Identification of a Bordetella pertussis regulatory factor required for transcription of the pertussis toxin operon in Escherichia coli

Interaction of Borrelia burgdorferi Hbb with the p66 promoter

Borrelia burgdorferi, an agent of Lyme disease, encodes the β₃-chain integrin ligand P66. P66 is expressed by B. burgdorferi in the mammal, in laboratory media, and as the bacteria are acquired or transmitted by the tick, but is not expressed by the bacterium in unfed ticks. Attempts to reveal factors influencing expression revealed that P66 was expressed in all in vitro conditions investigated. Candidate regulators identified in a search of the B. burgdorferi genome for homologs to other bacterial transcription factors were cloned and introduced into E. coli carrying a p66 promoter-signal sequence-phoA (alkaline phosphatase, or AP) fusion. Three candidate transcription factors—two that decreased AP activity (Hbb and BB0527), and one that increased AP activity (BBA23)—were identified. BBA23 and BB0527 did not bind to the p66 promoter at physiologically relevant concentrations. In contrast, several promoter fragments, including p66, were bound by Hbb (BB0232), with slightly different affinities. Consistent with results from other laboratories, Hbb appears to recognize multiple DNA sequences. Changes in the expression of p66 and bb0232 in the tick at various points with respect to feeding on mice, along with the results of the reporter experiment in the surrogate host E. coli, are consistent with Hbb/BB0232 being involved in regulating p66 expression.

Coenzyme A biosynthesis: an antimicrobial drug target

Pantothenic acid, a precursor of coenzyme A (CoA), is essential for the growth of pathogenic microorganisms. Since the structure of pantothenic acid was determined, many analogues of this essential metabolite have been prepared. Several have been demonstrated to exert an antimicrobial effect against a range of microorganisms by inhibiting the utilization of pantothenic acid, validating pantothenic acid utilization as a potential novel antimicrobial drug target. This review commences with an overview of the mechanisms by which various microorganisms acquire the pantothenic acid they require for growth, and the universal CoA biosynthesis pathway by which pantothenic acid is converted into CoA. A detailed survey of studies that have investigated the inhibitory activity of analogues of pantothenic acid and other precursors of CoA follows. The potential of inhibitors of both pantothenic acid utilization and biosynthesis as novel antibacterial, antifungal and antimalarial agents is discussed, focusing on inhibitors and substrates of pantothenate kinase, the enzyme catalysing the rate-limiting step of CoA biosynthesis in many organisms. The best strategies are considered for identifying inhibitors of pantothenic acid utilization and biosynthesis that are potent and selective inhibitors of microbial growth and that may be suitable for use as chemotherapeutic agents in humans.

BrgE is a regulator of Myxococcus xanthus development

We report here the identification and characterization of a member of the Myxococcus xanthus SdeK signal transduction pathway, BrgE. This protein was identified as an SdeK-interacting component using a yeast two-hybrid screen, and we further confirmed this interaction by the glutathione S-transferase (GST) pulldown assay. Additional yeast two-hybrid analyses revealed that BrgE preferentially interacts with the putative amino-terminal sensor domain of SdeK, but not with the carboxy-terminal kinase domain. A brgE insertion strain was shown to be blocked in development between aggregation and mound formation, and decreased by 50-fold in viable spore production compared with the parental wild type. These phenotypes are similar to those of sdeK mutants. The brgE mutation also altered expression of a sample of Tn5 lac developmental markers that are also SdeK regulated. Finally, we demonstrated that a brgE sdeK double mutant has a more severe sporulation defect than either of the two single mutants, suggesting that BrgE and SdeK act synergistically to regulate wild-type levels of sporulation. In sum, these data suggest that BrgE operates as an auxiliary factor to stimulate the SdeK signal transduction pathway by directly binding to the amino-terminal sensor domain of SdeK.

Characterization of a new pantothenate kinase isoform from Helicobacter pylori

Pantothenate kinase (PanK) catalyzes the first step in the biosynthesis of the essential and ubiquitous cofactor coenzyme A (CoA) in all organisms. Two well characterized isoforms of the enzyme are known: a prokaryotic PanK that predominates in eubacteria and a eukaryotic isoform that has primarily been characterized from mammalian and plant sources. Curiously, the genomes of certain pathogenic bacteria, including Helicobacter pylori and Pseudomonas aeruginosa, do not contain a PanK similar to either isoform, although these organisms possess all the other biosynthetic machinery required for CoA production. In this study we cloned, overexpressed and characterized an enzyme from Bacillus subtilis and its homologue from H. pylori and show that they catalyze the ATP-dependent phosphorylation of pantothenate. These enzymes do not share sequence homology with any known PanK, and unlike the bacterial and eukaryotic PanK isoforms their activity is not regulated by either CoA or acetyl-CoA. They also do not accept the pantothenic acid antimetabolite N-pentylpantothenamide as a substrate or are inhibited by it. Taken together, these results point to the identification of a third distinct isoform of PanK that accounts for the only known activity of the enzyme in pathogens such as H. pylori and P. aeruginosa.

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.

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.

The BvgAS two-component system of Bordetella spp.: a versatile modulator of virulence gene expression

Bordetella pertussis and the closely related species B. parapertussis and B. bronchiseptica colonize the respiratory tract and cause related diseases in man or mammalian species, respectively. Expression of virulence factors by these pathogens is coordinately regulated by the BvgAS two-component system according to changes in the growth conditions. Signal transduction by the BvgAS system is characterized by a complex His-Asp-His-Asp phosphorelay. This system controls the expression of two distinct subsets of genes either in a positive (vag genes) or in a negative (vrg genes) manner. Most of the known virulence factors such as several toxins and adhesins are encoded by vag genes, whereas the functions of most vrg genes and the biological significance of the vrg regulon are not yet clear. This review discusses the current knowledge about the molecular mechanisms of virulence regulation and their relevance for infection by these respiratory pathogens.

The Bvg accessory factor (Baf) enhances pertussis toxin expression in Escherichia coli and is essential for Bordetella pertussis viability

Pertussis toxin expression in the Gram-negative respiratory pathogen, Bordetella pertussis, is regulated by the BvgAS two-component system. Previous studies suggested that an additional gene encoding a Bvg accessory factor (Baf) was required, along with BvgAS, for expression of a ptx-lacZ fusion in Escherichia coli grown in rich medium. However, other studies showed that BvgAS is sufficient for ptx-lacZ expression in minimal medium. Here we show that Baf acts with BvgAS to further increase ptx-lacZ expression in E. coli grown in minimal media and this is concomitant with a two-fold increase in BvgA protein levels. Gene replacement experiments show that baf is essential for viability of B. pertussis, suggesting that Baf affects the expression of other genes in addition to ptx.

Molecular and functional analysis of the lipopolysaccharide biosynthesis locus wlb from Bordetella pertussis, Bordetella parapertussis and Bordetella bronchiseptica

The Bordetella pertussis wlb locus (wlbpe, formerly bpl) is required for the biosynthesis of a trisaccharide that, when attached to the B. pertussis lipopolysaccharide (LPS) core (band B), generates band A LPS. The equivalent loci in Bordetella bronchiseptica (wlbbr) and Bordetella parapertussis (wlbpa) were identified and cloned. The wlbbr and wlbpa loci differ from wlbpe in that they lack the insertion sequence that defines the right-hand terminus of wlbpe. Deletion of 12 kb of DNA containing the whole wlb locus (delta wlb) by allelic exchange in each of the three bordetellae had no effect on band B biosynthesis, whereas band A biosynthesis was prevented in B. pertussis and B. bronchiseptica. In B. bronchiseptica and B. parapertussis, delta wlb mutants also lacked O-antigen. Reintroduction of the wlbpe or wlbbr loci on a shuttle vector into the three delta wlb mutants restored the wild-type LPS phenotype in the B. pertussis and B. bronchiseptica mutants. In the case of B. parapertussis, which normally does not synthesize an apparent band A structure, introduction of the wlbpe or wlbbr loci now enabled the generation of band A. This suggests that the attachment point for band A trisaccharide on the LPS core is present in B. parapertussis, and further suggests that the wild-type wlbpa locus is not fully functional. Introduction of the wlbpa locus into the delta wlbpe, delta wlbbr and delta wlbpa mutants had interesting consequences. The B. bronchiseptica and B. parapertussis recipients were now able to biosynthesize O-antigen, but no band A was generated. In the B. pertussis recipient, a truncated band A was expressed consistent with a mutation in the wlbH gene, but on Western blotting the expression of a small amount of full-length band A was also seen. Evidence that the wlbHpa protein is not fully functional was provided by the failure of the wlbpa locus to fully complement a B. pertussis wlbH (delta wlbHpe) mutant. This was supported by DNA sequence data showing that a single amino acid, conserved between homologous proteins from a range of bacteria, is altered in the B. parapertussis WlbH protein.

Identification of Btr-regulated genes using a titration assay. Search for a role for this transcriptional regulator in the growth and virulence of Bordetella pertussis

Bordetella pertussis is the causative agent of the respiratory disease pertussis or whopoping cough. Btr, an oxygen-responsive transcriptional regulator of B. pertussis, is homologous to the FNR protein of E. coli. Using a murine respiratory model, we observed in the present study that Btr is important in growth and survival of B. pertussis in vivo. A titration assay was developed that identified genes containing Btr binding sites including B. pertussis sodB and btr, E. coli aspA and a new B. pertussis gene, brg1. The brg1 gene encodes a protein similar to the LysR family of transcriptional regulators and its expression is activated threefold by Btr under anaerobic growth conditions but unaffected by Btr aerobically. The nucleotide sequence flanking brg1 encodes proteins with similarity to various metabolic enzymes. Putative overlapping promoters and a Btr binding site (FNR box) were identified in the DNA sequence between brg1 and the adjacent genes. These intervening sequences may represent sites for regulation by Btr and Brg1.

A putative leucine zipper activator of Pasteurella haemolytica leukotoxin transcription and the potential for modulation of its synthesis by slipped-strand mispairing

A Pasteurella haemolytica cosmid clone that activates leukotoxin transcription in Escherichia coli has been isolated. The activator locus, alxA, is part of a continuous open reading frame that includes the type I hsdM methylase gene. AlxA and HsdM peptides are processed from a precursor, and translation of the polyprotein can be modulated by slipped-strand mispairing across a pentanucleotide repeat, ACAGC, within the 5' end of alxA-hsdM. Extracts containing AlxA can bind to a leukotoxin promoter fragment.

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.

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.