular survival of B. bronchiseptica, but a role for this regulatory locus in B. pertussis infection has not been established. Expression of the virulence determinants controlled by the bvg and ris loci is subject to modulation by different environmental signals, such as low temperature, which act through these two-component systems. Evidence indicates that, for B. bronchiseptica, bvg-controlled determinants expressed under modulating conditions, such as motility, facilitate adaptation and survival in environments outside the host. With B. pertussis, however, there is no apparent requirement for prolonged survival outside the host and this difference is reflected in the expression of different, as yet uncharacterized, determinants as a response to modulating signals. The nature of the gene products involved and their assumed role in the life cycle of B. pertussis remains to be determined. Thus, comparative analysis of these species provides an excellent model for understanding the genetic requirements for pathogenesis of respiratory infection and adaptation to changing environments, both within and outside the host.

Diversity in the Bordetella virulence regulon: transcriptional control of a Bvg-intermediate phase gene

The BvgAS signal transduction system controls the expression of at least three distinct phenotypic phases that lie along a continuum of gene expression states. The Bvg+ phase is characterized by the expression of adhesins and toxins, whereas the Bvg- phase is characterized by motility in Bordetella bronchiseptica and the expression of vrg loci in Bordetella pertussis. The Bvg-intermediate (Bvgi) phase is characterized by the absence of Bvg-repressed phenotypes, the expression of some, but not all, Bvg-activated virulence factors and the presence of a recently discovered set of antigens and phenotypes that are unique to this phase. We report here the transcriptional regulation of bipA, the first-identified Bvgi phase gene. We have mapped the bipA promoter and identified numerous BvgA binding sites in the transcriptional control region. Based on these data, we present a model in which phase-dependent expression of bipA results from the spatial distribution and relative affinities of multiple BvgA binding sites relative to the start site of transcription.

The virulence factors ofBordetella pertussis: a matter of control

Bordetella pertussis is the causative agent of whooping cough, a contagious childhood respiratory disease. Increasing public concern over the safety of whole-cell vaccines led to decreased immunisation rates and a subsequent increase in the incidence of the disease. Research into the development of safer, more efficacious, less reactogenic vaccine preparations was concentrated on the production and purification of detoxified B. pertussis virulence factors. These virulence factors include adhesins such as filamentous haemagglutinin, fimbriae and pertactin, which allow B. pertussis to bind to ciliated epithelial cells in the upper respiratory tract. Once attachment is initiated, toxins produced by the bacterium enable colonisation to proceed by interfering with host clearance mechanisms. B. pertussis co-ordinately regulates the expression of virulence factors via the Bordetella virulence gene (bvg) locus, which encodes a response regulator responsible for signal-mediated activation and repression. This strict regulation mechanism allows the bacterium to express different gene subsets in different environmental niches within the host, according to the stage of disease progression.

Genetic and biochemical analyses of BvgA interaction with the secondary binding region of the fha promoter of Bordetella pertussis

The BvgA-BvgS two-component signal transduction system regulates expression of virulence factors in Bordetella pertussis. The BvgA response regulator activates transcription by binding to target promoters, which include those for the genes encoding filamentous hemagglutinin (fha) and pertussis toxin (ptx). We have previously shown that at both promoters the phosphorylated form of BvgA binds multiple high- and low-affinity sites. Specifically, at the fha promoter, we proposed that there may be high- and a low-affinity binding sites for the BvgA dimer. In our present investigation, we used DNA binding analyses and in vitro and in vivo assays of promoters with substitutions and deletions to support and extend this hypothesis. Our observations indicate that (i) binding of BvgA approximately P to a primary (high-affinity) site and a secondary binding region (lower affinity) is cooperative, (ii) although both the primary binding site and the secondary binding region are required for full activity of the wild-type (undeleted) promoter, deletion of two helical turns within the secondary binding region can produce a fully active or hyperactive promoter, and (iii) BvgA binding to the secondary binding region shows limited DNA sequence specificity.

Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci

Salmonella typhimurium has three distinct Mg2+ transport systems, the constitutive high-capacity CorA transporter and two P-type ATPases, MgtA and MgtB, whose transcription is repressed by normal concentrations of Mg2+ in the growth medium. The latter Mg(2+)-transporting ATPase is part of a two-gene operon, mgtCB, with mgtC encoding a 23 kDa protein of unknown function. Transcriptional regulation using fusions of the promoter regions of mgtA and mgtCB to luxAB showed a biphasic time and Mg2+ concentration dependence. Between 1 and 6 h after transfer to nitrogen minimal medium containing defined concentrations of Mg2+, transcription increased about 200-fold for mgtCB and up to 400-fold for mgtA, each with a half-maximal dependence on Mg2+ of 0.5 mM. Continued incubation revealed a second phase of increased transcription, up to 2000-fold for mgtCB and up to 10,000-fold for mgtA. This secondary increase occurred between 6 and 9 h after transfer to defined medium for mgtCB but between 12 and 24 h for mgtA and had a distinct half-maximal dependence for Mg2+ of 0.01 mM. A concomitant increase of at least 1000-fold in uptake of cation was seen between 8 and 24 h incubation with either system, showing that the transcriptional increase was followed by functional incorporation of large amounts of the newly synthesized transporter into the membrane. Regulation of transcription by Mg2+ was not dependent on a functional stationary-phase sigma factor encoded by rpoS, but it was dependent on the presence of a functional phoPQ two-component regulatory system. Whereas mgtCB was completely dependent on regulation via phoPQ, the secondary late Mg(2+)-dependent phase of mgtA transcription was still evident in strains carrying a mutation in either phoP or phoQ, albeit substantially diminished. Several divalent cations blocked the early phase of the increase in transcription elicited by the decrease in Mg2+ concentration, including cations that inhibit Mg2+ uptake (Co2+, Ni2+ and Mn2+) and those which do not (Ca2+ and Zn2+). In contrast, the second later phase of the transcriptional increase was not well blocked by any cation except those which inhibit uptake. Overall, the data suggest that at least two distinct mechanisms for transcriptional regulation of the mgtA and mgtCB loci exist.

A new gene locus of Bordetella pertussis defines a novel family of prokaryotic transcriptional accessory proteins

Recently, a novel type of regulatory mutation causing differential effects on the expression of virulence genes due to a slight overexpression of the RNA polymerase alpha subunit (RpoA) was found in Bordetella pertussis (N. H. Carbonetti, T. M. Fuchs, A. A. Patamawenu, T. J. Irish, H. Deppisch, and R. Gross, J. Bacteriol. 176:7267-7273, 1994). To gather information on the molecular events behind this phenomenon, we isolated suppressor mutants of the RpoA-overexpressing strains after random mutagenesis. Genetic characterization of these suppressor strains revealed the existence of at least three distinct groups of dominant alleles. Mutations occurred either in the rpoA locus itself, in the bvg locus, or in unknown gene loci. One mutant of the latter group was further characterized. By the introduction of a cosmid library containing genomic B. pertussis DNA into this suppressor strain, we isolated a cosmid which suppressed the phenotype of the suppressor strain, thus restoring the negative effect on transcription of the ptx and cya toxin genes. Mutagenesis of the cosmid with Tn5 led to the identification of the gene locus responsible for this phenomenon. Its DNA sequence revealed the presence of an open reading frame (ORF) consisting of 2,373 bp coding for a hypothetical 86-kDa protein with extensive sequence similarities to ORFs with not yet identified functions of Escherichia coli, Haemophilus influenzae, and Neisseria meningitidis. The new gene, termed tex, for toxin expression, seems to be an essential factor for B. pertussis, as it cannot be deleted from the bacterial chromosome. All members of this new protein family show significant sequence similarities with the mannitol repressor protein MtlR and with the presumptive RNA-binding domains of the Pnp and ribosomal S1 proteins of E. coli in their N- and C-terminal parts, respectively. These sequence similarities and the fact that the tex gene was isolated by virtue of its effects on gene expression in B. pertussis indicate that the members of this new protein family may play an important role in the transcription machinery of prokaryotic organisms.

Synergistic binding of RNA polymerase and BvgA phosphate to the pertussis toxin promoter of Bordetella pertussis

Regulation of virulence factor expression in Bordetella pertussis is mediated by the BvgAS two-component regulatory system. Although previous studies have demonstrated that the transcriptional regulation of the filamentous hemagglutinin gene (fhaB) involves binding of the BvgA activator directly to the fhaB promoter region, the mechanism of pertussis toxin operon (ptx) regulation by BvgA has remained unclear. We demonstrate in vitro the specific binding of BvgA to a region upstream of the ptx promoter that encompasses a 20-bp directly repeated sequence (positions -157 to -117) previously shown to be critical for BvgA-dependent activation. This binding is strictly dependent on the phosphorylation of BvgA, which can be obtained by incubation of BvgA with acetyl phosphate. By DNase I protection studies, we demonstrate the synergistic binding of BvgA-phosphate and purified Escherichia coli RNA polymerase to the ptx promoter. In the presence of the polymerase holoenzyme, a greatly extended footprint encompassing the region between -163 and the putative polymerase binding site was observed. The implications of these observations for pertussis toxin expression and regulation are discussed.

BvgAS is sufficient for activation of the Bordetella pertussis ptx locus in Escherichia coli

BvgA and BvgS, which regulate virulence gene expression in Bordetella pertussis, are members of the two-component signal transduction family. The effects of growth conditions on the ability of BvgAS to activate transcription of fhaB (encoding filamentous hemagglutinin) and ptxA (encoding the S1 subunit of pertussis toxin) were assessed in Escherichia coli by using chromosomal fhaB-lacZYA and ptxA-lacZYA fusions. Although it had previously been reported that a ptxA-lacZYA transcriptional fusion was not activated by bvgAS in E. coli (J. F. Miller, C. R. Roy, and S. Falkow, J. Bacteriol. 171:6345-6348, 1989), we now present evidence that ptxA is activated by bvgAS in E. coli in a manner that is highly dependent on the growth conditions. Higher levels of beta-galactosidase were produced by ptxA-lacZYA in the presence of bvgAS during growth in Stainer-Scholte medium or M9 minimal salts medium with glucose than in Luria-Bertani medium. In contrast, the level of fhaB-lacZYA expression was high during growth in all media. Addition of modulating stimuli which inhibit BvgAS function eliminated expression of ptxA-lacZYA. Levels of beta-galactosidase expressed from the ptx-lacZYA fusion correlated with growth rate and with the final optical density at 600 nm, suggesting that the lower growth rate in M9-glucose and Stainer-Scholte media was responsible for greater accumulation of beta-galactosidase than was seen in Luria-Bertani medium. Overproduction of BvgA was not sufficient for activation of ptxA expression but was sufficient for fhaB expression. However, overproduction of a constitutive BvgA allele (bvgA-Cl) or overproduction of BvgA in the presence of BvgS was able to activate ptxA. Our results demonstrate Bvg-dependent activation of a ptxA-lacZYA fusion in E. coli and indicate that bvg is the only Bordetella locus required for ptxA activation in this heterologous system.

Mutations in the bvgA gene of Bordetella pertussis that differentially affect regulation of virulence determinants

By using chemical mutagenesis and genetic mapping, a search was undertaken for previously undescribed genes which may be involved in different regulatory mechanisms governing different virulence factors of Bordetella pertussis. Previous studies have shown that the fha locus encoding filamentous hemagglutinin is regulated directly by the bvgAS two component system, while regulation of ptx encoding pertussis toxin is less direct or occurs by a different mechanism. With a strain containing gene fusions to each of these regulated loci, screening was done for mutations which were defective for ptx expression but maintained normal or nearly normal levels of fha expression. Two mutations which had such a phenotype and were also deficient in adenylate cyclase toxin/hemolysin expression were found and characterized more fully. Both were found to affect residues in the C-terminal portion of the BvgA response regulator protein, a domain which shares sequence similarity with a family of regulatory proteins including FixJ, UhpA, MalT, RcsA, RcsB, and LuxR. The residues affected are within a region which, by extension from studies on the LuxR protein, may be involved in transcriptional activation.

Mutational analysis of the Bordetella pertussis fim/fha gene cluster: identification of a gene with sequence similarities to haemolysin accessory genes involved in export of FHA

The chromosome of Bordetella pertussis harbours a region of 27 contiguous kb, which contains the bvg, fha and fim genes, involved in the co-ordinate regulation of virulence genes, FHA production and fimbriae production, respectively. The linkage of FHA and fimbrial genes has resulted in some confusion concerning the existence and location of genes required for the production of FHA and the function of the fimbrial genes fimB-D, which were proposed to be involved in both FHA and fimbriae biosynthesis. Through the use of non-polar mutations in each of these genes, we found that fimB-D are required for the production of both serotype 2 and 3 fimbriae, but not for FHA biosynthesis. Furthermore, a large open reading frame, designated fhaC, was identified downstream of fimD. It was shown that fhaC is essential for FHA production but not for fimbriae biogenesis. We propose that insertion mutations in fimB-D affect FHA production because of polar effects on fhaC expression. An insertion in the region downstream of fhaC had only a slight effect on FHA and fimbriae production. The fhaC gene product shows homology with ShIB and HpmB, two outer membrane proteins involved in export and activation of the haemolysins, ShIA and HpmA, of Serratia marcescens and Proteus mirabilis, respectively. Homology is also observed between the N-termini of FHA, ShIA and HpmA. Export of the haemolysins requires the N-termini of these molecules, and when this region was removed from FHA by an in-frame deletion, FHA biosynthesis was abolished. These results suggest that the N-terminus of FHA interacts with FhaC, and that as a result FHA is transported across the outer membrane.

Environmental regulation of virulence factors in Bordetella species

Many bacteria respond in a coordinate manner to environmental changes. External stimuli, sensed by receptors, are transduced to regulatory proteins which participate in well defined pathways of gene expression by varying their structure and mode of action. The network of environmental signal transduction is responsible for a fine and continuous communication between the host and the pathogenic bacteria. As a result, the gene expression machinery of the pathogen is modified continuously, in order to establish the optimal conditions for bacterial survival and multiplication.

Modulation of adenylate cyclase toxin production as Bordetella pertussis enters human macrophages

During the course of human infection, Bordetella pertussis colonizes sequential niches in the respiratory tract that include intracellular and extracellular environments. In vitro the expression of virulence factors such as the adenylate cyclase toxin is coordinately regulated by the bvg locus, which is an example of a two-component sensory transduction system. With this toxin as a reporter, enzyme activities were compared between a wild-type and an altered strain to determine whether bacterial entry into human macrophages affected gene expression. BPRU140, a strain containing an inducible expression vector, produced enzyme activity independent of bvg. Samples of the parent, the induced, and the uninduced BPRU140 were incubated individually with macrophages for 30 min. Extracellular bacteria were then killed by gentamicin. The number of viable intracellular bacteria and the internalized bacterial enzyme activity were measured over time. By 2.5 hr all samples reached a steady-state concentration of 10(5) bacteria per 10(6) macrophages. Following an initial peak of enzyme activity, adenylate cyclase values for the parent and the uninduced BPRU140 decreased to a basal level, while the values for the induced strain remained at least 3-fold greater. Therefore, compared with the persistence of enzyme in the induced strain BPRU140, the decrease in enzyme production by the parent and the uninduced BPRU140 upon entry into macrophages indicates in vivo down-modulation of gene expression. These observations support the hypothesis that sensory transduction contributes to adaptations for bacterial survival in the infected host.