Identification of alcaligin as the siderophore produced by Bordetella pertussis and B. bronchiseptica

The Bordetella bhu locus is required for heme iron utilization

Bordetella pertussis and Bordetella bronchiseptica are capable of obtaining iron from hemin and hemoglobin. Genes encoding a putative bacterial heme iron acquisition system (bhu, for Bordetella heme utilization) were identified in a B. pertussis genomic sequence database, and the corresponding DNA was isolated from a virulent strain of B. pertussis. A B. pertussis bhuR mutant, predicted to lack the heme outer membrane receptor, was generated by allelic exchange. In contrast to the wild-type strain, bhuR mutant PM5 was incapable of acquiring iron from hemin and hemoglobin; genetic complementation of PM5 with the cloned bhuRSTUV genes restored heme utilization to wild-type levels. In parallel studies, B. bronchiseptica bhu sequences were also identified and a B. bronchiseptica bhuR mutant was constructed and confirmed to be defective in heme iron acquisition. The wild-type B. bronchiseptica parent strain grown under low-iron conditions produced the presumptive BhuR protein, which was absent in the bhuR mutant. Furthermore, production of BhuR by iron-starved B. bronchiseptica was markedly enhanced by culture in hemin-supplemented medium, suggesting that these organisms sense and respond to heme in the environment. Analysis of the genetic region upstream of the bhu cluster identified open reading frames predicted to encode homologs of the Escherichia coli ferric citrate uptake regulators FecI and FecR. These putative Bordetella regulators may mediate heme-responsive positive transcriptional control of the bhu genes.

Expression of the putative siderophore receptor gene bfrZ is controlled by the extracytoplasmic-function sigma factor BupI in Bordetella bronchiseptica

A new gene from Bordetella bronchiseptica, bfrZ encoding a putative siderophore receptor, was identified in a Fur-repressor titration assay. A bfrZ null mutant was constructed by allelic exchange. The protein profile of this mutant is similar to that of the wild-type parent strain. The BfrZ(-)-BfrZ(+) isogenic pair was tested for utilization of 132 different siderophores as iron sources. None of these iron sources acted as a ligand for BfrZ. Translational bfrZ::phoA and transcriptional bfrZ::lacZ fusions were introduced into the B. bronchiseptica bfrZ locus. No alkaline phosphatase or beta-galactosidase activity was detected. Sequence analysis of the bfrZ upstream region revealed the presence of two tightly linked genes, bupI and bupR. Both of these genes are located downstream from a Fur-binding sequence. BupI is homologous to Escherichia coli FecI and Pseudomonas putida PupI and belongs to the family of extracytoplasmic-function sigma factors involved in transcription of genes with extracytoplasmic functions. BupR is homologous to the FecR and PupR antisigma factors and is predicted to be localized in the inner membrane. Similar to the surface signaling receptors FecA and PupB, BfrZ bears an N-terminal extension. We found that bfrZ is not transcribed when bupI and bupR are expressed at the same level. However, overexpression of bupI from a multicopy plasmid triggers bfrZ transcription, and under these conditions BfrZ was detected in membrane fractions. By analogy with the FecI-FecR-FecA and PupI-PupR-PupB systems, our data suggest that bfrZ expression is inducible by binding of the cognate ligand to BfrZ and transduction of a signal through the envelope.

Reduced virulence of a Bordetella bronchiseptica siderophore mutant in neonatal swine

One means by which Bordetella bronchiseptica scavenges iron is through production of the siderophore alcaligin. A nonrevertible alcaligin mutant derived from the virulent strain 4609, designated DBB25, was constructed by insertion of a kanamycin resistance gene into alcA, one of the genes essential for alcaligin biosynthesis. The virulence of the alcA mutant in colostrum-deprived, caesarean-delivered piglets was compared with that of the parent strain in two experiments. At 1 week of age, piglets were inoculated with phosphate-buffered saline, 4609, or DBB25. Two piglets in each group were euthanatized on day 10 postinfection. The remainder were euthanatized at 21 days postinfection. Clinical signs, including fever, coughing, and sneezing, were present in both groups. Nasal washes performed 7, 14, and 21 days postinoculation demonstrated that strain DBB25 colonized the nasal cavity but did so at levels that were significantly less than those achieved by strain 4609. Analysis of colonization based on the number of CFU per gram of tissue recovered from the turbinate, trachea, and lung also demonstrated significant differences between DBB25 and 4609, at both day 10 and day 21 postinfection. Mild to moderate turbinate atrophy was apparent in pigs inoculated with strain 4609, while turbinates of those infected with strain DBB25 developed no or mild atrophy. We conclude from these results that siderophore production by B. bronchiseptica is not essential for colonization of swine but is required for maximal virulence. B. bronchiseptica mutants with nonrevertible defects in genes required for alcaligin synthesis may be candidates for evaluation as attenuated, live vaccine strains in conventionally reared pigs.

Iron metabolism in pathogenic bacteria

The ability of pathogens to obtain iron from transferrins, ferritin, hemoglobin, and other iron-containing proteins of their host is central to whether they live or die. To combat invading bacteria, animals go into an iron-withholding mode and also use a protein (Nramp1) to generate reactive oxygen species in an attempt to kill the pathogens. Some invading bacteria respond by producing specific iron chelators-siderophores-that remove the iron from the host sources. Other bacteria rely on direct contact with host iron proteins, either abstracting the iron at their surface or, as with heme, taking it up into the cytoplasm. The expression of a large number of genes (>40 in some cases) is directly controlled by the prevailing intracellular concentration of Fe(II) via its complexing to a regulatory protein (the Fur protein or equivalent). In this way, the biochemistry of the bacterial cell can accommodate the challenges from the host. Agents that interfere with bacterial iron metabolism may prove extremely valuable for chemotherapy of diseases.

Transcriptional activation of Bordetella alcaligin siderophore genes requires the AlcR regulator with alcaligin as inducer

Genetic and biochemical studies have established that Fur and iron mediate repression of Bordetella alcaligin siderophore system (alc) genes under iron-replete nutritional growth conditions. In this study, transcriptional analyses using Bordetella chromosomal alc-lacZ operon fusions determined that maximal alc gene transcriptional activity under iron starvation stress conditions is dependent on the presence of alcaligin siderophore. Mutational analysis and genetic complementation confirmed that alcaligin-responsive transcriptional activation of Bordetella alcaligin system genes is dependent on AlcR, a Fur-regulated AraC-like positive transcriptional regulator encoded within the alcaligin gene cluster. AlcR-mediated transcriptional activation is remarkably sensitive to inducer, occurring at extremely low alcaligin concentrations. This positive autogenous control circuit involving alcaligin siderophore as the inducer for AlcR-mediated transcriptional activation of alcaligin siderophore biosynthesis and transport genes coordinates environmental and intracellular signals for maximal expression of these genes under conditions in which the presence of alcaligin in the environment is perceived.

Kinetics and mechanism of iron(III) dissociation from the dihydroxamate siderophores alcaligin and rhodotorulic acid

The kinetics and mechanism of siderophore ligand dissociation from their fully chelated Fe(III) complexes is described for the highly preorganized cyclic tetradentate alcaligin and random linear tetradentate rhodotorulic acid in aqueous solution at 25 degrees C (Fe2L3 + 6H+ reversible 2 Fe3+ aq + 3 H2L). At siderophore:Fe(III) ratios where Fe(III) is hexacoordinated, kinetic data for the H(+)-driven ligand dissociation from the Fe2L3 species is consistent with a singly ligand bridged structure for both the alcaligin and rhodotorulic acid complexes. Proton-driven ligand dissociation is found to proceed via parallel reaction paths for rhodotorulic acid, in contrast with the single path previously observed for the linear trihydroxamate siderophore ferrioxamine B. Parallel paths are also available for ligand dissociation from Fe2(alcaligin)3, although the efficiency of one path is greatly diminished and dissociation of the bis coordinated complex Fe(alcaligin)(OH2)2+ is extremely slow (k = 10(-5) M-1 s-1) due to the high degree of preorganization in the alcaligin siderophore. Mechanistic interpretations were further confirmed by investigating the kinetics of ligand dissociation from the ternary complexes Fe(alcaligin)(L) in aqueous acid where L = N-methylacetohydroxamic acid and glycine hydroxamic acid. The existence of multiple ligand dissociation paths is discussed in the context of siderophore mediated microbial iron transport.

Microbial iron transport via a siderophore shuttle: a membrane ion transport paradigm

A mechanism of ion transport across membranes is reported. Microbial transport of Fe(3+) generally delivers iron, a growth-limiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe(3+) enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Identification of a human lactoferrin-binding protein in Gardnerella vaginalis

Previous studies have shown that Gardnerella vaginalis can utilize iron-loaded human lactoferrin as a sole source of iron. In this study, G. vaginalis cells were shown to bind digoxigenin (DIG)-labeled human lactoferrin in a dot blot assay. Using the DIG-labeled human lactoferrin, a 120-kDa human lactoferrin-binding protein was detected by Western blot analysis of G. vaginalis proteins. The lactoferrin-binding activity of this protein was found to be heat stable. Competition studies indicated that this binding activity was specific for human lactoferrin. Treatment of G. vaginalis cells with proteases suggested that this protein was surface exposed. An increase in lactoferrin binding by the 120-kDa protein was observed in G. vaginalis cells grown under iron-restrictive conditions, suggesting that this activity may be iron regulated.

Essential role of the iron-regulated outer membrane receptor FauA in alcaligin siderophore-mediated iron uptake in Bordetella species

Phenotypic analysis using heterologous host systems localized putative Bordetella pertussis ferric alcaligin transport genes and Fur-binding sequences to a 3.8-kb genetic region downstream from the alcR regulator gene. Nucleotide sequencing identified a TonB-dependent receptor family homolog gene, fauA, predicted to encode a polypeptide with high amino acid sequence similarity with known bacterial ferric siderophore receptors. In Escherichia coli, the fauA genes of both B. pertussis and Bordetella bronchiseptica directed the production of a 79-kDa polypeptide, approximating the predicted size of the mature FauA protein. B. bronchiseptica fauA insertion mutant BRM17 was unable to utilize ferric alcaligin, and in complementation analyses ferric alcaligin utilization was restored to this mutant by supplying the wild-type fauA gene in trans. Mutant BRM18, carrying a nonpolar in-frame fauA deletion mutation, was defective in ferric alcaligin utilization and (55)Fe-ferric alcaligin uptake and no longer produced a 79-kDa iron-regulated outer membrane protein. In complementation analyses, BRM18 merodiploids bearing the wild-type fauA gene in trans regained ferric alcaligin siderophore transport and utilization functions and produced the 79-kDa protein. Analysis of a plasmid-borne fauA-lacZ operon fusion confirmed that fauA is subject to iron regulation at the transcriptional level and that cis-acting transcriptional control elements mediating fauA iron repressibility reside within the 3.8-kb PstI fauA DNA region. Moreover, expression of the fauA-lacZ fusion gene under iron starvation conditions was shown to be alcR dependent. FauA is a 79-kDa iron-regulated outer membrane receptor protein required for transport and utilization of ferric alcaligin siderophore complexes by Bordetella species.

Disruption of tonB in Bordetella bronchiseptica and Bordetella pertussis prevents utilization of ferric siderophores, haemin and haemoglobin as iron sources

The Bordetella bronchiseptica tonB gene was cloned by detection of a chromosomal restriction fragment hybridizing with each of two degenerate oligonucleotides that corresponded to Pro-Glu and Pro-Lys repeats characteristic of known TonB proteins. The tonB(Bb) gene was situated upstream of exbB and exbD homologues and downstream of a putative Fur-regulated promoter. Hybridization results indicated that the tonB operon and flanking regions were highly conserved between B. bronchiseptica, Bordetella pertussis and Bordetella parapertussis. Disruption of tonB in B. bronchiseptica resulted in inability to grow in iron-limiting media, and inability to utilize alcaligin, enterobactin, ferrichrome, desferroxamine B, haemin and haemoglobin. Although it was not possible to inactivate tonB in a clinical B. pertussis isolate, tonB was disrupted in a laboratory B. pertussis strain previously selected for the ability to grow on Luria-Bertani medium. This B. pertussis tonB mutant shared a similar iron complex utilization deficient phenotype with the B. bronchiseptica tonB mutant. The B. bronchiseptica tonB operon present on a plasmid did not complement an Escherichia coli tonB mutant, but inefficient reconstitution of enterobactin utilization was observed in one fepA mutant harbouring plasmid copies of the B. pertussis fepA homologue and tonB(Bb) operon.

Electrochemical Behavior of the Fe(III) Complexes of the Cyclic Hydroxamate Siderophores Alcaligin and Desferrioxamine E

The redox behavior of Fe(III) complexes of the cyclic hydroxamate siderophores alcaligin and desferrioxamine E was investigated by cyclic voltammetry. The limiting, pH independent redox potential (E(1/2) vs NHE) is -446 mV for alcaligin above pH 9 and -477 mV for ferrioxamine E above pH 7.5. At lower pH values, the redox potential for both complexes shifts positive, with a loss of voltammetric reversibility which is interpreted to be the consequence of a secondary dissociation of Fe(II) from the reduced form of the complexes. These observations are of biological importance, since they suggest the possibility of a reductive mechanism in microbial cells which utilize these siderophores to acquire Fe. For comparison purposes, cyclic voltammograms were obtained for Fe(III) complexes with trihydroxamic acids of cyclic (ferrioxamine E) and linear (ferrioxamine B) structures, with dihydroxamic acids of cyclic (alcaligin) and linear (rhodotorulic and sebacic acids) structures, and with monohydroxamic acids (acetohydroxamic and N-methylacetohydroxamic acids) at identical conditions. The observed redox potentials allow us to estimate the overall stability constants for fully coordinated Fe(II) complexes as log beta(II)(Fe(2)alcaligin(3)) = 24.6 and log beta(II)(ferrioxamine E) = 12.1. A linear correlation between E(1/2) and pM was found, and the basis for this relationship is discussed in terms of structural (denticity and cyclic/acyclic) and electronic differences among the {alkyl-NOH-CO-alkyl} type of hydroxamic acid ligands studied.

Oxidative stress in microorganisms--I. Microbial vs. higher cells--damage and defenses in relation to cell aging and death

Oxidative stress in microbial cells shares many similarities with other cell types but it has its specific features which may differ in prokaryotic and eukaryotic cells. We survey here the properties and actions of primary sources of oxidative stress, the role of transition metals in oxidative stress and cell protective machinery of microbial cells, and compare them with analogous features of other cell types. Other features to be compared are the action of Reactive Oxygen Species (ROS) on cell constituents, secondary lipid- or protein-based radicals and other stress products. Repair of oxidative injury by microorganisms and proteolytic removal of irreparable cell constituents are briefly described. Oxidative damage of aerobically growing microbial cells by endogenously formed ROS mostly does not induce changes similar to the aging of multiplying mammalian cells. Rapid growth of bacteria and yeast prevents accumulation of impaired macromolecules which are repaired, diluted or eliminated. During growth some simple fungi, such as yeast or Podospora spp., exhibit aging whose primary cause seems to be fragmentation of the nucleolus or impairment of mitochondrial DNA integrity. Yeast cell aging seems to be accelerated by endogenous oxidative stress. Unlike most growing microbial cells, stationary-phase cells gradually lose their viability because of a continuous oxidative stress, in spite of an increased synthesis of antioxidant enzymes. Unlike in most microorganisms, in plant and animal cells a severe oxidative stress induces a specific programmed death pathway--apoptosis. The scant data on the microbial death mechanisms induced by oxidative stress indicate that in bacteria cell death can result from activation of autolytic enzymes (similarly to the programmed mother-cell death at the end of bacillary sporulation). Yeast and other simple eukaryotes contain components of a proapoptotic pathway which are silent under normal conditions but can be activated by oxidative stress or by manifestation of mammalian death genes, such as bak or bax. Other aspects, such as regulation of oxidative-stress response, role of defense enzymes and their control, acquisition of stress tolerance, stress signaling and its role in stress response, as well as cross-talk between different stress factors, will be the subject of a subsequent review.

A Preorganized Siderophore: Thermodynamic and Structural Characterization of Alcaligin and Bisucaberin, Microbial Macrocyclic Dihydroxamate Chelating Agents(1)

The iron coordination chemistry of two macrocyclic dihydroxamate siderophores, alcaligin (AG) and bisucaberin (BR), has been investigated thermodynamically and structurally. Alcaligin is a siderophore of freshwater bacteria as well as mammalian pathogens, including the bacterium that causes whooping cough in humans, while bisucaberin, a structural analogue of alcaligin, is produced by marine bacteria. Both alcaligin and bisucaberin form 1:1 ferric complexes (FeL(+)) in acidic conditions and 2:3 ferric complexes (Fe(2)L(3)) at and above neutral pH. The stability constants of these macrocyclic dihydroxamate siderophores differ significantly from that of rhodotorulic acid (RA), a linear dihydroxamate siderophore. Notably, K(FeL) of alcaligin is 32 times greater than that of rhodotorulic acid, while the subsequent stepwise formation constant for Fe(2)L(3) is 3 times less. The Fe(III) complexes of alcaligin are stereospecific; the absolute configuration of the Fe(2)L(3) complex (circular dichroism and X-ray structure) is Lambda. The structure of the Fe(2)L(3) alcaligin complex is a topological alternative to the triple-helicate structure of the rhodotorulic complex Fe(2)(RA)(3). The structures of the free ligand and the bisbidentate ligand in the FeL complex are essentially identical, indicating that alcaligin is highly preorganized for metal ion binding. This explains the difference in K(FeL) between alcaligin and rhodotorulic acid, as well as explaining the monobridged topology of the Fe(2)L(3) alcaligin complex. The protonation constants (log K(a1) and log K(a2)) are 9.42(5) and 8.61(1) for alcaligin and 9.49(2) and 8.76(3) for bisucaberin. The stepwise formation constants of the Fe(III) complexes (log K(ML) and log K(M)()2(L)()3) are 23.5(2) and 17.7(2) for alcaligin and 23.5(5) and 17.2(5) for bisucaberin. The overall formation constants (log beta(230)) of alcaligin and bisucaberin are 64.7(1) and 64.3(1). The solution chemistry of Fe(III) and alcaligin was further investigated at a lower ligand to metal ratio (1:1). At high pH, a novel 2:2 ferric bis-&mgr;-oxo-bridged complex of alcalagin forms (Fe(2)L(2)O(2)(2)(-)) with a log beta(22)(-)(4) of 16.7(2). This species exhibits behavior consistent with an iron bis-&mgr;-oxo complex, including antiferromagnetic coupling. Crystal data: Fe(2)(AG)(3).25H(2)O crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a =13.3374(4) Å, b = 16.1879(5) Å, c = 37.886(1) Å, V = 8179.7(4), Z = 4. For 5512 reflections with F(o)(2) > 3sigma(F(o)(2)) the final R (R(w)) = 0.053(0.068).

Iron starvation of Bordetella avium stimulates expression of five outer membrane proteins and regulates a gene involved in acquiring iron from serum

Iron starvation of Bordetella avium induced expression of five outer membrane proteins with apparent molecular masses of 95, 92, 91.5, 84, and 51 kDa. Iron-responsive outer membrane proteins (FeRPs) of similar sizes were detected in six of six strains of B. avium, suggesting that the five FeRPs are common constituents of the outer membrane of most, if not all, strains of B. avium. Iron-regulated genes of B. avium were targeted for mutagenesis with the transposon TnphoA. Two mutants with iron-responsive alkaline phosphatase activities were isolated from the transposon library. The transposon insertion did not alter the iron-regulated expression of the five FeRPs in mutant Pho-6. The mutant Pho-20 exhibited a loss in expression of the 95-kDa FeRP and the 84-kDa FeRP. Both Pho-6 and Pho-20 were able to use free iron as a nutrient source. However, Pho-20 was severely compromised in its ability to use iron present in turkey serum. The data indicated that the mutation in Pho-20 affected expression of one or more components of an uptake machinery that is involved in acquisition of iron from organic ferricomplexes.

Two iron-regulated putative ferric siderophore receptor genes in Bordetella bronchiseptica and Bordetella pertussis

Two iron-regulated genes with deduced homology to TonB-dependent ferric siderophore receptors were cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants. bfrB and bfrC were iron-repressed in B. bronchiseptica by a Fur-dependent mechanism, and were expressed from promoters overlapped by potential Fur-binding sites. Both genes were highly conserved among Bordetella species and were also iron-regulated in Bordetella pertussis. bfrB and bfrC mutants of both species and a bfrB-bfrC double mutant of B. bronchiseptica had no discernible defects in utilization of known iron sources for Bordetella.

Transcriptional analysis of the Bordetella alcaligin siderophore biosynthesis operon

The alc gene cluster of Bordetella pertussis includes three genes, alcA, alcB, and alcC, which are involved in alcaligin siderophore biosynthesis in response to iron starvation. The production of AlcA, AlcB, and AlcC in Bordetella cells and the transcriptional organization of alcA, alcB, and alcC were investigated by using a set of three alc'-'lacZ gene fusion constructs that were contiguous with the known promoter upstream of alcA and extended to fusion junctions within each alc cistron. All three alc'-'lacZ fusions exhibited iron-repressible reporter gene expression which was abolished by deletion of the 105-bp alcA promoter-operator region. In an immunoblot analysis using a monoclonal antibody specific for beta-galactosidase, the AlcA-LacZ, AlcB-LacZ, and AlcC-LacZ hybrid proteins were detected in Bordetella cells grown under iron-depleted conditions. A B. pertussis mutant in which the 105-bp alcA promoter-operator region was deleted by allelic exchange was unable to produce detectable levels of siderophore. Hybridization analysis using gene-specific probes showed that alc-specific transcript levels in the mutant were negligible compared with those of the wild-type parent. These results confirm that alcA, alcB, and alcC are cotranscribed from an iron-regulated control region immediately upstream of alcA. Transcript analysis using hybridization probes representing regions downstream of alcC demonstrated that alc transcription extends approximately 3.6 kb further downstream from the alcC coding region, suggesting the cotranscription of additional, uncharacterized alcaligin system genes.

Identification and characterization of alcR, a gene encoding an AraC-like regulator of alcaligin siderophore biosynthesis and transport in Bordetella pertussis and Bordetella bronchiseptica

A Bordetella bronchiseptica iron transport mutant was isolated following an enrichment procedure based on streptonigrin resistance. The mutant displayed a growth defect on iron-restricted medium containing ferric alcaligin as the sole iron source. In addition to the apparent inability to acquire iron from the siderophore, the mutant failed to produce alcaligin as well as two known iron-regulated proteins, one of which is the AlcC alcaligin biosynthesis protein. A 1.6-kb KpnI-PstI Bordetella pertussis DNA fragment mapping downstream of the alcaligin biosynthesis genes alcABC restored both siderophore biosynthesis and expression of the iron-regulated proteins to the mutant. Nucleotide sequencing of this complementing 1.6-kb region identified an open reading frame predicted to encode a protein with strong similarity to members of the AraC family of transcriptional regulators, for which we propose the gene designation alcR. Primer extension analysis localized an iron-regulated transcription initiation site upstream of the alcR open reading frame and adjacent to sequences homologous to the consensus Fur repressor binding site. The AlcR protein was produced by using an Escherichia coli expression system and visualized in electrophoretic gels. In-frame alcR deletion mutants of B. pertussis and B. bronchiseptica were constructed, and the defined mutants exhibited the alcR mutant phenotype, characterized by the inability to produce and transport alcaligin and express the two iron-repressed proteins. The cloned alcR gene provided in trans restored these siderophore system activities to the mutants. Together, these results indicate that AlcR is involved in the regulation of Bordetella alcaligin biosynthesis and transport genes and is required for their full expression.

Identification of AlcR, an AraC-type regulator of alcaligin siderophore synthesis in Bordetella bronchiseptica and Bordetella pertussis

A Fur titration assay was used to isolate DNA fragments bearing putative Fur binding sites (FBS) from a partial Bordetella bronchiseptica genomic DNA library. A recombinant plasmid bearing a 3.5-kb DNA insert was further studied. Successive deletions in the cloned fragment enabled us to map a putative FBS at about 2 kb from one end. Sequence analysis revealed the presence of an FBS upstream from a new gene encoding an AraC-type transcriptional regulator. The deduced protein displays similarity to PchR, an activator of pyochelin siderophore and ferripyochelin receptor synthesis in Pseudomonas aeruginosa. Homologous genes in Bordetella pertussis and Bordetella parapertussis were PCR amplified, and sequence comparisons indicated a very high conservation in the three species. The B. pertussis and B. bronchiseptica chromosomal genes were inactivated by allelic exchange. Under low-iron growth conditions, the mutants did not secrete the alcaligin siderophore and lacked AlcC, an alcaligin biosynthetic enzyme. Alcaligin production was restored after transformation with a plasmid bearing the wild-type gene. On the basis of its role in regulation of alcaligin biosynthesis, the new gene was designated alcR. Additional sequence determination showed that alcR is located about 2 kb downstream from the alcABC operon and is transcribed in the same orientation. Two tightly linked open reading frames, alcD and alcE, were identified between alcC and alcR. AlcE is a putative iron-sulfur protein; AlcD shows no homology with the proteins in the database. The production of major virulence factors and colonization in the mouse respiratory infection model are AlcR independent.

Analysis of the alcABC operon encoding alcaligin biosynthesis enzymes in Bordetella bronchiseptica

We previously cloned a B. bronchiseptica (Bb) genomic DNA fragment that complements a Bb alcaligin biosynthesis mutant, and reported the identification of a gene, alcA, with predicted protein sequence similarity to siderophore biosynthesis enzymes from other organisms. In the present study we show that further nt sequencing of this region revealed two open reading frames (ORFs) 3' to alcA that encode putative proteins AlcB and AlcC, with significant sequence similarity to the aerobactin biosynthesis enzymes IucB and IucC, respectively. RT-PCR analysis indicated that the three ORFs are encoded on a single transcript, and that this operon is repressed at the transcriptional level by Fe. Primer extension analysis placed the transcriptional start point (tsp) 35 nt from the 5' end of the Fur consensus sequence and 188 nt from the putative start of translation of AlcA.

Cloning and characterization of an Mn-containing superoxide dismutase (SodA) of Bordetella pertussis

The Fur titration assay (FURTA) recently developed by I. Stojiljkovic and coworkers (J. Mol. Biol. 236:531-545, 1994) was applied to clone iron-regulated genes of Bordetella pertussis. After sequence analysis, one of the clones obtained by this selection procedure was shown to contain an open reading frame with significant sequence similarities to Mn-containing superoxide dismutases (SodA). The open reading frame was preceded by a Fur consensus binding site, which according to primer extension analysis overlaps the -10 region of the sodA promoter. Southern blot analysis also revealed the presence of sodA homologous sequences in Bordetella bronchiseptica. On the transcriptional level, sodA expression is strictly iron regulated in both organisms and also in the heterologous host Escherichia coli harboring a plasmid with the sodA gene. Accordingly, SodA-mediated superoxide dismutase activity in Bordetella lysates was detected only after cultivation of the bacteria in iron-restricted media. A B. bronchiseptica fur mutant constitutively expressed SodA, thereby confirming the functional similarity of the iron regulatory systems in the two genera. Apart from iron regulation, sodA expression was affected by changes in DNA topology induced by coumermycin A but not by the global virulence regulatory Bvg system. B. pertussis and B. bronchiseptica sodA deletion mutants did not show significant changes in their growth properties. In contrast, mutation of the previously described Fe-containing SodB enzyme resulted in clones strongly impaired in viability. No direct involvement of SodA in bacterial virulence could be revealed because deletion of the sodA gene affected survival of Bordetella species neither in cultured macrophages nor in a mouse respiratory infection model.

An iron-regulated outer-membrane protein specific to Bordetella bronchiseptica and homologous to ferric siderophore receptors

The bfrA (Bordetella bronchiseptica ferric iron repressed outer-membrane protein) gene was cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants for iron-repressed fusions to phoA. The bfrA gene encoded an 80 kDa outer-membrane protein with a high level of amino acid sequence identity to several bacterial proteins belonging to the family of Ton B-dependent outer-membrane receptors. BfrA was especially homologous to Cir of Escherichia coli, IrgA of Vibrio cholerae and to three previously characterized ferric enterobactin receptors. DNA hybridization results indicated that bfrA was not present in other Bordetella species. Expression of the bfrA gene was induced by low iron availability from a promoter overlapped by a sequence resembling a consensus Fur-binding sequence, and bfrA expression was derepressed in a B. bronchiseptica fur mutant. Utilization of the Bordetella siderophore alcaligin and the exogenous siderophore enterobactin was unaffected in bfrA mutants. Upon attempting to find the specificity of BfrA, 2,3-dihydroxybenzoylserine (DHBS) was shown to be utilized in a bfeA (Bordetella ferric enterobactin receptor gene)-dependent manner by B. bronchiseptica and B. pertussis. In addition, the hydroxamate siderophores ferrichrome and desferrioxamine B, and the iron source haemin were shown to be utilized independently of bfeA and bfrA in B. bronchiseptica and B. pertussis.

Siderophore-mediated iron uptake in Alcaligenes eutrophus CH34 and identification of aleB encoding the ferric iron-alcaligin E receptor

Siderophore production in response to iron limitation was observed in Alcaligenes eutrophus CH34, and the corresponding siderophore was named alcaligin E. Alcaligin E was characterized as a phenolate-type siderophore containing neither catecholate nor hydroxamate groups. Alcaligin E promoted the growth of siderophore-deficient A. eutrophus mutants under iron-restricted conditions and promoted 59Fe uptake by iron-limited cells. However, the growth of the Sid- mutant AE1152, which was obtained from CH34 by Tn5-Tc mutagenesis, was completely inhibited by the addition of alcaligin E. AE1152 also showed strongly reduced 59Fe uptake in the presence of alcaligin E. This indicates that a gene, designated aleB, which is involved in transport of ferric iron-alcaligin E across the membrane is inactivated. The aleB gene was cloned, and its putative amino acid sequence showed strong similarity to those of ferric iron-siderophore receptor proteins. Both wild-type strain CH34 and aleB mutant AE1152 were able to use the same heterologous siderophores, indicating that AleB is involved only in ferric iron-alcaligin E uptake. Interestingly, no utilization of pyochelin, which is also a phenolate-type siderophore, was observed for A. eutrophus CH34. Genetic studies of different Sid- mutants, obtained after transposon mutagenesis, showed that the genes involved in alcaligin E and ferric iron-alcaligin E receptor biosynthesis are clustered in a 20-kb region on the A. eutrophus CH34 chromosome in the proximity of the cys-232 locus.

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.

Purification, spectroscopic analysis and biological activity of the macrocyclic dihydroxamate siderophore alcaligin produced by Bordetella pertussis and Bordetella bronchiseptica

Hydroxamate siderophores of virulent Bordetella pertussis and Bordetella bronchiseptica strains were purified using a simple large-scale isolation procedure, and identified by various spectroscopic techniques as the macrocyclic dihydroxamate siderophore trivially known as alcaligin, 1,8(S),11,18(S)- tetrahydroxy-1,6,11,16-tetraazacycloeicosane-2,5,12,15-tetrone+ ++, which was previously isolated from the taxonomically-related bacterial species Alcaligenes denitrificans subsp. xylosoxydans. Alcaligin purified from iron-depleted cultures of B. pertussis and B. bronchiseptica exhibited specific growth-promoting activity under iron-restricted conditions for Bordetella indicator strains, and ere active in [55Fe]ferric alcaligin transport assays. Evidence suggests that several C2-symmetric conformations of alcaligin exist simultaneously in both methanolic and aqueous solution.

The ornithine decarboxylase gene odc is required for alcaligin siderophore biosynthesis in Bordetella spp.: putrescine is a precursor of alcaligin

Chromosomal insertions defining Bordetella bronchiseptica siderophore phenotypic complementation group III mutants BRM3 and BRM5 were found to reside approximately 200 to 300 bp apart by restriction mapping of cloned genomic regions associated with the insertion markers. DNA hybridization analysis using B. bronchiseptica genomic DNA sequences flanking the cloned BRM3 insertion marker identified homologous Bordetella pertussis UT25 cosmids that complemented the siderophore biosynthesis defect of the group III B. bronchiseptica mutants. Subcloning and complementation analysis localized the complementing activity to a 2.8-kb B. pertussis genomic DNA region. Nucleotide sequencing identified an open reading frame predicted to encode a polypeptide exhibiting strong similarity at the primary amino acid level with several pyridoxal phosphate-dependent amino acid decarboxylases. Alcaligin production was fully restored to group III mutants by supplementation of iron-depleted culture media with putrescine (1,4-diaminobutane), consistent with defects in an ornithine decarboxylase activity required for alcaligin siderophore biosynthesis. Concordantly, the alcaligin biosynthesis defect of BRM3 was functionally complemented by the heterologous Escherichia coli speC gene encoding an ornithine decarboxylase activity. Enzyme assays confirmed that group III B. bronchiseptica siderophore-deficient mutants lack an ornithine decarboxylase activity required for the biosynthesis of alcaligin. Siderophore production by an analogous mutant of B. pertussis constructed by allelic exchange was undetectable. We propose the designation odc for the gene defined by these mutations that abrogate alcaligin siderophore production. Putrescine is an essential precursor of alcaligin in Bordetella spp.

bvg Repression of alcaligin synthesis in Bordetella bronchiseptica is associated with phylogenetic lineage

Recent studies have shown that Bordetella bronchiseptica utilizes a siderophore-mediated transport system for acquisition of iron from the host iron-binding proteins lactoferrin and transferrin. We recently identified the B. bronchiseptica siderophore as alcaligin, which is also produced by B. pertussis. Alcaligin production by B. bronchiseptica is repressed by exogenous iron, a phenotype of other microbes that produce siderophores. In this study, we report that alcaligin production by B. bronchiseptica RB50 and GP1SN was repressed by the Bordetella global virulence regulator, bvg, in addition to being Fe repressed. Modulation of bvg locus expression with 50 mM MgSO4 or inactivation of bvg by deletion allowed strain RB50 to produce alcaligin. In modulated organisms, siderophore production remained Fe repressed. These observations contrasted with our previous data indicating that alcaligin production by B. bronchiseptica MBORD846 and B. pertussis was repressed by Fe but bvg independent. Despite bvg repression of alcaligin production, strain RB50 was still able to acquire Fe from purified alcaligin, suggesting that expression of the bacterial alcaligin receptor was not repressed by bvg. We tested 114 B. bronchiseptica strains and found that bvg repression of alcaligin production was strongly associated with Bordetella phylogenetic lineage and with host species from which the organisms were isolated.

Nonradioactive colony lift-hybridization assay for detection of Bordetella bronchiseptica infection in swine

Current methods for the isolation and identification of Bordetella bronchiseptica from clinical samples are time-consuming and are based, in part, on subjective observations. We describe the use of a Bordetella-specific DNA probe in a nonradioactive colony lift-hybridization assay for the identification of B. bronchiseptica. Eleven of 82 clinical specimens were found to contain B. bronchiseptica by this method, while only 5 of these were reported to contain the organism when the specimens were analyzed by traditional methods. The chromosomal fragment containing a sequence complementary to the probe appeared to be conserved in B. bronchiseptica isolates from swine from a variety of sources. The assay is more rapid than culture and biochemical testing since it can be performed directly on primary culture plates, even when they are heavily contaminated with other bacterial species. Only minimal training is required to accomplish the assay successfully, and the results are easy to interpret.