Regulation of the Yersinia pestis Yfe and Ybt iron transport systems

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Oral vaccination with live attenuated Yersinia pseudotuberculosis strains delivering a FliC180-LcrV fusion antigen confers protection against pulmonary Y. Pestis infection

We incorporated the ΔPfur::TT araC PBADfur deletion-insertion mutation on top of a previous Yersinia pseudotuberculosis mutant (Δasd ΔyopJ ΔyopK) to construct a new mutant designated as Yptb5, which manifests the arabinose-dependent regulated delayed fur (encoding ferric uptake regulator) shut-off. The Yptb5 strain was used to deliver an adjuvanted fusion protein, FliC180-LcrV. Levels of FliC180-LcrV synthesis were same in Yptb5 either harboring pSMV4, a p15A ori plasmid or pSMV8, a pSC101 ori plasmid containing the fliC180-lcrV fusion gene driven by Ptrc promoter. Tissue burdens of both Yptb5(pSMV4) and Yptb5(pSMV8) in mice had similar patterns. Mice vaccinated orally with 5 × 108 CFU of either Yptb5(pSMV4) or Yptb5(pSMV8) strain were primed high antibody titers with a balanced Th1/Th2 response, also developed potent T-cell responses with significant productions of IFN-γ, IL-17A and TNF-α. Immunization with each mutant strain conferred complete protection against pulmonary challenge with 5.5 × 103 CFU (55 LD50) of Y. pestis, but partial protection (50% survival) against 100 LD50 of Y. pestis. Our results demonstrate that arabinose-dependent regulated delayed fur shut-off is an effective strategy to develop live attenuated bacterial vaccines while retaining strong immunogenicity.

Zinc transporters YbtX and ZnuABC are required for the virulence of Yersinia pestis in bubonic and pneumonic plague in mice

A number of bacterial pathogens require the ZnuABC Zinc (Zn²⁺) transporter and/or a second Zn²⁺ transport system to overcome Zn²⁺ sequestration by mammalian hosts. Previously we have shown that in addition to ZnuABC, Yersinia pestis possesses a second Zn²⁺ transporter that involves components of the yersiniabactin (Ybt), siderophore-dependent iron transport system. Synthesis of the Ybt siderophore and YbtX, a member of the major facilitator superfamily, are both critical components of the second Zn²⁺ transport system. Here we demonstrate that a ybtX znu double mutant is essentially avirulent in mouse models of bubonic and pneumonic plague while a ybtX mutant retains high virulence in both plague models. While sequestration of host Zn is a key nutritional immunity factor, excess Zn appears to have a significant antimicrobial role in controlling intracellular bacterial survival. Here, we demonstrate that ZntA, a Zn²⁺ exporter, plays a role in resistance to Zn toxicity in vitro, but that a zntA zur double mutant retains high virulence in both pneumonic and bubonic plague models and survival in macrophages. We also confirm that Ybt does not directly bind Zn²⁺in vitro under the conditions tested. However, we detect a significant increase in Zn²⁺-binding ability of filtered supernatants from a Ybt⁺ strain compared to those from a strain unable to produce the siderophore, supporting our previously published data that Ybt biosynthetic genes are involved in the production of a secreted Zn-binding molecule (zincophore). Our data suggest that Ybt or a modified Ybt participate in or promote Zn-binding activity in culture supernatants and is involved in Zn acquisition in Y. pestis.

Strain-specific impact of the high-pathogenicity island on virulence in extra-intestinal pathogenic Escherichia coli

In order to clarify the role of the high-pathogenicity island (HPI) in the experimental virulence of Escherichia coli, we constructed different deletion mutants of the entire HPI and of three individual genes (irp2, fyuA and ybtA), encoding for three main functions within the HPI. Those mutants were constructed for three phylogroup B2 strains (536-STc127, CFT073-STc73, and NU14-STc95), representative of the main B2 subgroups causing extra-intestinal infections. Transcriptional profiles obtained for the selected HPI genes irp2, fyuA and ybtA revealed similar patterns for all strains, both under selective iron-deplete conditions and in intracellular bacterial communities in vitro, with a high expression of irp2. Deletion of irp2 and ybtA abrogated yersiniabactin production, whereas the fyuA knockout was only slightly impaired for siderophore synthesis. The experimental virulence of the strains was then tested in amoeba Dictyostelium discoideum and mouse septicaemia models. No effect of any HPI mutant was observed for the two more virulent strains 536 and CFT073. In contrast, the virulence of the less virulent NU14 strain was dramatically diminished by the complete deletion of the HPI and irp2 gene whereas a lesser reduction in virulence was observed for the fyuA and ybtA deletion mutants. The two experimental virulence models gave similar results. It appears that the role of the HPI in experimental virulence is depending on the genetic background of the strains despite similar inter-strain transcriptional patterns of HPI genes, as well as of the functional class of the studied gene. Altogether, these data indicate that the intrinsic extra-intestinal virulence in the E. coli species is multigenic, with epistatic interactions between the genes.

The role of transition metal transporters for iron, zinc, manganese, and copper in the pathogenesis of Yersinia pestis

Yersinia pestis, the causative agent of bubonic, septicemic and pneumonic plague, encodes a multitude of Fe transport systems. Some of these are defective due to frameshift or IS element insertions, while others are functional in vitro but have no established role in causing infections. Indeed only 3 Fe transporters (Ybt, Yfe and Feo) have been shown to be important in at least one form of plague. The yersiniabactin (Ybt) system is essential in the early dermal/lymphatic stages of bubonic plague, irrelevant in the septicemic stage, and critical in pneumonic plague. Two Mn transporters have been characterized (Yfe and MntH). These two systems play a role in bubonic plague but the double yfe mntH mutant is fully virulent in a mouse model of pneumonic plague. The same in vivo phenotype occurs with a mutant lacking two (Yfe and Feo) of four ferrous transporters. A role for the Ybt siderophore in Zn acquisition has been revealed. Ybt-dependent Zn acquisition uses a transport system completely independent of the Fe-Ybt uptake system. Together Ybt components and ZnuABC play a critical role in Zn acquisition in vivo. Single mutants in either system retain high virulence in a mouse model of septicemic plague while the double mutant is completely avirulent.

The Yersinia pestis siderophore, yersiniabactin, and the ZnuABC system both contribute to zinc acquisition and the development of lethal septicaemic plague in mice

Bacterial pathogens must overcome host sequestration of zinc (Zn(2+) ), an essential micronutrient, during the infectious disease process. While the mechanisms to acquire chelated Zn(2+) by bacteria are largely undefined, many pathogens rely upon the ZnuABC family of ABC transporters. Here we show that in Yersinia pestis, irp2, a gene encoding the synthetase (HMWP2) for the siderophore yersiniabactin (Ybt) is required for growth under Zn(2+) -deficient conditions in a strain lacking ZnuABC. Moreover, growth stimulation with exogenous, purified apo-Ybt provides evidence that Ybt may serve as a zincophore for Zn(2+) acquisition. Studies with the Zn(2+) -dependent transcriptional reporter znuA::lacZ indicate that the ability to synthesize Ybt affects the levels of intracellular Zn(2+) . However, the outer membrane receptor Psn and TonB as well as the inner membrane (IM) ABC transporter YbtPQ, which are required for Fe(3+) acquisition by Ybt, are not needed for Ybt-dependent Zn(2+) uptake. In contrast, the predicted IM protein YbtX, a member of the Major Facilitator Superfamily, was essential for Ybt-dependent Zn(2+) uptake. Finally, we show that the ZnuABC system and the Ybt synthetase HMWP2, presumably by Ybt synthesis, both contribute to the development of a lethal infection in a septicaemic plague mouse model.

The Yfe and Feo transporters are involved in microaerobic growth and virulence of Yersinia pestis in bubonic plague

The Yfe/Sit and Feo transport systems are important for the growth of a variety of bacteria. In Yersinia pestis, single mutations in either yfe or feo result in reduced growth under static (limited aeration), iron-chelated conditions, while a yfe feo double mutant has a more severe growth defect. These growth defects were not observed when bacteria were grown under aerobic conditions or in strains capable of producing the siderophore yersiniabactin (Ybt) and the putative ferrous transporter FetMP. Both fetP and a downstream locus (flp for fet linked phenotype) were required for growth of a yfe feo ybt mutant under static, iron-limiting conditions. An feoB mutation alone had no effect on the virulence of Y. pestis in either bubonic or pneumonic plague models. An feo yfe double mutant was still fully virulent in a pneumonic plague model but had an ∼90-fold increase in the 50% lethal dose (LD(50)) relative to the Yfe(+) Feo(+) parent strain in a bubonic plague model. Thus, Yfe and Feo, in addition to Ybt, play an important role in the progression of bubonic plague. Finally, we examined the factors affecting the expression of the feo operon in Y. pestis. Under static growth conditions, the Y. pestis feo::lacZ fusion was repressed by iron in a Fur-dependent manner but not in cells grown aerobically. Mutations in feoC, fnr, arcA, oxyR, or rstAB had no significant effect on transcription of the Y. pestis feo promoter. Thus, the factor(s) that prevents repression by Fur under aerobic growth conditions remains to be identified.

Manganese transporters Yfe and MntH are Fur-regulated and important for the virulence of Yersinia pestis

Yersinia pestis has a flea-mammal-flea transmission cycle, and is a zoonotic pathogen that causes the systemic diseases bubonic and septicaemic plague in rodents and humans, as well as pneumonic plague in humans and non-human primates. Bubonic and pneumonic plague are quite different diseases that result from different routes of infection. Manganese (Mn) acquisition is critical for the growth and pathogenesis of a number of bacteria. The Yfe/Sit and/or MntH systems are the two prominent Mn transporters in Gram-negative bacteria. Previously we showed that the Y. pestis Yfe system transports Fe and Mn. Here we demonstrate that a mutation in yfe or mntH did not significantly affect in vitro aerobic growth under Mn-deficient conditions. A yfe mntH double mutant did exhibit a moderate growth defect which was alleviated by supplementation with Mn. No short-term energy-dependent uptake of (54)Mn was observed in this double mutant. Like the yfeA promoter, the mntH promoter was repressed by both Mn and Fe via Fur. Sequences upstream of the Fur binding sequence in the yfeA promoter converted an iron-repressible promoter to one that is also repressed by Mn and Fe. To our knowledge, this is the first report identifying cis promoter elements needed to alter cation specificities involved in transcriptional repression. Finally, the Y. pestis yfe mntH double mutant had an ~133-fold loss of virulence in a mouse model of bubonic plague but no virulence loss in the pneumonic plague model. This suggests that Mn availability, bacterial Mn requirements or Mn transporters used by Y. pestis are different in the lungs (pneumonic plague) compared with systemic disease.

Yersiniabactin iron uptake: mechanisms and role in Yersinia pestis pathogenesis

Yersiniabactin (Ybt) is a siderophore-dependent iron uptake system encoded on a pathogenicity island that is widespread among pathogenic bacteria including the Yersiniae. While biosynthesis of the siderophore has been elucidated, the secretion mechanism and a few components of the uptake/utilization pathway are unidentified. ybt genes are transcriptionally repressed by Fur but activated by YbtA, likely in combination with the siderophore itself. The Ybt system is essential for the ability of Yersinia pestis to cause bubonic plague and important in pneumonic plague as well. However, the ability to cause fatal septicemic plague is independent of Ybt.

Znu is the predominant zinc importer in Yersinia pestis during in vitro growth but is not essential for virulence

Little is known about Zn homeostasis in Yersinia pestis, the plague bacillus. The Znu ABC transporter is essential for zinc (Zn) uptake and virulence in a number of bacterial pathogens. Bioinformatics analysis identified ZnuABC as the only apparent high-affinity Zn uptake system in Y. pestis. Mutation of znuACB caused a growth defect in Chelex-100-treated PMH2 growth medium, which was alleviated by supplementation with submicromolar concentrations of Zn. Use of transcriptional reporters confirmed that Zur mediated Zn-dependent repression and that it can repress gene expression in response to Zn even in the absence of Znu. Virulence testing in mouse models of bubonic and pneumonic plague found only a modest increase in survival in low-dose infections by the znuACB mutant. Previous studies of cluster 9 (C9) transporters suggested that Yfe, a well-characterized C9 importer for manganese (Mn) and iron in Y. pestis, might function as a second, high-affinity Zn uptake system. Isothermal titration calorimetry revealed that YfeA, the solute-binding protein component of Yfe, binds Mn and Zn with comparably high affinities (dissociation constants of 17.8 ± 4.4 nM and 6.6 ± 1.2 nM, respectively), although the complete Yfe transporter could not compensate for the loss of Znu in in vitro growth studies. Unexpectedly, overexpression of Yfe interfered with the znu mutant's ability to grow in low concentrations of Zn, while excess Zn interfered with the ability of Yfe to import iron at low concentrations; these results suggest that YfeA can bind Zn in the bacterial cell but that Yfe is incompetent for transport of the metal. In addition to Yfe, we have now eliminated MntH, FetMP, Efe, Feo, a substrate-binding protein, and a putative nickel transporter as the unidentified, secondary Zn transporter in Y. pestis. Unlike other bacterial pathogens, Y. pestis does not require Znu for high-level infectivity and virulence; instead, it appears to possess a novel class of transporter, which can satisfy the bacterium's Zn requirements under in vivo metal-limiting conditions. Our studies also underscore the need for bacterial cells to balance binding and transporter specificities within the periplasm in order to maintain transition metal homeostasis.

Stereospecific recognition of pyochelin and enantio-pyochelin by the PchR proteins in fluorescent pseudomonads

The siderophore pyochelin of Pseudomonas aeruginosa promotes growth under iron limitation and induces the expression of its biosynthesis genes via the transcriptional AraC/XylS-type regulator PchR. Pseudomonas fluorescens strain CHA0 makes the optical antipode of pyochelin termed enantio-pyochelin, which also promotes growth and induces the expression of its biosynthesis genes when iron is scarce. Growth promotion and signalling by pyochelin and enantio-pyochelin are highly stereospecific and are known to involve the pyochelin and enantio-pyochelin outer-membrane receptors FptA and FetA, respectively. Here we show that stereospecificity in signalling is also based on the stereospecificity of the homologous PchR proteins of P. aeruginosa and P. fluorescens towards their respective siderophore effectors. We found that PchR functioned in the heterologous species only if supplied with its native ligand and that the FptA and FetA receptors enhanced the efficiency of signalling. By constructing and expressing hybrid and truncated PchR regulators we showed that the weakly conserved N-terminal domain of PchR is responsible for siderophore specificity. Thus, both uptake and transcriptional regulation confer stereospecificity to pyochelin and enantio-pyochelin biosynthesis.

The yersiniabactin transport system is critical for the pathogenesis of bubonic and pneumonic plague

Iron acquisition from the host is an important step in the pathogenic process. While Yersinia pestis has multiple iron transporters, the yersiniabactin (Ybt) siderophore-dependent system plays a major role in iron acquisition in vitro and in vivo. In this study, we determined that the Ybt system is required for the use of iron bound by transferrin and lactoferrin and examined the importance of the Ybt system for virulence in mouse models of bubonic and pneumonic plague. Y. pestis mutants unable to either transport Ybt or synthesize the siderophore were both essentially avirulent via subcutaneous injection (bubonic plague model). Surprisingly, via intranasal instillation (pneumonic plague model), we saw a difference in the virulence of Ybt biosynthetic and transport mutants. Ybt biosynthetic mutants displayed an approximately 24-fold-higher 50% lethal dose (LD(50)) than transport mutants. In contrast, under iron-restricted conditions in vitro, a Ybt transport mutant had a more severe growth defect than the Ybt biosynthetic mutant. Finally, a Delta pgm mutant had a greater loss of virulence than the Ybt biosynthetic mutant, indicating that the 102-kb pgm locus encodes a virulence factor, in addition to Ybt, that plays a role in the pathogenesis of pneumonic plague.

Reduced synthesis of the Ybt siderophore or production of aberrant Ybt-like molecules activates transcription of yersiniabactin genes in Yersinia pestis

Synthesis of the siderophore yersiniabactin (Ybt) proceeds by a mixed nonribosomal peptide synthetase/polyketide synthase mechanism. Transcription of ybt genes encoding biosynthetic and transport functions is repressed under excess iron conditions by Fur, but is also activated by Ybt via the transcriptional regulator YbtA. While mutations in most biosynthetic genes and ybtA negate transcription activation from the regulated promoters, three biosynthetic mutations do not reduce this transcriptional activation. Here we show that two of these mutants, one lacking the putative type II thioesterase (TE) YbtT and the other with a mutation in the TE domain of HMWP1, produce reduced levels of authentic Ybt that are capable of signalling activity. Alanine substitutions in two residues of YbtT that are essential for catalytic activity in other type II TEs reduced the ability of Yersinia pestis to grow under iron-chelated conditions. The third mutant, which lacks the salicylate synthase YbtS, did not make authentic Ybt but did produce a signalling molecule. Finally, a Δpgm strain of Y. pestis, which lacks essential Ybt biosynthetic genes, also produced a signalling molecule that can activate transcription of ybt genes. The non-Ybt signal molecules from these two mutants are likely separate compounds. While these compounds are not biologically relevant to normal Ybt regulation, a comparison of the structures of Ybt and other signalling molecules will help in determining the chemical structures recognized as a Ybt signal.

Proteomic analysis of iron acquisition, metabolic and regulatory responses of Yersinia pestis to iron starvation

Background

The Gram-negative bacterium Yersinia pestis is the causative agent of the bubonic plague. Efficient iron acquisition systems are critical to the ability of Y. pestis to infect, spread and grow in mammalian hosts, because iron is sequestered and is considered part of the innate host immune defence against invading pathogens. We used a proteomic approach to determine expression changes of iron uptake systems and intracellular consequences of iron deficiency in the Y. pestis strain KIM6+ at two physiologically relevant temperatures (26°C and 37°C).

Results

Differential protein display was performed for three Y. pestis subcellular fractions. Five characterized Y. pestis iron/siderophore acquisition systems (Ybt, Yfe, Yfu, Yiu and Hmu) and a putative iron/chelate outer membrane receptor (Y0850) were increased in abundance in iron-starved cells. The iron-sulfur (Fe-S) cluster assembly system Suf, adapted to oxidative stress and iron starvation in E. coli, was also more abundant, suggesting functional activity of Suf in Y. pestis under iron-limiting conditions. Metabolic and reactive oxygen-deactivating enzymes dependent on Fe-S clusters or other iron cofactors were decreased in abundance in iron-depleted cells. This data was consistent with lower activities of aconitase and catalase in iron-starved vs. iron-rich cells. In contrast, pyruvate oxidase B which metabolizes pyruvate via electron transfer to ubiquinone-8 for direct utilization in the respiratory chain was strongly increased in abundance and activity in iron-depleted cells.

Conclusions

Many protein abundance differences were indicative of the important regulatory role of the ferric uptake regulator Fur. Iron deficiency seems to result in a coordinated shift from iron-utilizing to iron-independent biochemical pathways in the cytoplasm of Y. pestis. With growth temperature as an additional variable in proteomic comparisons of the Y. pestis fractions (26°C and 37°C), there was little evidence for temperature-specific adaptation processes to iron starvation.

The Bordetella bfe system: growth and transcriptional response to siderophores, catechols, and neuroendocrine catecholamines

Ferric enterobactin utilization by Bordetella bronchiseptica and Bordetella pertussis requires the BfeA outer membrane receptor. Under iron-depleted growth conditions, transcription of bfeA is activated by the BfeR regulator by a mechanism requiring the siderophore enterobactin. In this study, enterobactin-inducible bfeA transcription was shown to be TonB independent. To determine whether other siderophores or nonsiderophore catechols could be utilized by the Bfe system, various compounds were tested for the abilities to promote the growth of iron-starved B. bronchiseptica and induce bfeA transcription. The BfeA receptor transported ferric salmochelin, corynebactin, and the synthetic siderophores TRENCAM and MECAM. Salmochelin and MECAM induced bfeA transcription in iron-starved Bordetella cells, but induction by corynebactin and TRENCAM was minimal. The neuroendocrine catecholamines epinephrine, norepinephrine, and dopamine exhibited a remarkable capacity to induce transcription of bfeA. Norepinephrine treatment of B. bronchiseptica resulted in BfeR-dependent bfeA transcription, elevated BfeA receptor production, and growth stimulation. Pyrocatechol, carbidopa, and isoproterenol were similarly strong inducers of bfeA transcription, whereas tyramine and 3,4-dihydroxymandelic acid demonstrated low inducing activity. The results indicate that the inducer structure requires a catechol group for function and that the ability to induce bfeA transcription does not necessarily correlate with the ability to stimulate bacterial growth. The expanded range of catechol siderophores transported by the BfeA receptor demonstrates the potential versatility of the Bordetella Bfe iron retrieval system. The finding that catecholamine neurotransmitters activate bfeA transcription and promote growth suggests that Bordetella cells can perceive and may benefit from neuroendocrine catecholamines on the respiratory epithelium.

The BfeR regulator mediates enterobactin-inducible expression of Bordetella enterobactin utilization genes

Utilization of the enterobactin siderophore by the respiratory pathogens Bordetella pertussis and Bordetella bronchiseptica is dependent on the BfeA outer membrane receptor. This study determined that production of BfeA was increased significantly in iron-starved bacteria upon supplementation of cultures with enterobactin. A 1.01-kb open reading frame, designated bfeR, encoding a predicted positive transcriptional regulator of the AraC family was identified upstream and divergently oriented from bfeA. In iron-depleted cultures containing enterobactin, a Bordetella bfeR mutant exhibited markedly decreased BfeA receptor production compared to that of the wild-type strain. Additionally, B. pertussis and B. bronchiseptica bfeR mutants exhibited impaired growth with ferric enterobactin as the sole source of iron, demonstrating that effective enterobactin utilization is bfeR dependent. Transcriptional analysis using bfeA-lacZ reporter fusions in wild-type strains demonstrated that bfeA transcription was stimulated in iron-depleted conditions in the presence of enterobactin, compared to modest expression levels in cultures lacking enterobactin. In contrast, bfeA transcription in B. pertussis and B. bronchiseptica bfeR mutants was completely unresponsive to the enterobactin inducer. bfeA transcriptional analyses of a bfeA mutant demonstrated that induction by enterobactin did not require BfeA receptor-mediated uptake of the siderophore. These studies establish that bfeR encodes an enterobactin-dependent positive regulator of bfeA transcription in these Bordetella species.