The bvg-dependent promoters show similar behaviour in different Bordetella species and share sequence homologies

The regulation of bacterial two-partner secretion systems

Two-partner secretion (TPS) systems, also known as Type Vb secretion systems, allow the translocation of effector proteins across the outer membrane of Gram-negative bacteria. By secreting different classes of effectors, including cytolysins and adhesins, TPS systems play important roles in bacterial pathogenesis and host interactions. Here, we review the current knowledge on TPS systems regulation and highlight specific and common regulatory mechanisms across TPS functional classes. We discuss in detail the specific regulatory networks identified in various bacterial species and emphasize the importance of understanding the context-dependent regulation of TPS systems. Several regulatory cues reflecting host environment during infection, such as temperature and iron availability, are common determinants of expression for TPS systems, even across relatively distant species. These common regulatory pathways often affect TPS systems across subfamilies with different effector functions, representing conserved global infection-related regulatory mechanisms.

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.

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.

Molecular characterization of the BvgA response regulator of Bordetella holmesii

The BvgAS system controls the expression of most virulence factors in Bordetella pertussis. Recently, we identified an orthologous system in the related human pathogen Bordetella holmesii. However, while we found that the orthologous histidine kinases BvgS could be functionally exchanged between the two species, the B. holmesii response regulator BvgA(BH) could not substitute for its B. pertussis counterpart in vivo and, accordingly, was not able to bind to B. pertussis virulence promoters in vitro. Here we show that a hybrid response regulator consisting of the B. pertussis derived DNA-binding output domain of BvgA(BP) combined with the B. holmesii receiver domain binds to BvgA(BP) regulated virulence promoters of B. pertussis in vitro and is functional in B. pertussis in vivo. This shows that the inability of BvgA(BH) to complement BvgA(BP) in B. pertussis is due to the small number of sequence variations present in its output domain. However, by mutation analysis we show that four amino acid exchanges present in the helix-turn-helix motif of BvgA(BH) as compared to BvgA(BP) are not the only reason for its inability to substitute for BvgA(BP) but additional mutations present in the output domain must play a role.

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.

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.

Bordetella pertussis autoregulates pertussis toxin production through the metabolism of cysteine

Pertussis toxin (Ptx) expression and secretion in Bordetella pertussis are regulated by a two-component signal transduction system encoded by the bvg regulatory locus. However, it is not known whether the metabolic pathways and growth state of the bacterium influence synthesis and secretion of Ptx and other virulence factors. We have observed a reduction in the concentration of Ptx per optical density unit midway in fermentation. Studies were conducted to identify possible factors causing this reduction and to develop culture conditions that optimize Ptx expression. Medium reconstitution experiments demonstrated that spent medium and a fraction of this medium containing components with a molecular weight of <3,000 inhibited the production of Ptx. A complete flux analysis of the intermediate metabolism of B. pertussis revealed that the sulfur-containing amino acids methionine and cysteine and the organic acid pyruvate accumulated in the media. In fermentation, a large amount of internal sulfate (SO4(2-)) was observed in early stage growth, followed by a rapid decrease as the cells entered into logarithmic growth. This loss was later followed by the accumulation of large quantities of SO4(2-) into the media in late-stage fermentation. Release of SO4(2-) into the media by the cells signaled the decoupling of cell growth and Ptx production. Under conditions that limited cysteine, a fivefold increase in Ptx production was observed. Addition of barium chloride (BaCl2) to the culture further increased Ptx yield. Our results suggest that B. pertussis is capable of autoregulating the activity of the bvg regulon through its metabolism of cysteine. Reduction of the amount of cysteine in the media results in prolonged vir expression due to the absence of the negative inhibitor SO4(2-). Therefore, the combined presence and metabolism of cysteine may be an important mechanism in the pathogenesis of B. pertussis.

Molecular characterization of Bordetella bronchiseptica filamentous haemagglutinin and its secretion machinery

Two closely related pathogens, Bordetella pertussis and Bordetella bronchiseptica, share a number of virulence factors. Filamentous haemagglutinin (FHA) is widely regarded as the dominant adhesin of B. pertussis, and its multiple binding activities have been well characterized. This large protein is produced and secreted at high levels by B. pertussis and significantly lower levels by B. bronchiseptica strains. FHA secretion is mediated by a single outer-membrane accessory protein, FhaC. The genes encoding FHA and FhaC in B. bronchiseptica were characterized by sequencing and functional analyses and are highly similar to those of B. pertussis. The most distinctive feature of B. bronchiseptica FHA is additional repeats in the N-terminal portion of the predicted protein. Interestingly, a point mutation in the fhaB promoter region of the B. bronchiseptica GP1 isolate, relative to other isolates, was found to be detrimental to promoter activity and to FHA production. FhaC and the N-terminal secretion domain of FHA of B. bronchiseptica were fully functional for secretion in B. pertussis. Thus, the different levels of FHA secretion by these Bordetella species might reflect differences in physiology, composition and structure of cell envelope, or differential protein degradation. Characterization of FHA expression and function may provide clues as to the basis of host species tropism, tissue localization and receptor recognition.

Identification and characterization of iron-regulated Bordetella pertussis alcaligin siderophore biosynthesis genes

Bordetella bronchiseptica mutants BRM1, BRM6, and BRM9 fail to produce the native dihydroxamate siderophore alcaligin. A 4.5-kb BamHI-Smal Bordetella pertussis genomic DNA fragment carried multiple genes required to restore alcaligin production to these siderophore-deficient mutants. Phenotypic complementation analysis using subclones of the 4.5-kb genomic region demonstrated that the closely linked BRM1 and BRM9 mutations were genetically separable from the BRM6 mutation, and both insertions exerted strong polar effects on expression of the downstream gene defined by the BRM6 mutation, suggesting a polycistronic transcriptional organization of these alcaligin biosynthesis genes. Subcloning and complementation experiments localized the putative Bordetella promoter to a 0.7-kb BamHI-SphI subregion of the cloned genomic DNA fragment. Nucleotide sequencing, phenotypic analysis of mutants, and protein expression by the 4.5-kb DNA fragment in Escherichia coli suggested the presence of three alcaligin system genes, namely, alcA, alcB, and alcC. The deduced protein products of alcA, alcB, and alcC have significant primary amino acid sequence similarities with known microbial siderophore biosynthesis enzymes. Primer extension analysis mapped the transcriptional start site of the putative alcaligin biosynthesis operon containing alcABC to a promoter region overlapping a proposed Fur repressor-binding site and demonstrated iron regulation at the transcriptional level.

Differential binding of BvgA to two classes of virulence genes of Bordetella pertussis directs promoter selectivity by RNA polymerase

Transcription of virulence genes of Bordetella pertussis is co-ordinately regulated by the BvgA and BvgS proteins, which are members of the two-component family of bacterial signal-transduction proteins. BvgS is the transmembrane sensor and BvgA the transcriptional regulator. By gel mobility shift assays we demonstrate that phosphorylated BvgA (BvgA approximately P) forms distinct complexes with the filamentous haemagglutinin (PFHA) promoter DNA at different BvgA approximately P: DNA ratios. DNase I protection analyses show that phosphorylation of BvgA not only enhances affinity of the protein for the binding sites of the PFHA and bvgP1 promoters, but it extends significantly the bound region towards position -35 of these promoters. Conversely, a 10-fold higher amount of BvgA approximately P is required for binding to a large DNA region, from -168 to -60, of the pertussis toxin (Ptox) promoter sequence. These findings suggest that the molecular interaction of BvgA approximately P with the Ptox promoter is different from its interaction with the PFHA and bvgP1 promoters. The sigma 70 Escherichia coli RNA polymerase (RNP) does not bind to the bvg-regulated promoters. However, following the formation of a BvgA approximately P-promoter complex, the E. coli RNP specifically recognizes and binds to the bvg-regulated promoters. Thus, BvgA approximately P exerts its action at the level of promoter recognition by directing promoter selectivity by RNP.

Tracheal colonization factor: a Bordetella pertussis secreted virulence determinant

We report here the identification of a virulence-associated factor, Tcf, (tracheal colonization factor), produced by strains of Bordetella pertussis but not Bordetella parapertussis or Bordetella bronchiseptica. This protein is encoded by the tcfA gene. When a strain of B. pertussis 18323 lacking this protein is used to infect mice with an aerosol challenge, the number of bacteria isolated from the tracheas is decreased 10-fold when compared with the parent 18323. The derived amino acid sequence of tcfA predicts a 68 kDa RGD-containing, proline-rich protein, which after cleavage of a typical prokaryotic signal sequence would be 64 kDa. Amino acid sequence analysis demonstrates that the C-terminal 30 kDa of this protein shows 50% identity to the 30 kDa C-terminus of another Bordetella protein, the pertactin precursor. The N-terminal 34 kDa region contains the three amino-acid motif RGD and is 16.5% proline. Coupled in vitro transcription and translation analysis indicates that the tcfA gene product migrates as two bands of approximately 90 kDa. A fusion protein of the N-terminal, 34 kDa portion of Tcf to maltose-binding protein migrates, on SDS-PAGE, 30 kDa higher than expected from the combined molecular weights. Polyclonal antisera raised against the unique N-terminal portion of Tcf recognizes 90 kDa and 60 kDa bands in immunoblots of whole-cell lysates of strains of B. pertussis; it does not recognize any protein in whole-cell lysates of B. bronchiseptica or B. parapertussis. Supernatants of cultures of B. pertussis 18323 contain the 60 kDa form of the protein. Southern blot analysis of chromosomal DNA from strains of B. bronchiseptica and B. parapertussis, using a probe derived from tcfA, shows strong hybridization only to B. pertussis DNA. Thus, Tcf appears to be a unique virulence factor of B. pertussis.

DNA topology affects transcriptional regulation of the pertussis toxin gene of Bordetella pertussis in Escherichia coli and in vitro

The bvg locus of Bordetella pertussis encodes an environmentally inducible operon essential for the expression of virulence genes. We show that in Escherichia coli, the PTOX promoter cloned in cis of the bvg locus is activated and environmentally regulated. Cotransformation of E. coli with the bvg locus cloned in a low-copy-number plasmid and with the PTOX promoter cloned in a high-copy-number plasmid can give rise to two different results. If the PTOX promoter is cloned in the pGem-3 vector, transcription is absent. If the PTOX promoter is cloned in the plasmid pKK232, containing the PTOX promoter between two ribosomal gene terminators of transcription, transcription occurs, although regulation of transcription is abolished. Under these conditions, the intracellular amount of RNA transcripts is increased by adding to the culture medium novobiocin, an inhibitor of bacterial gyrases. In vitro, the transcription of the PTOX promoter is activated on E. coli RNA polymerase supplemented with cell extracts from wild-type B. pertussis. Addition of DNA gyrase to the mixture dramatically reduces the amount of RNA synthesized. Our data show that the products of the bvg locus, BvgA and BvgS, are directly involved in the regulation of the PTOX promoter in E. coli and that DNA topology may play a role in the induction of transcription.

Flagellin gene transcription in Bordetella bronchiseptica is regulated by the BvgAS virulence control system

The products of the bvgAS locus activate expression of a majority of the known Bordetella virulence factors but also exert negative control over a class of genes called vrg genes (bvg-repressed genes). BvgAS negatively controls the production of flagella and the phenotype of motility in Bordetella bronchiseptica. In this study flaA, the flagellin gene, was cloned and characterized to facilitate studies of this negative control pathway. An internal flaA probe detected hybridizing sequences on genomic Southern blots of Bordetella pertussis, Bordetella parapertussis, and Bordetella avium, although B. pertussis and B. parapertussis are nonmotile. FlaA is similar to the FliC flagellins of Salmonella typhimurium and Escherichia coli, and flaA complemented an E. coli flagellin mutant. Insertional inactivation of the chromosomal flaA locus eliminated motility, which was restored by complementation with the wild-type locus. Analysis of flaA mRNA production by Northern (RNA) blotting and primer extension indicated that negative regulation by BvgAS occurs at the level of transcription. The transcriptional start site of flaA mapped near a consensus site for the alternative sigma factor, sigma F, encoded by fliA in E. coli and S. typhimurium. Consistent with a role for a fliA analog in B. bronchiseptica, transcriptional activation of a flaA-lacZ fusion in E. coli required fliA and a flaA-linked locus designated frl.frl also efficiently complemented mutations in the flagellar master regulatory locus, flhDC, of E. coli. Our analysis of the motility phenotype of B. bronchiseptica suggests that the Bordetella virulence control system mediates transcriptional control of flaA through a regulatory hierarchy that includes the frl locus and an alternative sigma factor.

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.

Characterization of a Bordetella pertussis fimbrial gene cluster which is located directly downstream of the filamentous haemagglutinin gene

The biosynthesis of fimbriae is a complex process requiring multiple genes which are generally found clustered on the chromosome. In Bordetella pertussis, only major fimbrial subunit genes have been identified, and no evidence has yet been found that they are located in a fimbrial gene cluster. To locate additional genes involved in the biosynthesis of B. pertussis fimbriae, we used TnphoA mutagenesis. A PhoA+ mutant (designated B176) was isolated which was affected in the production of both serotype 2 and 3 fimbriae. Cloning and sequencing of the DNA region harbouring the transposon insertion revealed the presence of at least three additional fimbrial genes, designated fimB, fimC and fimD. The transposon was found to be located in fimD. Analysis of PhoA activity indicated that the fimbrial gene cluster was positively regulated by the bvg locus. A potential binding site for BvgA was observed upstream of fimB. FimB showed homology with the so-called chaperone-like fimbrial proteins, while FimC was homologous with a class of fimbrial proteins located in the outer membrane and presumed to be involved in transport and anchorage of fimbrial subunits. An insertion mutation in fimB abolished the expression of fimbrial subunits, implicating this gene in the biosynthesis of both serotype 2 and 3 fimbriae. Upstream of fimB a pseudogene (fimA) was observed which showed homology with the three major fimbrial subunit genes, fim2, fim3 and fimX. The construction of a phylogenetic tree suggested that fimA may be the primordial major fimbrial subunit gene from which the other three were derived by gene duplication. Interestingly, the fimbrial gene cluster was found to be located directly downstream from the gene coding for the filamentous haemagglutinin, an important B. pertussis adhesin, possibly suggesting co-operation between the two loci in the pathogenesis of pertussis.