Identification and characterization of genes encoding the human transferrin-binding proteins from Haemophilus influenzae

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.

Characterization of the Pasteurella haemolytica transferrin receptor genes and the recombinant receptor proteins

The tbpA and tbpB genes encoding the transferrin receptor proteins, TbpA and TbpB, from Pasteurella haemolytica A1 were cloned, sequenced and expressed in Escherichia coli. The genes were organized in a putative operon arrangement of tbpB- tbpA. The tbpB gene was preceded by putative promoter and regulatory sequences, and followed by a 96 base pair intergenic sequence in which no promoter regions were found, suggesting that the two genes are coordinately transcribed. The deduced amino acid sequences of the TbpA and TbpB proteins had regions of homology with the corresponding Neisseria meningitidis, N. gonorrhoeae, Haemophilus influenzae and Actinobacillus pleuropneumoniae Tbp and Lbp proteins. The intact tbpB gene was expressed in a T7 expression system and the resulting recombinant TbpB protein retained the functional bovine transferrin binding characteristics. The availability of the recombinant TbpB enabled us to demonstrate its specificity for ruminant transferrins, its ability to bind both the C-and N-terminal lobes of bovine transferrin and its preference for the iron-loaded form of this protein. Several attempts at expressing the cloned tbpA gene were unsuccessful, suggesting that the product of the gene may be toxic to E. coli.

A protective epitope of Moraxella catarrhalis is encoded by two different genes

The high-molecular-weight UspA protein of Moraxella catarrhalis has been described as being both present on the surface of all M. catarrhalis disease isolates examined to date and a target for a monoclonal antibody (MAb 17C7) which enhanced pulmonary clearance of this organism in a mouse model system (M. E. Helminen et al., J. Infect. Dis. 170:867-872, 1994). A recombinant bacteriophage that formed plaques which bound MAb 17C7 was shown to contain a M. catarrhalis gene, designated uspA1, that encoded a protein with a calculated molecular weight of 88,271. Characterization of an isogenic uspA1 mutant revealed that elimination of expression of UspA1 did not eliminate the reactivity of M. catarrhalis with MAb 17C7. In addition, N-terminal amino acid analysis of internal peptides derived from native UspA protein and Southern blot analysis of M. catarrhalis chromosomal DNA suggested the existence of a second UspA-like protein. A combination of epitope mapping and ligation-based PCR methods identified a second M. catarrhalis gene, designated uspA2, which also encoded the MAb 17C7-reactive epitope. The UspA2 protein had a calculated molecular weight of 62,483. Both the isogenic uspA1 mutant and an isogenic uspA2 mutant possessed the ability to express a very-high-molecular-weight antigen that bound MAb 17C7. Southern blot analysis indicated that disease isolates of M. catarrhalis likely possess both uspA1 and uspA2 genes. Both UspA1 and UspA2 most closely resembled adhesins produced by other bacterial pathogens.

Identification of human transferrin-binding sites within meningococcal transferrin-binding protein B

Transferrin-binding protein B (TbpB) from Neisseria meningitidis binds human transferrin (hTf) at the surface of the bacterial cell as part of the iron uptake process. To identify hTf binding sites within the meningococcal TbpB, defined regions of the molecule were produced in Escherichia coli by a translational fusion expression system and the ability of the recombinant proteins (rTbpB) to bind peroxidase-conjugated hTf was characterized by Western blot and dot blot assays. Both the N-terminal domain (amino acids [aa] 2 to 351) and the C-terminal domain (aa 352 to 691) were able to bind hTf, and by a peptide spot synthesis approach, two and five hTf binding sites were identified in the N- and C-terminal domains, respectively. The hTf binding activity of three rTbpB deletion variants constructed within the central region (aa 346 to 543) highlighted the importance of a specific peptide (aa 377 to 394) in the ligand interaction. Taken together, the results indicated that the N- and C-terminal domains bound hTf approximately 10 and 1000 times less, respectively, than the full-length rTbpB (aa 2 to 691), while the central region (aa 346 to 543) had a binding avidity in the same order of magnitude as the C-terminal domain. In contrast with the hTf binding in the N-terminal domain, which was mediated by conformational epitopes, linear determinants seemed to be involved in the hTf binding in the C-terminal domain. The host specificity for transferrin appeared to be mediated by the N-terminal domain of the meningococcal rTbpB rather than the C-terminal domain, since we report that murine Tf binds to the C-terminal domain. Antisera raised to both N- and C-terminal domains were bactericidal for the parent strain, indicating that both domains are accessible at the bacterial surface. We have thus identified hTf binding sites within each domain of the TbpB from N. meningitidis and propose that the N- and C-terminal domains together contribute to the efficient binding of TbpB to hTf with their respective affinities and specificities for determinants of their ligand.

Sequence analysis of the structural tbpA gene: protein topology and variable regions within neisserial receptors for transferrin iron acquisition

The gene coding for the 98-kDa meningococcal outer membrane transferrin binding protein 1 (TbpA) from strain B385 was cloned and sequenced. Sequence comparison among its deduced aminoacid sequence and those from TbpA and the closely related LbpA (lactoferrin binding protein) gene from three different meningococcal strains, and four isolates from two other bacterial pathogens, showed that TbpA variability is confined to five specific segments, designated VR1 (199-287), VR2 (306-381), VR3 (480-546), VR4 (618-651) and VR5 (681-708). The third VR was the most variable among strains both at the nucleotide and amino acid levels. Six additional tbpA genes from different meningococcal strains were cloned and its VR3 sequence determined. On the basis of this data we were able to cluster tbpA genes in two groups: D (bearing a deletion in VR3) and N (nondeleted); all N and D strains belonging to the groups of high or low molecular weight transferrin receptor isotype, respectively. However, by phenogram analysis, the prototypical strain M982 (Group II) was clustered with M990 (B16B6 isotype, Group I). These results point to the existence of important exposed regions as well as to the possibility of horizontal gene exchange involving this locus. A topology model with 14 exposed loops and 28 membrane spanning segments was postulated. According to this tentative analysis, TbpA as well as LbpA proteins should form a gated channel in the neisserial outer membrane. The variable regions were located in the fifth, sixth, eighth, 10th and 11th loops respectively. Among TbpAs VR1, VR2, and VR3 resulted the most relevant regions.

Elemental iron does repress transferrin, haemopexin and haemoglobin receptor expression in Haemophilus influenzae

The iron repressible nature of Haemophilus influenzae transferrin binding proteins suggests a regulatory role for elemental iron in their expression. The existence of a Haemophilus ferric uptake repressor (Fur) binding motif identified in the promoter region of both tbpA and tbpB further supports this hypothesis. However, a recent study using brain heart infusion growth medium suggested that transferrin binding protein synthesis in H. influenzae was haem-rather than iron-regulated. The present study re-investigates this observation and using a chemically defined medium, we demonstrate that elemental iron haem or protoporphyrin IX can each regulate Haemophilus influenzae transferrin, haemopexin and haemoglobin receptor expression.

A diffusible cytotoxin of Haemophilus ducreyi

Little is known about the virulence mechanisms employed by Haemophilus ducreyi in the production of genital ulcers. This Gram-negative bacterium previously has been shown to produce a soluble cytotoxic activity that kills HeLa and HEp-2 cells. We have now identified a cluster of three H. ducreyi genes that encode this cytotoxic activity. The predicted proteins encoded by these genes are most similar to the products of the Escherichia coli cdtABC genes that comprise the cytolethal distending toxin (CDT) of this enteric pathogen. Eleven of 12 H. ducreyi strains were shown to possess this gene cluster and culture supernatants from these strains readily killed HeLa cells. The culture supernatant from a single strain of H. ducreyi that lacked these genes was unable to kill HeLa cells. When the H. ducreyi cdtABC gene cluster was cloned into E. coli, culture supernatant from the recombinant E. coli clone killed HeLa cells. A monoclonal antibody that neutralized this soluble cytotoxic activity of H. ducreyi was shown to bind to the H. ducreyi cdtC gene product. This soluble H. ducreyi cytotoxin may play a role in the development or persistence of the ulcerative lesions characteristic of chancroid.

Characterization of a large transferrin-binding protein from Actinobacillus pleuropneumoniae serotype 7

The binding of transferrin at the surface of Actinobacillus pleuropneumoniae (A. pp.) is mediated by two proteins of approximately 60 and 100 kDa. The 60 kDa protein has been shown to be highly divergent among different serotypes and to induce a serotype-specific protective immune response. In this study we have characterized the 100 kDa transferrin-binding protein of A. pp. serotype 7 and designated it as TfbB. The tfbB gene was found to be located immediately downstream of the tfbA gene. It was cloned and sequenced, and antibodies raised against the isolated recombinant protein detected, with a constant intensity, a 100 kDa protein in A. pp. serotypes 2, 4, 6, 7, 8, 9, 10 and 11, and a polypeptide of approximately 103 kDa in serotypes 1, 3, 5A and 12. In addition, comparative analysis of the deduced amino acid sequence showed more than 40% identity with the large transferrin-binding proteins of Neisseria meningitidis and Haemophilus influenzae. The TfbB protein was expressed in E. coli outer membranes in a conformation eliciting porcine transferrin-specific binding activity. Sera of pigs immunized with these TfbB-containing E. coli membranes recognized functional membrane-associated TfbB protein whereas no such reaction was observed upon immunization with isolated recombinant TfbB protein. A preliminary animal experiment showed that TfbB-containing outer membrane preparations from recombinant E. coli can reduce significantly the mortality of an A.pp. infection with the homologous strain.

The major outer membrane protein of Haemophilus ducreyi consists of two OmpA homologs

The major outer membrane protein (MOMP) of Haemophilus ducreyi is an OmpA homolog that migrates on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels as three species with apparent molecular weights ranging from 37,000 to 43,000. Monoclonal antibodies directed against this macromolecule were used to identify recombinant clones containing fragments of the gene encoding this protein. Nucleotide sequence analysis of these fragments confirmed that the MOMP encoded by the intact gene (momp) was a member of the OmpA family of outer membrane proteins. Construction of an isogenic H. ducreyi mutant unable to express the MOMP led to the discovery of a second outer membrane protein which migrated at the same rate on SDS-PAGE gels as the MOMP. N-terminal amino acid sequence analysis of this second protein revealed that its N terminus was nearly identical to that of the MOMP and also had homology with members of the OmpA family. Nucleotide sequence analysis of the region downstream from the momp gene revealed the presence of a partial open reading frame encoding a predicted OmpA-like protein. A modification of anchored PCR technology was used to obtain the nucleotide sequence of this downstream gene which was shown to encode a second OmpA homolog (OmpA2). The N-terminal amino acid sequence of OmpA2 was identical to that of the OmpA-like protein detected in the momp mutant. The H. ducreyi MOMP and OmpA2 proteins, which comigrated on SDS-PAGE gels and which were encoded by the tandem arranged momp and ompA2 genes, were 72% identical.

Characterization of the diversity and the transferrin-binding domain of gonococcal transferrin-binding protein 2

The molecular weight heterogeneities of Tbp1 and Tbp2 among a panel of 45 gonococcal isolates were assessed. The tbpB genes from four of these strains were sequenced to characterize the Tbp2 sequence diversity among gonococci. By expressing truncated versions of gonococcal Tbp2, we delimited the extent of Tbp2 necessary for transferrin binding in a Western blot.

Rapid identification and cloning of bacterial transferrin and lactoferrin receptor protein genes

The sequences of genes encoding the transferrin and lactoferrin receptor proteins from several bacterial species were analyzed for areas of identity in the predicted protein sequences. Degenerate oligonucleotide primers were designed and tested for their ability to amplify portions of the receptor genes. Primer pairs capable of amplifying products of the tbpA/lbpA or tbpB/lbpB genes from all species possessing these receptors were identified.

Lack of expression of the global regulator OxyR in Haemophilus influenzae has a profound effect on growth phenotype

A pBR322-based library of chromosomal DNA from the nontypeable Haemophilus influenzae TN106 was screened for the expression of transferrin-binding activity in Escherichia coli. A recombinant clone expressing transferrin-binding activity contained a 3.7-kb fragment of nontypeable H. influenzae DNA. Nucleotide sequence analysis of this insert revealed the presence of two complete open reading frames encoding proteins of approximately 26 and 34 kDa. Mini-Tn10kan transposon mutagenesis at different sites within the open reading frame encoding the 34-kDa protein resulted in the abolition of transferrin-binding activity in the recombinant E. coli clone. The deduced amino acid sequence of the 34-kDa protein had 70% identity with the OxyR protein of E. coli; this latter macromolecule is a member of the LysR family of transcriptional activators. When a mutated H. influenzae oxyR gene was introduced into the chromosome of the wild-type H. influenzae strain by allelic exchange, the resulting oxyR mutant still exhibited wild-type levels of transferrin-binding activity but was unable to grow on media containing the heme precursor protoporphyrin IX (PPIX) in place of heme. This mutant also exhibited reduced growth around disks impregnated with heme sources. Supplementation of the PPIX-based growth media with catalase or sodium pyruvate resulted in normal growth of the H. influenzae oxyR mutant. Provision of the wild-type H. influenzae oxyR gene in trans also permitted the growth of this mutant on a PPIX-based medium. Exogenously supplied catalase restored the growth of this mutant with heme sources to nearly wild-type levels. These results indicate that expression of a wild-type OxyR protein by H. influenzae is essential to allow this organism to protect itself against oxidative stresses in vitro.

Identification of an outer membrane protein involved in utilization of hemoglobin-haptoglobin complexes by nontypeable Haemophilus influenzae

A recombinant plasmid containing a 6.5-kb fragment of nontypeable Haemophilus influenzae (NTHI) chromosomal DNA was shown to confer a hemoglobin-haptoglobin-binding phenotype on Escherichia coli. Use of a mini-Tn10kan transposon for random insertion mutagenesis of this recombinant plasmid allowed localization of the NTHI DNA responsible for this hemoglobin-haptoglobin-binding phenotype to a 3.5-kb PstI-XhoI fragment within the 6.5-kb NTHI DNA insert. When this mutagenized NTHI DNA fragment was used to transform the wild-type NTHI strain, the resultant kanamycin-resistant mutant exhibited significantly decreased abilities to bind hemoglobin-haptoglobin and utilize it as a source of heme for aerobic growth in vitro. This mutant also lacked expression of a 115-kDa outer membrane protein that was present in the wild-type parent strain. Transformation of this mutant with wild-type NTHI chromosomal DNA restored the abilities to bind and utilize hemoglobin-haptoglobin and to express the 115-kDa outer membrane protein. Nucleotide sequence analysis of the relevant NTHI DNA revealed the presence of a gene, designated hhuA, that encoded a predicted 117,145-Da protein. The HhuA protein exhibited features typical of a TonB-dependent outer membrane receptor and had significant identity with the hemoglobin receptors of both Haemophilus ducreyi and Neisseria meningitidis.

Cloning of a DNA fragment encoding a heme-repressible hemoglobin-binding outer membrane protein from Haemophilus influenzae

Haemophilus influenzae is able to use hemoglobin as a sole source of heme, and heme-repressible hemoglobin binding to the cell surface has been demonstrated. Using an affinity purification methodology, a hemoglobin-binding protein of approximately 120 kDa was isolated from H. influenzae type b strain HI689 grown in heme-restricted but not in heme-replete conditions. The isolated protein was subjected to N-terminal amino acid sequencing, and the derived amino acid sequence was used to design corresponding oligonucleotides. The oligonucleotides were used to probe a Southern blot of EcoRI-digested HI689 genomic DNA. A hybridizing band of approximately 4.2 kb was successfully cloned into pUC19. Using a 1.9-kb internal BglII fragment of the 4.2-kb clone as a probe, hybridization was seen in both typeable and nontypeable H. influenzae but not in other bacterial species tested. Following partial nucleotide sequencing of the 4.2-kb insert, a putative open reading frame was subcloned into an expression vector. The host Escherichia coli strain in which the cloned fragment was expressed bound biotinylated human hemoglobin, whereas binding of hemoglobin was not detected in E. coli with the vector alone. In conclusion, we hypothesize that the DNA fragment encoding an approximately 120-kDa heme-repressible hemoglobin-binding protein mediates one step in the acquisition of hemoglobin by H. influenzae in vivo.

Bacterial transferrin and lactoferrin receptors

Pathogenic members of the Neisseriaceae and Pasteurellaceae express outer-membrane receptor proteins involved in the direct assimilation of iron from the host glycoproteins transferrin and lactoferrin. The critical requirement of iron for growth suggests that this function is an important component of colonization and infection. A model describing this novel process is presented.

Cloning, sequencing and expression of the transferrin-binding protein 1 gene from Actinobacillus pleuropneumoniae

Two outer-membrane proteins are involved in the uptake of iron from transferrin by certain Gram-negative bacteria, transferrin-binding proteins 1 and 2. The gene encoding transferrin-binding protein 1 from a serotype 1 isolate of the Gram-negative pathogen Actinobacillus pleuropneumoniae was cloned, and a fragment encoding 700 amino acids of Tbp1 was expressed in Escherichia coli. We also report here sequencing of the tbpl gene and a comparison of the deduced amino acid sequence with Tbpls from related species. The predicted polypeptide product of tbpl is a 106 kDa protein with a 22-residue signal peptide.

Lipoprotein e(P4) is essential for hemin uptake by Haemophilus influenzae

Heme uptake is a common means of iron and porphyrin acquisition by many pathogenic bacteria. The genus Haemophilus includes several important pathogenic bacterial species that characteristically require hemin-, protoporphyrin-, or heme-substituted proteins as essential growth factors under aerobic conditions. However, the mechanism of heme transport is not understood for Haemophilus. We have cloned a DNA fragment from H. influenzae that allows an Escherichia coli hemA mutant to employ exogenous hemin or protoporphyrin IX as sole sources of porphyrin. DNA sequencing of the cloned DNA fragment suggested that a previously characterized gene (hel) encoding an antigenic, outer membrane lipoprotein e(P4) was responsible for the complementation activity. Construction of hel insertion mutations in strain H. influenzae Rd demonstrated that hel is essential for growth under aerobic conditions but not under anaerobic conditions. The aerobic growth defect of hel mutants could be reversed by providing exogenous hemin in the presence of outer membrane. The analysis of hybrids between e(P4) and beta-lactamase demonstrated that a domain of e(P4) near its NH2' terminus was required for its function in hemin use. Within this domain is a short amino acid sequence that displays similarity to H. influenzae hemin binding protein HbpA, hemin-binding motifs present in eukaryotic transcription activator heme-activated protein, and the heme containing proteins hemoglobin (alpha-chain) and cytochrome C3, suggesting that this region may be involved in hemin binding and/or transport.

Progress towards a vaccine for nontypable Haemophilus influenzae

Nontypable Haemophilus influenzae is a common cause of human disease and is associated with significant morbidity and considerable societal cost. At present, measures to prevent nontypable H. influenzae disease are limited to prophylactic antibiotics and, on occasion, exogenous antibody preparations. However, because these interventions are often inadequate, there is interest in developing an effective vaccine. Given the marked diversity among epidemiologically unrelated strains and the frequent strain specificity of the immune response to infection, efforts have focused on identifying bacterial antigens that are highly conserved and capable of stimulating protective antibody. With the recent identification of several such antigens, attention must now turn toward selecting the appropriate combination of these molecules and determining the optimal strategy for their presentation to the immune system. The ultimate goal is to induce broad-based and long-lasting protection.

Production and characterization of chimeric transferrins for the determination of the binding domains for bacterial transferrin receptors

Pathogenic bacteria in the Neisseriaceae and Pasteurellaceae possess outer membrane proteins that specifically bind transferrin from the host as the first step in the iron acquisition process. As a logical progression from prior studies of the ligand-receptor interaction using biochemical approaches, we have initiated an approach involving the production of recombinant chimeric transferrins to further identify the regions of transferrin involved in receptor binding. In order to prepare bovine/human hybrids, the bovine transferrin gene was cloned, sequenced, and compared with the existing human transferrin gene sequence. After identification of potential splice sites, hybrid transferrin genes were constructed using the polymerase chain reaction-based approach of splicing by overlap extension. Five hybrid genes containing sequences from both bovine and human transferrin were constructed. Recombinant transferrins were produced in a baculovirus expression vector system and affinity-purified using concanavalin A-Sepharose. The recombinant proteins were analyzed for reactivity against polyclonal and monoclonal antibodies and assessed for binding to Neisseria meningitidis transferrin receptor proteins in solid-phase binding assays and affinity isolation experiments. These experiments enabled us to localize the regions of human transferrin predominantly involved in binding to the N. meningitidis receptor to amino acid residues 346-588. The construction of these chimeras provides unique tools for the investigation of transferrin binding to receptors from both human and bovine bacterial pathogens.

Biochemical analysis of lactoferrin receptors in the Neisseriaceae: identification of a second bacterial lactoferrin receptor protein

Bacterial transferrin receptors that have been described in the families Pasteurellaceae and Neisseriaceae are composed of two receptor proteins, transferrin binding proteins 1 and 2 (Tbp1 and Tbp2). In contrast, bacterial lactoferrin receptors have only been described for human pathogens in the family Neisseriaceae, and were believed to consist of a single protein, Lbp1, which is highly homologous to Tbp1. We describe a modified affinity isolation procedure that facilities isolation of a second lactoferrin receptor protein Lbp2 (a presumptive Tbp2 homologue) from Neisseria meningitidis, Moraxella catarrhalis and Moraxella bovis using immobilized lactoferrin. Antiserum specific for either the M. catarrhalis Tbp1+2 molecules, the M. catarrhalis Lbp1 molecule, or for a commercial preparation of human lactoferrin did not react on western blots with the same organisms' affinity purified Lbp2. In addition, the M. catarrhalis Lbp2 could be isolated in a functional form without contaminating Lbp1 or Tbp1+2. We also demonstrate that the bovine pathogen, M. bovis, produces functional transferrin and lactoferrin receptors specific for the bovine forms of these glycoproteins. A putative lbpB gene, recently speculated to reside immediately upstream of the N. meningitidis Lbp1 structural gene, lbpA, likely encodes the newly isolated Lbp2 protein from this bacterial species.

Biochemical characterization of a Haemophilus influenzae periplasmic iron transport operon

Bacterial iron transport is critical for growth of pathogens in the host environment, where iron is limited as a form of nonspecific immunity. For Gram-negative bacteria such as Haemophilus influenzae, iron first must be transported across the outer membrane and into the periplasmic space, then from the periplasm to the cytosol. H. influenzae express a periplasmic iron-binding protein encoded by the hitA gene. This gene is organized as the first of a three-gene operon purported to encode a classic high affinity iron acquisition system that includes hitA, a cytoplasmic permease (hitB), and a nucleotide binding protein (hitC). In this study we describe the cloning, overexpression, and purification of the H. influenzae hitA gene product. The function of this protein is unambiguously assigned by demonstrating its ability to compete for iron bound to the chemical iron chelator 2,2'-dipyridyl, both in vitro and within the periplasmic space of a siderophore-deficient strain of Escherichia coli. Finally, the importance of a functional hitABC operon for iron acquisition is demonstrated by complementation of this siderophore-deficient E. coli to growth on dipyridyl-containing medium. These studies represent a detailed genetic, biochemical, and physiologic description of an active transport system that has evolved to efficiently transport iron and consequently is widely distributed among Gram-negative pathogenic bacteria.

Characterization of transferrin binding proteins 1 and 2 in invasive type b and nontypeable strains of Haemophilus influenzae

Haemophilus influenzae has the ability to obtain iron from human transferrin via two bacterial cell surface transferrin binding proteins, Tbp1 and Tbp2. Although a wide array of strains have been shown to express these receptor proteins, two studies have recently identified a series of isolates which appeared to lack the ability to bind transferrin. Included in this group were the members of a cryptic genospecies of nontypeable biotype IV strains which appear to possess a tropism for female urogenital tissues and are major etiologic agents of neonatal and postpartum bacteremia due to H. influenzae. The present study employed oligonucleotide primers specific for genes encoding the Tbp proteins of a type b biotype I strain of H. influenzae to probe the genomic DNAs of isolates from the previous studies. The tbpA and tbpB genes which encode Tbp1 and Tbp2, respectively, were detected in all of the strains tested either by PCR amplification directly or by Southern hybridization analysis. All of the strains displayed a transferrin binding phenotype, and affinity isolation of receptor proteins with transferrin-conjugated Sepharose recovered Tbp1 and/or Tbp2 from 11 of 14 strains, including 2 of the nontypeable biotype IV strains. In addition, all of the strains were capable of growing on human transferrin specifically, indicating that the mechanism of iron assimilation from transferrin is functional and is not siderophore mediated. These results confirm the presence of tbp genes in all of the invasive H. influenzae isolates characterized to date, suggesting that Tbp-mediated iron acquisition is important in disease which initiates from either the respiratory or urogenital mucosa.

Mapping of functional regions on the transferrin-binding protein (TfbA) of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae can use porcine transferrin as the sole source of iron. Two proteins with molecular masses of approximately 60 kDa (TfbA) and 110 kDa have been shown to specifically bind porcine transferrin; from the TfbA protein, three isoforms from A. pleuropneumoniae serotypes 1, 5, and 7 have been identified and characterized by nucleotide sequence analysis. Here we defined the transferrin-binding region(s) of the TfbA protein of A. pleuropneumoniae serotype 7 by TnphoA mutagenesis, random mutagenesis, and peptide spot synthesis. The amino-terminal half of the TfbA molecule, which has only 36% amino acid sequence identity among the three isoforms, was shown to be responsible for transferrin binding by TnphoA mutagenesis. This result was confirmed by analysis of six random mutants with decreased transferrin binding affinity. The subsequent analysis of overlapping 16-mer peptides comprising the amino-terminal half of the TfbA molecule revealed three domains of 13 or 14 amino acids in length with transferrin-binding activity. They overlapped, or were very close to, point mutations decreasing transferrin-binding ability. The first and third domains were unique to the TfbA protein of A. pleuropneumoniae serotype 7. In contrast, the sequence of the second domain was present in almost identical forms (12 of 14 residues) in the TfbA proteins of A. pleuropneumoniae serotypes 1 and 5; in addition, a sequence consisting of functionally homologous amino acids was present in the otherwise completely distinct small transferrin-binding proteins of Neisseria gonorrhoeae (TbpB), N. meningitidis (Tbp2), and Haemophilus influenzae (Tbp2).