Heme utilization in Bordetella avium is regulated by RhuI, a heme-responsive extracytoplasmic function sigma factor

The diversity of heme sensor systems - heme-responsive transcriptional regulation mediated by transient heme protein interactions

Heme is a versatile molecule that is vital for nearly all cellular life by serving as prosthetic group for various enzymes or as nutritional iron source for diverse microbial species. However, elevated levels of heme molecule are toxic to cells. The complexity of this stimulus has shaped the evolution of diverse heme sensor systems, which are involved in heme-dependent transcriptional regulation in eukaryotes and prokaryotes. The functions of these systems are manifold - ranging from the specific control of heme detoxification or uptake systems to the global integration of heme and iron homeostasis. This review focuses on heme sensor systems, regulating heme homeostasis by transient heme protein interaction. We provide an overview of known heme-binding motifs in prokaryotic and eukaryotic transcription factors. Besides the central ligands, the surrounding amino acid environment was shown to play a pivotal role in heme binding. The diversity of heme-regulatory systems therefore illustrates that prediction based on pure sequence information is hardly possible and requires careful experimental validation. Comprehensive understanding of heme-regulated processes is not only important for our understanding of cellular physiology, but also provides a basis for the development of novel antibacterial drugs and metabolic engineering strategies.

RNA-mediated thermoregulation of iron-acquisition genes in Shigella dysenteriae and pathogenic Escherichia coli

The initiation, progression and transmission of most bacterial infections is dependent upon the ability of the invading pathogen to acquire iron from each of the varied environments encountered during the course of a natural infection. In total, 95% of iron within the human body is complexed within heme, making heme a potentially rich source of host-associated nutrient iron for invading bacteria. As heme is encountered only within the host, pathogenic bacteria often regulate synthesis of heme utilization factors such that production is maximal under host-associated environmental conditions. This study examines the regulated production of ShuA, an outer-membrane receptor required for the utilization of heme as a source of nutrient iron by Shigella dysenteriae, a pathogenic bacterium that causes severe diarrheal diseases in humans. Specifically, the impact of the distinct environmental temperatures encountered during infection within a host (37°C) and transmission between hosts (25°C) on shuA expression is investigated. We show that shuA expression is subject to temperature-dependent post-transcriptional regulation resulting in increased ShuA production at 37°C. The observed thermoregulation is mediated by nucleic acid sequences within the 5' untranslated region. In addition, we have identified similar nucleotide sequences within the 5' untranslated region of the orthologous chuA transcript of enteropathogenic E. coli and have demonstrated that it also functions to confer temperature-dependent post-transcriptional regulation. In both function and predicted structure, the regulatory element within the shuA and chuA 5' untranslated regions closely resembles a FourU RNA thermometer, a zipper-like RNA structure that occludes the Shine-Dalgarno sequence at low temperatures. Increased production of ShuA and ChuA in response to the host body temperature allows for maximal production of these heme acquisition factors within the environment where S. dysenteriae and pathogenic E. coli strains would encounter heme, a host-specific iron source.

The Bartonella quintana extracytoplasmic function sigma factor RpoE has a role in bacterial adaptation to the arthropod vector environment

Bartonella quintana is a vector-borne bacterial pathogen that causes fatal disease in humans. During the infectious cycle, B. quintana transitions from the hemin-restricted human bloodstream to the hemin-rich body louse vector. Because extracytoplasmic function (ECF) sigma factors often regulate adaptation to environmental changes, we hypothesized that a previously unstudied B. quintana ECF sigma factor, RpoE, is involved in the transition from the human host to the body louse vector. The genomic context of B. quintana rpoE identified it as a member of the ECF15 family of sigma factors found only in alphaproteobacteria. ECF15 sigma factors are believed to be the master regulators of the general stress response in alphaproteobacteria. In this study, we examined the B. quintana RpoE response to two stressors that are encountered in the body louse vector environment, a decreased temperature and an increased hemin concentration. We determined that the expression of rpoE is significantly upregulated at the body louse (28°C) versus the human host (37°C) temperature. rpoE expression also was upregulated when B. quintana was exposed to high hemin concentrations. In vitro and in vivo analyses demonstrated that RpoE function is regulated by a mechanism involving the anti-sigma factor NepR and the response regulator PhyR. The ΔrpoE ΔnepR mutant strain of B. quintana established that RpoE-mediated transcription is important in mediating the tolerance of B. quintana to high hemin concentrations. We present the first analysis of an ECF15 sigma factor in a vector-borne human pathogen and conclude that RpoE has a role in the adaptation of B. quintana to the hemin-rich arthropod vector environment.

HmuP is a coactivator of Irr-dependent expression of heme utilization genes in Bradyrhizobium japonicum

Utilization of heme as an iron source by Bradyrhizobium japonicum involves induction of the outer membrane heme receptor gene hmuR and other genes within the heme utilization locus. Here, we discovered the hmuP gene located upstream of hmuR and transcribed divergently from it along with hmuTUV. hmuP encodes a small protein that accumulated under iron limitation and is transcriptionally controlled by the global iron-responsive regulator Irr, as were all genes within the heme utilization locus. Cross-linking and immunoprecipitation experiments showed that Irr occupies the hmuR-hmuP promoter in vivo. An hmuP mutant did not grow on heme as an iron source, but retained the ability to use ferric chloride. Correspondingly, induction of hmuR mRNA under iron limitation was severely diminished in an hmuP strain, but other genes within the Irr regulon were unaffected. HmuP occupied the hmuR-hmuP promoter, and thus it plays a direct regulatory role in gene expression. HmuP was not required for Irr occupancy, nor was ectopic expression of hmuP from an Irr-independent promoter sufficient to induce the hmuR gene. Thus, both HmuP and Irr occupancy are necessary for hmuR induction. We suggest that HmuP is a coactivator of Irr-dependent expression of hmuR.

Novel iron-regulated and Fur-regulated small regulatory RNAs in Aggregatibacter actinomycetemcomitans

Iron can regulate biofilm formation via non-coding small RNA (sRNA). To determine if iron-regulated sRNAs are involved in biofilm formation by the periodontopathogen Aggregatibacter actinomycetemcomitans, total RNA was isolated from bacteria cultured with iron supplementation or chelation. Transcriptional analysis demonstrated that the expression of four sRNA molecules (JA01-JA04) identified by bioinformatics was significantly upregulated in iron-limited medium compared with iron-rich medium. A DNA fragment encoding each sRNA promoter was able to titrate Escherichia coli ferric uptake regulator (Fur) from a Fur-repressible reporter fusion in an iron uptake regulator titration assay. Cell lysates containing recombinant AaFur shifted the mobility of sRNA-specific DNAs in a gel shift assay. Potential targets of these sRNAs, determined in silico, included genes involved in biofilm formation. The A. actinomycetemcomitans overexpressing JA03 sRNA maintained a rough phenotype on agar, but no longer adhered to uncoated polystyrene or glass, although biofilm determinant gene expression was only modestly decreased. In summary, these sRNAs have the ability to modulate biofilm formation, but their functional target genes remain to be confirmed.

The Neisseria meningitidis ZnuD zinc receptor contributes to interactions with epithelial cells and supports heme utilization when expressed in Escherichia coli

Neisseria meningitidis employs redundant heme acquisition mechanisms, including TonB receptor-dependent and receptor-independent uptakes. The TonB-dependent zinc receptor ZnuD shares significant sequence similarity to HumA, a heme receptor of Moraxella catarrhalis, and contains conserved motifs found in many heme utilization proteins. We present data showing that, when expressed in Escherichia coli, ZnuD allowed heme capture on the cell surface and supported the heme-dependent growth of an E. coli hemA strain. Heme agarose captured ZnuD in enriched outer membrane fractions, and this binding was inhibited by excess free heme, supporting ZnuD's specific interaction with heme. However, no heme utilization defect was detected in the meningococcal znuD mutant, likely due to unknown redundant TonB-independent heme uptake mechanisms. Meningococcal replication within epithelial cells requires a functional TonB, and we found that both the znuD and tonB mutants were defective not only in survival within epithelial cells but also in adherence to and invasion of epithelial cells. Ectopic complementation rescued these phenotypes. Interestingly, while znuD expression was repressed by Zur with zinc as a cofactor, it also was induced by iron in a Zur-independent manner. A specific interaction of meningococcal Fur protein with the znuD promoter was demonstrated by electrophoretic mobility shift assay (EMSA). Thus, the meningococcal ZnuD receptor likely participates in both zinc and heme acquisition, is regulated by both Zur and Fur, and is important for meningococcal interaction with epithelial cells.

Change is good: variations in common biological mechanisms in the epsilonproteobacterial genera Campylobacter and Helicobacter

Microbial evolution and subsequent species diversification enable bacterial organisms to perform common biological processes by a variety of means. The epsilonproteobacteria are a diverse class of prokaryotes that thrive in diverse habitats. Many of these environmental niches are labeled as extreme, whereas other niches include various sites within human, animal, and insect hosts. Some epsilonproteobacteria, such as Campylobacter jejuni and Helicobacter pylori, are common pathogens of humans that inhabit specific regions of the gastrointestinal tract. As such, the biological processes of pathogenic Campylobacter and Helicobacter spp. are often modeled after those of common enteric pathogens such as Salmonella spp. and Escherichia coli. While many exquisite biological mechanisms involving biochemical processes, genetic regulatory pathways, and pathogenesis of disease have been elucidated from studies of Salmonella spp. and E. coli, these paradigms often do not apply to the same processes in the epsilonproteobacteria. Instead, these bacteria often display extensive variation in common biological mechanisms relative to those of other prototypical bacteria. In this review, five biological processes of commonly studied model bacterial species are compared to those of the epsilonproteobacteria C. jejuni and H. pylori. Distinct differences in the processes of flagellar biosynthesis, DNA uptake and recombination, iron homeostasis, interaction with epithelial cells, and protein glycosylation are highlighted. Collectively, these studies support a broader view of the vast repertoire of biological mechanisms employed by bacteria and suggest that future studies of the epsilonproteobacteria will continue to provide novel and interesting information regarding prokaryotic cellular biology.

Expression of BfrH, a putative siderophore receptor of Bordetella bronchiseptica, is regulated by iron, Fur1, and the extracellular function sigma factor EcfI

Iron (Fe) in soluble elemental form is found in the tissues and fluids of animals at concentrations insufficient for sustaining growth of bacteria. Consequently, to promote colonization and persistence, pathogenic bacteria evolved a myriad of scavenging mechanisms to acquire Fe from the host. Bordetella bronchiseptica, the etiologic agent of upper respiratory infections in a wide range of mammalian hosts, expresses a number of proteins for acquisition of Fe. Using proteomic and genomic approaches, three Fe-regulated genes were identified in the bordetellae: bfrH, a gene encoding a putative siderophore receptor; ecfI, a gene encoding a putative extracellular function (ECF) sigma factor; and ecfR, a gene encoding a putative EcfI modulator. All three genes are highly conserved in B. pertussis, B. parapertussis, and B. avium. Genetic analysis revealed that transcription of bfrH was coregulated by ecfI, ecfR, and fur1, one of two fur homologues carried by B. bronchiseptica. Overexpression of ecfI decoupled bfrH from Fe-dependent regulation. In contrast, expression of bfrH was significantly reduced in an ecfI deletion mutant. Deletion of ecfR, however, was correlated with a significant increase in expression of bfrH, due in part to a cis-acting nucleotide sequence within ecfR which likely reduces the frequency of readthrough transcription of bfrH from the Fe-dependent ecfIR promoter. Using a murine competition infection model, bfrH was shown to be required for optimal virulence of B. bronchiseptica. These experiments revealed ecfIR-bfrH as a locus encoding a new member of the growing family of Fe and ECF sigma factor-modulated regulons in the bordetellae.

The heme sensor system of Staphylococcus aureus

The important human pathogen Staphylococcus aureus is able to satisfy its nutrient iron requirement by acquiring heme from host hemoglobin in the context of infection. However, heme acquisition exposes S. aureus to heme toxicity. In order to detect the presence of toxic levels of exogenous heme, S. aureus is able to sense heme through the heme sensing system (HssRS) two-component system. Upon sensing heme, HssRS directly regulates the expression of the heme-regulated ABC transporter HrtAB, which alleviates heme toxicity. Importantly, the inability to sense or respond to heme alters the virulence of S. aureus, highlighting the importance of heme sensing and detoxification to staphylococcal pathogenesis. Furthermore, potential orthologues of the Hss and Hrt systems are found in many species of Gram-positive bacteria, a possible indication that heme stress is a challenge faced by bacteria whose habitats include host tissues rich in heme.

Temporal signaling and differential expression of Bordetella iron transport systems: the role of ferrimones and positive regulators

The bacterial respiratory pathogens Bordetella pertussis and Bordetella bronchiseptica employ multiple alternative iron acquisition pathways to adapt to changes in the mammalian host environment during infection. The alcaligin, enterobactin, and heme utilization pathways are differentially expressed in response to the cognate iron source availability by a mechanism involving substrate-inducible positive regulators. As inducers, the iron sources function as chemical signals termed ferrimones. Ferrimone-sensing allows the pathogen to adapt and exploit early and late events in the infection process.

Heme uptake by Microscilla marina and evidence for heme uptake systems in the genomes of diverse marine bacteria

The ability to acquire diverse and abundant forms of iron would be expected to confer a survival advantage in the marine environment, where iron is scarce. Marine bacteria are known to use siderophores and inorganic iron, but their ability to use heme, an abundant intracellular iron form, has only been examined preliminarily. Microscilla marina, a cultured relative of a bacterial group frequently found on marine particulates, was used as a model organism to examine heme uptake. Searches of the genome revealed analogs to known heme transport proteins, and reverse transcription-quantitative PCR analysis of these genes showed that they were expressed and upregulated under iron stress and during growth on heme. M. marina was found to take up heme-bound iron and could grow on heme as a sole iron source, supporting the genetic evidence for heme transport. Similar putative heme transport components were identified in the genomes of diverse marine bacteria. These systems were found in the genomes of many bacteria thought to be particle associated but were lacking in known free-living organisms (e.g., Pelagibacter ubique and marine cyanobacteria). This distribution of transporters is consistent with the hydrophobic, light-sensitive nature of heme, suggesting that it is primarily available on phytoplankton or detritus or in nutrient-rich environments.

Expression of hurP, a gene encoding a prospective site 2 protease, is essential for heme-dependent induction of bhuR in Bordetella bronchiseptica

Expression of the hurIR bhuRSTUV heme utilization locus in Bordetella bronchiseptica is coordinately controlled by the global iron-dependent regulator Fur and the extracytoplasmic function sigma factor HurI. Activation of HurI requires transduction of a heme-dependent signal via HurI, HurR, and BhuR, a three-component heme-dependent regulatory system. In silico searches of the B. bronchiseptica genome to identify other genes that encode additional participants in this heme-dependent regulatory cascade revealed hurP, an open reading frame encoding a polypeptide with homology to (i) RseP, a site 2 protease (S2P) of Escherichia coli required for modifying the cytoplasmic membrane protein RseA, and (ii) YaeL, an S2P of Vibrio cholerae required for modification of the cytoplasmic membrane protein TcpP. A mutant of B. bronchiseptica defective for hurP was incapable of regulating expression of BhuR in a heme-dependent manner. Furthermore, the hurP mutant was unable to utilize hemin as a sole source of nutrient Fe. These defects in hemin utilization and heme-dependent induction of BhuR were restored when recombinant hurP (or recombinant rseP) was introduced into the mutant. Introduction of hurP into a yaeL mutant of V. cholerae also complemented its S2P defect. These data provided strong evidence that protease activity and cleavage site recognition was conserved in HurP, RseP, and YaeL. The data are consistent with a model in which HurP functionally modifies HurR, a sigma factor regulator that is essential for heme-dependent induction of bhuR.

Profiling theBordetellapertussisProteome during Iron Starvation

Regulation of gene expression in response to local iron concentration is commonly observed in bacterial pathogens that face this nutrient limitation during host infection. In this study, a proteomic approach was used to analyze the differential protein expression of Bordetella pertussis under iron limitation. Whole cell lysates (WCL) and outer membrane fractions of bacteria grown either under iron-starvation or iron-excess conditions were analyzed by two-dimensional (2-D) gel electrophoresis. Statistical analysis revealed 36 proteins displaying differential expression, 9 with higher expression under iron-excess and 27 with increased expression under iron-starvation. These proteins were subjected to tryptic digestion and MALDI-TOF MS. Apart from those previously reported, we identified new low-iron-induced proteins that might help to explain the increased virulence of this phenotype. Additionally, we found evidence that at least one of the identified proteins, solely expressed under iron starvation, is highly immunogenic in infected individuals.

Intracellular metalloporphyrin metabolism in Staphylococcus aureus

The bacterial pathogen Staphylococcus aureus is responsible for a significant amount of human morbidity and mortality, and the ability of S. aureus to cause disease is absolutely dependent on the acquisition of iron from the host. The most abundant iron source to invading staphylococci is in the form of the porphyrin heme. S. aureus is capable of acquiring nutrient iron from heme and hemoproteins via two heme-acquisition systems, the iron-regulated surface determinant system (Isd) and the heme transport system (Hts). Heme acquisition through these systems is involved in staphylococcal pathogenesis suggesting that the intracellular fate of heme plays a significant role in the infectious process. The valuable heme molecule presents a paradox to invading bacteria because although heme is an abundant source of nutrient iron, the extreme reactivity of heme makes it toxic at high concentrations. Therefore, bacteria must regulate the levels of intracellular heme to avoid toxicity. Although the molecular mechanisms responsible for staphylococcal heme acquisition are beginning to emerge, the mechanisms by which S. aureus regulate intracellular heme homeostasis are largely unknown. In this review we describe three potential fates of host-derived heme acquired by S. aureus during infection: (i) degradation for use as a nutrient iron source, (ii) incorporation into bacterial heme-binding proteins for use as an enzyme cofactor, or (iii) efflux through a dedicated ABC-type transport system. We hypothesize that the ultimate fate of exogenously acquired heme in S. aureus is dependent upon the intracellular and extracellular availability of both iron and heme.

The ChrA-ChrS and HrrA-HrrS signal transduction systems are required for activation of the hmuO promoter and repression of the hemA promoter in Corynebacterium diphtheriae

Transcription of the Corynebacterium diphtheriae hmuO gene, which encodes a heme oxygenase involved in heme iron utilization, is activated in a heme- or hemoglobin-dependent manner in part by the two-component system ChrA-ChrS. Mutation of either the chrA or the chrS gene resulted in a marked reduction of hemoglobin-dependent activation at the hmuO promoter in C. diphtheriae; however, it was observed that significant levels of hemoglobin-dependent expression were maintained in the mutants, suggesting that an additional activator is involved in regulation. A BLAST search of the C. diphtheriae genome sequence revealed a second two-component system, encoded by DIP2268 and DIP2267, that shares similarity with ChrS and ChrA, respectively; we have designated these genes hrrS (DIP2268) and hrrA (DIP2267). Analysis of hmuO promoter expression demonstrated that hemoglobin-dependent activity was fully abolished in strains from which both the chrA-chrS and the hrrA-hrrS two-component systems were deleted. Similarly, deletion of the sensor kinase genes chrS and hrrS or the genes encoding both of the response regulators chrA and hrrA also eliminated hemoglobin-dependent activation at the hmuO promoter. We also show that the regulators ChrA-ChrS and HrrA-HrrS are involved in the hemoglobin-dependent repression of the promoter upstream of hemA, which encodes a heme biosynthesis enzyme. Evidence for cross talk between the ChrA-ChrS and HrrA-HrrS systems is presented. In conclusion, these findings demonstrate that the ChrA-ChrS and HrrA-HrrS regulatory systems are critical for full hemoglobin-dependent activation at the hmuO promoter and also suggest that these two-component systems are involved in the complex mechanism of the regulation of heme homeostasis in C. diphtheriae.

Bordetella iron transport and virulence

Bordetella pertussis, Bordetella parapertussis, and Bordetella bronchiseptica are pathogens with a complex iron starvation stress response important for adaptation to nutrient limitation and flux in the mammalian host environment. The iron starvation stress response is globally regulated by the Fur repressor using ferrous iron as the co-repressor. Expression of iron transport system genes of Bordetella is coordinated by priority regulation mechanisms that involve iron source sensing. Iron source sensing is mediated by distinct transcriptional activators that are responsive to the cognate iron source acting as the inducer.

Staphylococcus aureus redirects central metabolism to increase iron availability

Staphylococcus aureus pathogenesis is significantly influenced by the iron status of the host. However, the regulatory impact of host iron sources on S. aureus gene expression remains unknown. In this study, we combine multivariable difference gel electrophoresis and mass spectrometry with multivariate statistical analyses to systematically cluster cellular protein response across distinct iron-exposure conditions. Quadruplicate samples were simultaneously analyzed for alterations in protein abundance and/or post-translational modification state in response to environmental (iron chelation, hemin treatment) or genetic (Δfur) alterations in bacterial iron exposure. We identified 120 proteins representing several coordinated biochemical pathways that are affected by changes in iron-exposure status. Highlighted in these experiments is the identification of the heme-regulated transport system (HrtAB), a novel transport system which plays a critical role in staphylococcal heme metabolism. Further, we show that regulated overproduction of acidic end-products brought on by iron starvation decreases local pH resulting in the release of iron from the host iron-sequestering protein transferrin. These findings reveal novel strategies used by S. aureus to acquire scarce nutrients in the hostile host environment and begin to define the iron and heme-dependent regulons of S. aureus.

Virtually all bacterial pathogens require iron to successfully infect their human hosts. This presents a problem to invading bacteria because the majority of iron in humans is tightly bound by iron-binding proteins. To counteract this host defense, bacterial pathogens have developed elaborate mechanisms to acquire nutrient iron during infection. To gain insight into how the amount of available iron impacts the human pathogen Staphylococcus aureus, the authors identified proteins that increase or decrease abundance upon alterations in iron status. The authors found that under conditions of iron starvation, the Fur regulatory protein of S. aureus coordinates a redirection of the central metabolic pathways causing the bacteria to produce large amounts of acidic end-products. The accumulation of these acidic end-products facilitates the release of iron from host iron-binding proteins, in effect increasing the availability of this precious nutrient source. These findings provide a mechanistic explanation for how S. aureus alters its local microenvironment during infection to increase the availability of nutrient iron. Based on the well-established role for bacterial iron acquisition during pathogenesis, systems involved in iron acquisition represent excellent potential therapeutic targets against bacterial infection.

Characterization of a highly conserved island in the otherwise divergent Bordetella holmesii and Bordetella pertussis genomes

The recently discovered pathogen Bordetella holmesii has been isolated from the airways and blood of diseased humans. Genetic events contributing to the emergence of B. holmesii are not understood, and its phylogenetic position among the bordetellae remains unclear. To address these questions, B. holmesii strains were analyzed by comparative genomic hybridization (CGH) to a Bordetella pertussis microarray and by multilocus sequence typing. Both methods indicated substantial sequence divergence between B. pertussis and B. holmesii. However, CGH identified a putative pathogenicity island of 66 kb that is highly conserved between these species and contains several IS481 elements that may have been laterally transferred from B. pertussis to B. holmesii. This island contains, among other genes, a functional, iron-regulated locus encoding the biosynthesis, export, and uptake of the siderophore alcaligin. The acquisition of this genomic island by B. holmesii may have significantly contributed to its emergence as a human pathogen. Horizontal gene transfer between B. pertussis and B. holmesii may also explain the unusually high sequence identity of their 16S rRNA genes.

Analysis of a heme-dependent signal transduction system in Corynebacterium diphtheriae: deletion of the chrAS genes results in heme sensitivity and diminished heme-dependent activation of the hmuO promoter

The Corynebacterium diphtheriae hmuO gene encodes a heme oxygenase that is involved in the utilization of heme as an iron source. Transcription of hmuO is activated by heme or hemoglobin and repressed by iron and DtxR. Previous studies with Escherichia coli showed that heme-dependent transcriptional activation of an hmuO promoter-lacZ fusion was dependent on the cloned C. diphtheriae chrA and chrS genes (chrAS), which encode the response regulator and sensor kinase, respectively, of a two-component signal transduction system. In this study, nonpolar deletions in the chrAS genes were constructed on the chromosome of C. diphtheriae. Mutations in chrAS resulted in marked reduction in heme-dependent transcription of hmuO, which indicates that the ChrA/S system is a key regulator at the hmuO promoter. However, low but significant levels of heme-specific transcriptional activity were observed at the hmuO promoter in the chrAS mutants, suggesting that an additional heme-dependent activator is involved in hmuO expression. The chrAS mutants were also sensitive to heme, which was observed only in stationary-phase cultures and correlated with reduced cell viability. The heme sensitivity of the mutants was not due to reduced expression of hmuO, and these results suggest that additional factors controlled by the ChrA/S system may be involved in protection against heme toxicity. Transcriptional analysis of the chrAS operon revealed that it was not autoregulated or affected by iron or heme levels.

Transcriptional control of the rhuIR-bhuRSTUV heme acquisition locus in Bordetella avium

Iron (Fe) is an essential nutrient for most bacterial pathogens. In these organisms, a variety of regulatory systems that respond to specific Fe complexes found within their vertebrate hosts have evolved. In Bordetella avium, the heme utilization locus encoded by rhuIR-bhuRSTUV mediates efficient acquisition of Fe from heme and hemoproteins. Control of bhuRSTUV expression is promulgated at two levels. When Fe is abundant, expression is repressed in a Fur-dependent manner which is partially relieved when Fe is limiting. In the presence of heme or hemoproteins, expression of the bhuRSTUV operon is induced via a three-component signal transduction cascade composed of RhuI, RhuR, and BhuR. Herein, we report the identification of two promoters (PrhuI and PbhuR) that control expression of the rhuIR-bhuRSTUV cluster. Primer extension analysis identified the transcriptional start site of PrhuI within a putative Fur box. Transcriptional initiation of PbhuR mapped within the rhuR-bhuR intergenic region. Maximal transcription from PbhuR required Fe-limiting conditions, the presence of heme (or hemoglobin), and rhuI; however, analysis of transcripts produced from the rhuIR-bhuRSTUV locus revealed a pattern of low-level bhuR transcription in the absence of heme which originated from both PbhuR and PrhuI. Transcription from PrhuI was repressed by Fe in the presence of fur and somewhat enhanced by the addition of hemin to Fe-limited media. The nature of this hemin-associated PrhuI stimulation was rhuI independent and therefore not induced by heme via the BhuR-RhuR-RhuI signal cascade. Fe also repressed transcription from PbhuR in a fur-dependent manner; however, activation from this promoter, in the presence or absence of heme, did not occur without rhuI.

Heme binds to and inhibits the DNA-binding activity of the global regulator FurA fromAnabaenasp. PCC 7120

Heme is an iron-containing cofactor that aside from serving as the active group of essential proteins is a key element in the control of many molecular and cellular processes. In prokaryotes, the family of Fur (ferric uptake regulator) proteins governs processes essential for the survival of microorganims such as the iron homeostasis. We show that purified recombinant FurA from Anabaena sp. PCC 7120 interacts strongly with heme in the micromolar range and this interaction affects the in vitro ability of FurA to bind DNA, inhibiting that process in a concentration-dependent fashion. Our results provide the first evidence of the possible involvement of heme in the regulatory function of cyanobacterial Fur.

Stress and transcriptional regulation of tick ferritin HC

We previously identified a partial Dermacentor variabilis cDNA encoding ferritin HC (HC) subunit homolog (DVFER) that was differentially upregulated in Rickettsia montanensis infected ticks (Mulenga et al., 2003a). We have used rapid amplification of cDNA ends to clone full-length DVFER cDNA and its apparent ortholog from the wood tick, D. andersoni (DAFER), both of which show high sequence similarity to vertebrate than insect ferritin. Both DVFER and DAFER contain the stem-loop structure of a putative iron responsive element in the 5' untranslated region (nucleotide positions, 16-42) and the feroxidase centre loop typical for vertebrate ferritin HC subunits. Quantitative Western and Northern blotting analyses of protein and RNA from unfed and partially fed whole tick as well as dissected tick tissues demonstrated that DVFER is constitutively and ubiquitously expressed. Based on densitometric analysis of detected protein and mRNA bands, DVFER is predominantly expressed in the midgut, and to a lesser extent in the salivary glands, ovary and fatbody. Sham treatment (mechanical injury) and Escherichia coli challenge of D. variabilis ticks stimulated statistically significant (approximately 1.5- and approximately 3.0-fold, respectively) increases in DVFER mRNA abundance over time point matched naive control ticks. These data suggest that DVFER mRNA is nonspecifically up regulated in response to mechanical injury or bacterial infection induced stress.

Heme-iron increases levels of AP65-mediated adherence by Trichomonas vaginalis

Trichomonas vaginalis is a protozoan responsible for the number one, non-viral sexually transmitted disease. Surface proteins (AP65, AP51, AP33 and AP23) mediate adherence to vaginal epithelial cells (VECs). Iron increases growth of trichomonads and synthesis and surface placement of adhesins. We observed by immunofluorescence using monoclonal antibody (mAb) 12G4 the placement of AP65 on surfaces of trichomonads supplemented with hemoglobin or hemin as a source of iron. We, therefore, tested the hypothesis that heme-bound iron is an alternative source of iron important to trichomonal growth and regulation of expression of the adhesin genes. Here we show that the inhibition of parasite growth by the iron chelator 2,2-dipyridal is rescued by hemoglobin or hemin, but not protoporphyrin IX. Importantly, trichomonads grown in iron-limiting medium supplemented with free iron, hemoglobin and hemin had elevated levels of ap65 transcript that were 12.6-, 12.3- and 9.2-fold higher, respectively, than low-iron organisms, as determined by RT-PCR. Similarly, the amounts of AP65 were 8.9-, 11.2-, and 4.8-fold higher in parasites grown in free iron, hemoglobin and hemin, respectively, than organisms in low-iron medium. The heme-iron-regulated AP65 increased adherence of parasites to immortalized VECs. Not surprisingly, parasites pretreated with anti-AP65 serum IgG had decreased adherence compared to organisms incubated with prebleed serum IgG. These data illustrate that heme-bound iron is a source of iron similar to lactoferrin. This work extends our findings about the multiple sources of iron for regulating virulence genes of T. vaginalis.

Integration of environmental signals controls expression of Bordetella heme utilization genes

The Bordetella pertussis heme utilization gene cluster hurIR bhuRSTUV encodes regulatory and transport functions required for assimilation of iron from heme and hemoproteins. Expression of the bhu genes is iron regulated and heme inducible. The putative extracytoplasmic function (ECF) sigma factor, HurI, is required for heme-responsive bhu gene expression. In this study, transcriptional activation of B. pertussis bhu genes in response to heme compounds was shown to be dose dependent and specific for heme; protoporphyrin IX and other heme structural analogs did not activate bhu gene expression. Two promoters controlling expression of the heme utilization genes were mapped by primer extension analysis. The hurI promoter showed similarity to sigma(70)-like promoters, and its transcriptional activity was iron regulated and heme independent. A second promoter identified upstream of bhuR exhibited little similarity to previously characterized ECF sigma factor-dependent promoters. Expression of bhuR was iron regulated, heme responsive, and hurI dependent in B. pertussis, as shown in a previous study with Bordetella bronchiseptica. Further analyses showed that transcription originating at a distal upstream site and reading through the hurR-bhuR intergenic region contributes to bhuR expression under iron starvation conditions in the absence of heme inducer. The pattern of regulation of the readthrough transcript was consistent with transcription from the hurI promoter. The positions and regulation of the two promoters within the hur-bhu gene cluster influence the production of heme transport machinery so that maximal expression of the bhu genes occurs under iron starvation conditions only in the presence of heme iron sources.

RhuR, an extracytoplasmic function sigma factor activator, is essential for heme-dependent expression of the outer membrane heme and hemoprotein receptor of Bordetella avium

Genes involved in iron (Fe) acquisition often are regulated in response to the local availability of Fe. In many bacteria, Fe-dependent responsiveness is mediated by Fur, a global Fe-dependent transcriptional repressor. Tighter regulatory control of Fur-responsive genes is afforded by incorporating additional regulators into Fur-dependent regulatory cascades. RhuI, a Fur-dependent extracytoplasmic function sigma factor of Bordetella avium, in response to the dual stimulation of Fe starvation and the presence of heme (or hemoproteins), regulates P(bhuR), a heme-responsive promoter which directs expression of the bhuRSTUV heme utilization operon. While BhuR, the outer membrane heme receptor, and RhuI have been shown to be indispensable for heme-dependent activation of P(bhuR), collateral components of the regulatory cascade have not been described. In this investigation, RhuR, an integral cytoplasmic membrane protein with homology to anti-sigma factors, is shown to be an essential activator of P(bhuR) expression. The functional domain of RhuR required for heme-dependent activation of P(bhuR) expression was mapped to the N-terminal 97 amino acids of the protein by use of a chimeric RhuR-BlaM fusion. Expression of the chimera in a rhuR mutant rendered P(bhuR) constitutive, thereby decoupling the promoter from heme dependency. Growth studies confirmed that B. avium requires RhuR for optimal utilization of hemoglobin, but not hemin, as a sole source of nutrient Fe. These data imply that B. avium expresses, in addition to the BhuR heme/hemoprotein utilization system, an alternative RhuR-independent heme utilization mechanism. A model is proposed in which RhuR is the functional bridge between BhuR and RhuI in a heme-dependent regulatory cascade.

Regulation of the FecI-type ECF sigma factor by transmembrane signalling

Induction of the ferric citrate transport genes of Escherichia coli K-12 involves a signalling cascade that starts at the cell surface and proceeds to the cytoplasm. Three specific proteins are involved: FecA in the outer membrane, FecR in the cytoplasmic membrane, and FecI in the cytoplasm. The binding of dinuclear ferric citrate to FecA causes substantial structural changes in FecA, triggering the signal cascade. The amino-proximal end of FecA interacts with the carboxy-proximal end of FecR in the periplasm. FecR then transmits the signal across the cytoplasmic membrane into the cytoplasm and activates the FecI sigma factor, which binds to the RNA polymerase core enzyme and directs the RNA polymerase to the promoter upstream of the fecABCDE transport genes to initiate transcription. Transcription of the fecIR regulatory genes and the fec transport genes is repressed by the Fur protein loaded with Fe(2+). Therefore, transcription of the fec transport genes is subjected to double control: cells first detect iron deficiency and respond by synthesis of the regulatory proteins FecI and FecR, which initiate transcription of the fec transport genes, provided ferric citrate is available. FecI belongs to the extracytoplasmic function sigma factors, which are widespread among bacteria. With the recent sequencing of complete microbial genomes, it has become apparent that the FecIRA cascade is now a paradigm for the regulatory control of FecI family sigmas in Gram-negative bacteria.

Heme-responsive transcriptional activation of Bordetella bhu genes

Bordetella pertussis and Bordetella bronchiseptica, gram-negative respiratory pathogens of mammals, possess a heme iron utilization system encoded by the bhuRSTUV genes. Preliminary evidence suggested that expression of the BhuR heme receptor was stimulated by the presence of heme under iron-limiting conditions. The hurIR (heme uptake regulator) genes were previously identified upstream of the bhuRSTUV gene cluster and are predicted to encode homologs of members of the iron starvation subfamily of extracytoplasmic function (ECF) regulators. In this study, B. pertussis and B. bronchiseptica DeltahurI mutants, predicted to lack an ECF sigma factor, were constructed and found to be deficient in the utilization of hemin and hemoglobin. Genetic complementation of DeltahurI strains with plasmid-borne hurI restored wild-type levels of heme utilization. B. bronchiseptica DeltahurI mutant BRM23 was defective in heme-responsive production of the BhuR heme receptor; hurI in trans restored heme-inducible BhuR expression to the mutant and resulted in BhuR overproduction. Transcriptional analyses with bhuR-lacZ fusion plasmids confirmed that bhuR transcription was activated in iron-starved cells in response to heme compounds. Heme-responsive bhuR transcription was not observed in mutant BRM23, indicating that hurI is required for positive regulation of bhu gene expression. Furthermore, bhuR was required for heme-inducible bhu gene activation, supporting the hypothesis that positive regulation of bhuRSTUV occurs by a surface signaling mechanism involving the heme-iron receptor BhuR.

BhuR, a virulence-associated outer membrane protein of Bordetella avium, is required for the acquisition of iron from heme and hemoproteins

Iron (Fe) is an essential element for most organisms which must be obtained from the local environment. In the case of pathogenic bacteria, this fundamental element must be acquired from the fluids and tissues of the infected host. A variety of systems have evolved in bacteria for efficient acquisition of host-bound Fe. The gram-negative bacterium Bordetella avium, upon colonization of the avian upper respiratory tract, produces a disease in birds that has striking similarity to whooping cough, a disease caused by the obligate human pathogen Bordetella pertussis. We describe a B. avium Fe utilization locus comprised of bhuR and six accessory genes (rhuIR and bhuSTUV). Genetic manipulations of B. avium confirmed that bhuR, which encodes a putative outer membrane heme receptor, mediates efficient acquisition of Fe from hemin and hemoproteins (hemoglobin, myoglobin, and catalase). BhuR contains motifs which are common to bacterial heme receptors, including a consensus FRAP domain, an NPNL domain, and two TonB boxes. An N-terminal 32-amino-acid segment, putatively required for rhuIR-dependent regulated expression of bhuR, is present in BhuR but not in other bacterial heme receptors. Two forms of BhuR were observed in the outer membrane of B. avium: a 91-kDa polypeptide consistent in size with the predicted mature protein and a smaller 82-kDa polypeptide which lacks the 104 amino acids found at the N terminus of the 91-kDa form. A mutation in hemA was engineered in B. avium to demonstrate that the bacterium transports heme into the cytoplasm in a BhuR-dependent manner. The role of BhuR in virulence was established in turkey poults by use of a competitive-infection model.

Functional interaction of region 4 of the extracytoplasmic function sigma factor FecI with the cytoplasmic portion of the FecR transmembrane protein of the Escherichia coli ferric citrate transport system

Transcriptional regulation of the ferric citrate transport genes of Escherichia coli is initiated by the binding of ferric citrate to the outer membrane protein FecA. This binding elicits a signal that is transmitted by FecR across the cytoplasmic membrane into the cytoplasm, where the sigma factor FecI directs the RNA polymerase to the promoter upstream of the fecABCDE genes. An in vivo deletion analysis using a bacterial two-hybrid system assigned the interaction of the FecR and FecI proteins to the cytoplasmic portion of the FecR transmembrane protein and region 4 of FecI. Missense mutations randomly generated by PCR were localized to region 4 of FecI, and the mutants were impaired with regard to the interaction of FecR with FecI and fecB-lacZ transcription. The cloned region 4 of FecI interfered with fecB-lacZ transcription. Interaction of N-proximal regions of predicted FecR homologs with region 4 of predicted FecI homologs of Pseudomonas aeruginosa was demonstrated. The interaction was specific in that only cognate protein pairs interacted with each other; no interactions occurred between heterologous combinations of the P. aeruginosa proteins and between a P. aeruginosa FecI homolog and E. coli FecR. The results demonstrate that region 4 of FecI specifically binds FecR and that this binding is necessary for FecI to function as a sigma factor.