Iron acquisition systems in the pathogenic Neisseria

Interactions of haemoglobin with the Neisseria meningitidis receptor HpuAB: the role of TonB and an intact proton motive force

We have characterized the interaction of the Neisseria meningitidis TonB-dependent receptor HpuAB with haemoglobin (Hb). Protease accessibility assays indicated that HpuA and HpuB are surface exposed, HpuB interacts physically with HpuA, and TonB energization affects the conformation of HpuAB. Binding assays using [125I]-Hb revealed that the bipartite receptor has a single binding site for Hb (Kd 150 nM). Competitive binding assays using heterologous Hbs revealed that HpuAB Hb recognition was not species specific. The binding kinetics of Hb to HpuAB were dramatically altered in a TonB- mutant and in wild-type meningococci treated with the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP), indicating that TonB and an intact proton motive force are required for normal Hb binding and release from HpuAB. Our results support a model in which both HpuA and HpuB are required to form a receptor complex in the outer membrane with a single binding site, whose structure and ligand interactions are significantly affected by the TonB-mediated energy state of the receptor.

A Streptococcus pneumoniae pathogenicity island encoding an ABC transporter involved in iron uptake and virulence

Restricted iron availability is a major obstacle to growth and survival of pathogenic bacteria during infection. In contrast to Gram-negative pathogens, little is known about how Gram-positive pathogens obtain this essential metal. We have identified two Streptococcus pneumoniae genetic loci, pit1 and pit2, encoding homologues of ABC iron transporters that are required for iron uptake by this organism. S. pneumoniae strains containing disrupted copies of either pit1 or pit2 had decreased sensitivity to the iron-dependent antibiotic streptonigrin, and a strain containing disrupted copies of both pit1 and pit2 was unable to use haemoglobin as an iron source and had a reduced rate of iron uptake. The pit2- strain was moderately and the pit1-/pit2- strain strongly attenuated in virulence in mouse models of pulmonary and systemic infection, showing that the pit loci play a critical role during in vivo growth of S. pneumoniae. The pit2 locus is contained within a 27 kb region of chromosomal DNA that has several features of Gram-negative bacterial pathogenicity islands. This probable pathogenicity island (PPI-1) is the first to be described for S. pneumoniae, and its acquisition is likely to have played a significant role in the evolution of this important human pathogen.

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.

Cloning and expression of two novel hemin binding protein genes from Treponema denticola

Treponema denticola does not appear to produce siderophores, so it must acquire iron by other pathways. Indeed, T. denticola has been shown to have an iron-regulated 44-kDa outer membrane protein (HbpA) with hemin binding ability. To characterize the HbpA protein, its gene was cloned from genomic DNA libraries of T. denticola. Sequence analysis of the hbpA open reading frame indicated that it encoded a 42.8-kDa protein with a 23-amino-acid signal peptide. HbpA has no significant homology to any proteins in the databases. Southern blot analysis demonstrated that hbpA is present in several T. denticola ATCC strains and clinical isolates, but not in Treponema pectinovorum, Treponema socranskii, or Escherichia coli. HbpA, expressed as a recombinant protein in E. coli and purified by antibody affinity chromatography, has hemin binding activity as determined by lithium dodecyl sulfate-polyacrylamide gel electrophoresis with tetramethylbenzidine staining. Northern blot analysis showed that there were two hbpA-containing transcripts, of approximately 1.3 and 2.6 kb, and that the RNA levels were low-iron induced. Interestingly, the 2.6-kb mRNA also encoded a second protein with significant homology to hbpA. This downstream gene, called hbpB, was cloned and sequenced and its product was expressed as a fusion protein in E. coli. The hbpB gene product is 49% identical to HbpA and binds hemin. Thus, T. denticola has two novel hemin binding proteins which may be part of a previously unrecognized iron acquisition pathway.

Characterization of in vitro interactions between a truncated TonB protein from Escherichia coli and the outer membrane receptors FhuA and FepA

High-affinity iron uptake in gram-negative bacteria depends upon TonB, a protein which couples the proton motive force in the cytoplasmic membrane to iron chelate receptors in the outer membrane. To advance studies on TonB structure and function, we expressed a recombinant form of Escherichia coli TonB lacking the N-terminal cytoplasmic membrane anchor. This protein (H(6)-'TonB; M(r), 24,880) was isolated in a soluble fraction of lysed cells and was purified by virtue of a hexahistidine tag located at its N terminus. Sedimentation experiments indicated that the H(6)-'TonB preparation was almost monodisperse and the protein was essentially monomeric. The value found for the Stokes radius (3.8 nm) is in good agreement with the value calculated by size exclusion chromatography. The frictional ratio (2.0) suggested that H(6)-'TonB adopts a highly asymmetrical form with an axial ratio of 15. H(6)-'TonB captured both the ferrichrome-iron receptor FhuA and the ferric enterobactin receptor FepA from detergent-solubilized outer membranes in vitro. Capture was enhanced by preincubation of the receptors with their cognate ligands. Cross-linking assays with the purified proteins in vitro demonstrated that there was preferential interaction between TonB and ligand-loaded FhuA. Purified H(6)-'TonB was found to be stable and thus shows promise for high-resolution structural studies.

Neisserial TonB-dependent outer-membrane proteins: detection, regulation and distribution of three putative candidates identified from the genome sequences

Computer searches were carried out of the gonococcal and meningococcal genome databases for previously unknown members of the TonB-dependent family (Tdf) of outer-membrane receptor proteins. Seven putative non-contiguous genes were found and three of these (identified in gonococcal strain FA1090) were chosen for further study. Consensus motif analysis of the peptide sequences was consistent with the three genes encoding TonB-dependent receptors. In view of the five previously characterized TonB-dependent proteins of pathogenic neisseriae, the putative genes were labelled tdfF, tdfG and tdfH. TdfF had homology with the siderophore receptors FpvA of Pseudomonas aeruginosa and FhuE of Escherichia coli, whereas TdfG and TdfH had homology with the haemophore receptor HasR of Serratia marcescens. The aim of this project was to characterize these proteins and determine their expression, regulation, distribution and surface exposure. Strain surveys of iron-stressed commensal and pathogenic neisseriae revealed that TdfF is unlikely to be expressed, TdfG is expressed by gonococci only and that TdfH is expressed by both meningococci and gonococci. Expression of TdfH was unaffected by iron availability. Susceptibility of TdfH to cleavage by proteases in live gonococci was consistent with surface exposure of this protein. TdfH may function as a TonB-dependent receptor for a non-iron nutrient source. Furthermore, TdfH is worthy of future investigation as a potential meningococcal vaccine candidate as it is a highly conserved, widely distributed and surface-exposed outer-membrane protein.

Iron uptake mechanisms and their regulation in pathogenic bacteria

In the human body iron is present in growth-limiting amounts for bacteria. For this reason intricate iron transport and iron regulatory systems evolved in bacteria to guarantee a sufficient iron supply. The few principal mechanisms that underly the large variety of iron supply systems will be presented, as well as cases, in which defined iron supply systems are related to virulence.

ABC transporter-mediated uptake of iron, siderophores, heme and vitamin B12

Microbes have developed a number of different strategies to utilize iron, which is a vital element for most organisms but not always readily available from the environment. Based on experimental studies and sequence analysis data, this article gives a short overview of ABC transporters related to iron uptake: components of three distinct families mediate the translocation of iron, siderophores, heme and vitamin B12 across the cytoplasmic membrane of bacteria.

Dependence of Staphylococcus epidermidis on non-transferrin-bound iron for growth

The ability of Staphylococcus epidermidis strains to grow in the presence of human transferrin and varying amounts of ferric iron was studied. At initial bacterial densities up to 10(4) cfu ml(-1), none of the three strains grew when transferrin iron saturation was below the full saturation point, whereas the bacteria grew consistently when transferrin was fully iron-saturated and there was non-transferrin-bound iron in the medium. Precultivation of the bacteria under iron-restricted conditions to induce siderophore production did not abolish the growth dependence on non-transferrin-bound iron. At initial bacterial densities of 10(6) cfu ml(-1), the bacteria proliferated consistently also in the presence of partially saturated transferrin. The results indicate that at low bacterial densities, S. epidermidis cannot utilise transferrin-bound iron for growth and that its proliferation is dependent on non-transferrin-bound iron.

Interactions of pathogenic neisseriae with epithelial cell membranes

The closely related bacterial pathogens Neisseria gonorrhoeae (gonococci, GC) and N. meningitidis (meningococci, MC) initiate infection at human mucosal epithelia. Colonization begins at apical epithelial surfaces with a multistep adhesion cascade, followed by invasion of the host cell, intracellular persistence, transcytosis, and exit. These activities are modulated by the interaction of a panoply of virulence factors with their cognate host cell receptors, and signals are sent from pathogen to host and host to pathogen at multiple stages of the adhesion cascade. Recent advances place us on the verge of understanding the colonization process at a molecular level of detail. In this review we describe the Neisseria virulence factors in the context of epithelial cell biology, placing special emphasis on the signaling functions of type IV pili, pilus-based twitching motility, and the Opa and Opc outermembrane adhesin/invasin proteins. We also summarize what is known about bacterial intracellular trafficking and growth. With the accelerated integration of tools from cell biology, biochemistry, biophysics, and genomics, experimentation in the next few years should bring unprecedented insights into the interactions of Neisseriae with their host.

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.

Characterization of a novel transferrin receptor in bovine strains of Pasteurella multocida

Analysis of bovine respiratory isolates of Pasteurella multocida demonstrated that six of nine strains tested were capable of growth dependent upon bovine transferrin and of specifically binding ruminant transferrins. A single 82-kDa protein was affinity isolated from the P. multocida strains with immobilized bovine transferrin. In contrast to what has been observed in other species, binding of this protein to immobilized transferrin was specifically blocked by the N-lobe subfragment of bovine transferrin. A single gene encoding the 82-kDa protein was flanked by a leucyl-tRNA synthetase gene and an IS1060 element, in contrast to other species where genes encoding the two receptor proteins (TbpB and TbpA) are found in an operonic arrangement. A similar gene arrangement was observed in all of the receptor-positive strains, in spite of the observation that they belonged to different genomic groups. Analysis of the deduced amino acid sequence of the receptor protein indicated that it is a member of the TonB-dependent outer membrane receptor family, and although it is related to transferrin and lactoferrin receptor proteins (TbpAs and LbpAs) from other species, it differs substantially from other members of this group. Amino acid alignments suggest that the reduced size (20 kDa smaller) of the P. multocida TbpA is primarily due to the absence of larger predicted external loops. Collectively these results suggest that P. multocida has a single, novel receptor protein (TbpA) that is capable of efficiently mediating iron acquisition from bovine transferrin without the involvement of a second receptor protein (TbpB).

Meningococcal pathogenesis: at the boundary between the pre- and post-genomic eras

Meningococcal disease remains an important public health burden worldwide and, indeed, cause of death, particularly in poorer countries. The rapidly progressive nature of infections means that antibiotic therapy often comes too late. Vaccines are of limited efficacy in infants, one of the most vulnerable age groups, and do not exist for bacteria of serogroup B. Hence, much remains to be achieved in terms of vaccine design and the understanding of the pathogenesis of meningococcal disease. The causative bacterium, Neisseria meningitidis, is usually a commensal of the nasopharynx. Factors that lead to the invasion of the bloodstream, often followed by the crossing of the blood-brain barrier and meningitis, may be partly host- and partly bacterium-dependent, but are ill-understood. It is hoped that, taken together with the fundamental knowledge gained from biochemical and genetic studies, the huge amount of new information made available with the recent publication of the genome sequences will help to unlock more of the secrets of the lifestyle and pathogenic potential of this still poorly understood pathogen.

Human lactoferrin proteolytic activity: analysis of the cleaved region in the IgA protease of Haemophilus influenzae

Human lactoferrin proteolytically cleaves and inactivates two colonization factors of non-typable Haemophilus influenzae, the IgA protease precursor protein (Iga), and Hap, the non-pilus adhesin by which microoganisms adhere to host epithelial cells and form microcolonies. Iga and Hap are homologous proteins that are members of the autotransporter family of secreted proteins expressed by gram-negative bacteria. Studies of Iga cleaved by lactoferrin, reported here, show that proteolysis occurred within the helper region of Iga (Iga(beta)) domain which anchors the autotransporter within the Haemophilus outer membrane. The amino-terminus of the extracted Iga protein was not modified. The location of the proteolytic active site in human lactoferrin is under study. Lactoferrin proteolysis may attenuate pathogenicity of H. influenzae, an important cause of otitis media.

Degradation of heme in gram-negative bacteria: the product of the hemO gene of Neisseriae is a heme oxygenase

A full-length heme oxygenase gene from the gram-negative pathogen Neisseria meningitidis was cloned and expressed in Escherichia coli. Expression of the enzyme yielded soluble catalytically active protein and caused accumulation of biliverdin within the E. coli cells. The purified HemO forms a 1:1 complex with heme and has a heme protein spectrum similar to that previously reported for the purified heme oxygenase (HmuO) from the gram-positive pathogen Corynebacterium diphtheriae and for eukaryotic heme oxygenases. The overall sequence identity between HemO and these heme oxygenases is, however, low. In the presence of ascorbate or the human NADPH cytochrome P450 reductase system, the heme-HemO complex is converted to ferric-biliverdin IXalpha and carbon monoxide as the final products. Homologs of the hemO gene were identified and characterized in six commensal Neisseria isolates, Neisseria lactamica, Neisseria subflava, Neisseria flava, Neisseria polysacchareae, Neisseria kochii, and Neisseria cinerea. All HemO orthologs shared between 95 and 98% identity in amino acid sequences with functionally important residues being completely conserved. This is the first heme oxygenase identified in a gram-negative pathogen. The identification of HemO as a heme oxygenase provides further evidence that oxidative cleavage of the heme is the mechanism by which some bacteria acquire iron for further use.

Human lactoferrin binds and removes the hemoglobin receptor protein of the periodontopathogen Porphyromonas gingivalis

Porphyromonas gingivalis possesses a hemoglobin receptor (HbR) protein on the cell surface as one of the major components of the hemoglobin utilization system in this periodontopathogenic bacterium. HbR is intragenically encoded by the genes of an arginine-specific cysteine proteinase (rgpA), lysine-specific cysteine proteinase (kgp), and a hemagglutinin (hagA). Here, we have demonstrated that human lactoferrin as well as hemoglobin have the abilities to bind purified HbR and the cell surface of P. gingivalis through HbR. The interaction of lactoferrin with HbR led to the release of HbR from the cell surface of P. gingivalis. This lactoferrin-mediated HbR release was inhibited by the cysteine proteinase inhibitors effective to the cysteine proteinases of P. gingivalis. P. gingivalis could not utilize lactoferrin for its growth as an iron source and, in contrast, lactoferrin inhibited the growth of the bacterium in a rich medium containing hemoglobin as the sole iron source. Lactoferricin B, a 25-amino acid-long peptide located at the N-lobe of bovine lactoferrin, caused the same effects on P. gingivalis cells as human lactoferrin, indicating that the effects of lactoferrin might be attributable to the lactoferricin region. These results suggest that lactoferrin has a bacteriostatic action on P. gingivalis by binding HbR, removing it from the cell surface, and consequently disrupting the iron uptake system from hemoglobin.

Repeat-associated phase variable genes in the complete genome sequence of Neisseria meningitidis strain MC58

Phase variation, mediated through variation in the length of simple sequence repeats, is recognized as an important mechanism for controlling the expression of factors involved in bacterial virulence. Phase variation is associated with most of the currently recognized virulence determinants of Neisseria meningitidis. Based upon the complete genome sequence of the N. meningitidis serogroup B strain MC58, we have identified tracts of potentially unstable simple sequence repeats and their potential functional significance determined on the basis of sequence context. Of the 65 potentially phase variable genes identified, only 13 were previously recognized. Comparison with the sequences from the other two pathogenic Neisseria sequencing projects shows differences in the length of the repeats in 36 of the 65 genes identified, including 25 of those not previously known to be phase variable. Six genes that did not have differences in the length of the repeat instead had polymorphisms such that the gene would not be expected to be phase variable in at least one of the other strains. A further 12 candidates did not have homologues in either of the other two genome sequences. The large proportion of these genes that are associated with frameshifts and with differences in repeat length between the neisserial genome sequences is further corroborative evidence that they are phase variable. The number of potentially phase variable genes is substantially greater than for any other species studied to date, and would allow N. meningitidis to generate a very large repertoire of phenotypes through expression of these genes in different combinations. Novel phase variable candidates identified in the strain MC58 genome sequence include a spectrum of genes encoding glycosyltransferases, toxin related products, and metabolic activities as well as several restriction/modification and bacteriocin-related genes and a number of open reading frames (ORFs) for which the function is currently unknown. This suggests that the potential role of phase variation in mediating bacterium-host interactions is much greater than has been appreciated to date. Analysis of the distribution of homopolymeric tract lengths indicates that this species has sequence-specific mutational biases that favour the instability of sequences associated with phase variation.

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.

Alteration of epithelial cell transferrin-iron homeostasis by Neisseria meningitidis and Neisseria gonorrhoeae

Iron is an essential element for nearly all organisms. In mammals, iron is transported to body tissues by the serum glycoprotein transferrin. Transferrin-iron is internalized by binding to specific receptors followed by endocytosis. In vitro, Neisseria meningitidis and Neisseria gonorrhoeae can use iron from a variety of iron-containing compounds, including human transferrin. In vivo, transferrin is an important source of iron for N. gonorrhoeae: a mutant that is unable to bind and use transferrin-iron is unable to colonize the urethra of men or initiate disease at this site. As pathogenic Neisseria and its human host derive much of their iron from transferrin, we reasoned that a competition may exist between microbe and host epithelial cells for transferrin-iron at certain stages of infection. We therefore tested the hypothesis that N. meningitidis and N. gonorrhoeae may actively interfere with host transferrin-iron metabolism. We report that Neisseria-infected human epithelial cells have reduced levels of transferrin receptor messenger RNA and cycling transferrin receptors. The ability of infected cells to internalize transferrin receptor is also reduced. Finally, the relative distribution of surface and cycling transferrin receptors is altered in an infected cell. We conclude that Neisseria infection alters epithelial cell transferrin-iron homeostasis at multiple levels.

A high-affinity iron permease essential for Candida albicans virulence

Microbial pathogens must compete with the iron-withholding defense systems of their host to acquire this essential nutrient. Here, two high-affinity iron permease genes, CaFTR1 and CaFTR2, were isolated. CaFTR1 expression was induced under iron-limited conditions and repressed when iron supply was sufficient, whereas the expression of CaFTR2 was regulated in a reversed manner. Mutants lacking CaFTR1 but not CaFTR2 exhibited a severe growth defect in iron-deficient medium and were unable to establish systemic infection in mice. Thus, CaFTR1-mediated iron-uptake mechanism constitutes a virulence factor of Candida albicans and may be a target for the development of anti-Candida therapies.

Epidemiology and pathogenesis of Neisseria meningitidis

Neisseria meningitidis, an exclusive pathogen of humans, remains the leading worldwide cause of meningitis and fatal sepsis, usually in otherwise healthy individuals. In recent years, significant advances have improved our understanding of the epidemiology and genetic basis of meningococcal disease and led to progress in the development of the next generation of meningococcal vaccines. This review summarizes current knowledge of the human susceptibility to and the epidemiology and molecular pathogenesis of meningococcal disease.

Bacterial heme sources: the role of heme, hemoprotein receptors and hemophores

The major mechanisms by which Gram-negative bacteria acquire heme from host heme-carrier proteins involve either direct binding to specific outer membrane receptors or release of bacterial hemophores that take up heme from host heme carriers and shuttle it back to specific receptors. The ability to interact with and remove heme from carrier proteins distinguishes heme from conceptually similar siderophore and vitamin B12 receptors. Recent genetic, biochemical and crystallization studies have started to unravel the mechanism and molecular interactions between heme-carrier proteins and components of bacterial heme assimilation systems.

Complete genome sequence of Neisseria meningitidis serogroup B strain MC58

The 2,272,351-base pair genome of Neisseria meningitidis strain MC58 (serogroup B), a causative agent of meningitis and septicemia, contains 2158 predicted coding regions, 1158 (53.7%) of which were assigned a biological role. Three major islands of horizontal DNA transfer were identified; two of these contain genes encoding proteins involved in pathogenicity, and the third island contains coding sequences only for hypothetical proteins. Insights into the commensal and virulence behavior of N. meningitidis can be gleaned from the genome, in which sequences for structural proteins of the pilus are clustered and several coding regions unique to serogroup B capsular polysaccharide synthesis can be identified. Finally, N. meningitidis contains more genes that undergo phase variation than any pathogen studied to date, a mechanism that controls their expression and contributes to the evasion of the host immune system.

Association of iron-regulated outer membrane proteins of Neisseria meningitidis with the RmpM (class 4) protein

The RmpM (class 4) protein of Neisseria meningitidis has previously been shown to be associated with the outer membrane porins. In the present study, we demonstrate that this protein forms complexes with the lactoferrin receptor LbpA, the transferrin receptor TbpA and the siderophore receptor FrpB as well. This complexation apparently resulted in a stabilization of oligomeric forms of these iron-regulated proteins. In vitro experiments further revealed a reduced ability to acquire iron from human lactoferrin in the rmpM mutant. Furthermore, all TonB-dependent receptors investigated here appeared to exist as oligomers (probably dimers), suggesting that this is a general feature of this class of proteins.

Use of heme compounds as iron sources by pathogenic neisseriae requires the product of the hemO gene

Heme compounds are an important source of iron for neisseriae. We have identified a neisserial gene, hemO, that is essential for heme, hemoglobin (Hb), and haptoglobin-Hb utilization. The hemO gene is located 178 bp upstream of the hmbR Hb receptor gene in Neisseria meningitidis isolates. The product of the hemO gene is homologous to enzymes that degrade heme; 21% of its amino acid residues are identical, and 44% are similar, to those of the human heme oxygenase-1. DNA sequences homologous to hemO were ubiquitous in commensal and pathogenic neisseriae. HemO genetic knockout strains of Neisseria gonorrhoeae and N. meningitidis were unable to use any heme source, while the assimilation of transferrin-iron and iron-citrate complexes was unaffected. A phenotypic characterization of a conditional hemO mutant, constructed by inserting an isopropyl-beta-D-thiogalactopyranoside (IPTG)-regulated promoter upstream of the ribosomal binding site of hemO, confirmed the indispensability of the HemO protein in heme utilization. The expression of HemO also protected N. meningitidis cells against heme toxicity. hemO mutants were still able to transport heme into the cell, since both heme and Hb could complement an N. meningitidis hemA hemO double mutant for growth. The expression of the HmbR receptor was reduced significantly by the inactivation of the hemO gene, suggesting that hemO and hmbR are transcriptionally linked. The expression of the unlinked Hb receptor, HpuAB, was not altered. Comparison of the polypeptide patterns of the wild type and the hemO mutant led to detection of six protein spots with an altered expression pattern, suggesting a more general role of HemO in the regulation of gene expression in Neisseriae.