The cytochrome bd oxidase of Escherichia coli prevents respiratory inhibition by endogenous and exogenous hydrogen sulfide

The function, biogenesis and regulation of the electron transport chains in Campylobacter jejuni: New insights into the bioenergetics of a major food-borne pathogen

Campylobacter jejuni is a zoonotic Epsilonproteobacterium that grows in the gastrointestinal tract of birds and mammals, and is the most frequent cause of food-borne bacterial gastroenteritis worldwide. As an oxygen-sensitive microaerophile, C. jejuni has to survive high environmental oxygen tensions, adapt to oxygen limitation in the host intestine and resist host oxidative attack. Despite its small genome size, C. jejuni is a versatile and metabolically active pathogen, with a complex and highly branched set of respiratory chains allowing the use of a wide range of electron donors and alternative electron acceptors in addition to oxygen, including fumarate, nitrate, nitrite, tetrathionate and N- or S-oxides. Several novel enzymes participate in these electron transport chains, including a tungsten containing formate dehydrogenase, a Complex I that uses flavodoxin and not NADH, a periplasmic facing fumarate reductase and a cytochrome c tetrathionate reductase. This review presents an updated description of the composition and bioenergetics of these various respiratory chains as they are currently understood, including recent work that gives new insights into energy conservation during electron transport to various alternative electron acceptors. The regulation of synthesis and assembly of the electron transport chains is also discussed. A deeper appreciation of the unique features of the respiratory systems of C. jejuni may be helpful in informing strategies to control this important pathogen.

The functional diversity of the prokaryotic sulfur carrier protein TusA

Persulfide groups participate in a wide array of biochemical pathways and are chemically very versatile. The TusA protein has been identified as a central element supplying and transferring sulfur as persulfide to a number of important biosynthetic pathways, like molybdenum cofactor biosynthesis or thiomodifications in nucleosides of tRNAs. In recent years, it has furthermore become obvious that this protein is indispensable for the oxidation of sulfur compounds in the cytoplasm. Phylogenetic analyses revealed that different TusA protein variants exists in certain organisms, that have evolved to pursue specific roles in cellular pathways. The specific TusA-like proteins thereby cannot replace each other in their specific roles and are rather specific to one sulfur transfer pathway or shared between two pathways. While certain bacteria like Escherichia coli contain several copies of TusA-like proteins, in other bacteria like Allochromatium vinosum a single copy of TusA is present with an essential role for this organism. Here, we give an overview on the multiple roles of the various TusA-like proteins in sulfur transfer pathways in different organisms to shed light on the remaining mysteries of this versatile protein.

The Cytochrome bd Complex Is Essential for Chromate and Sulfide Resistance and Is Regulated by a GbsR-Type Regulator, CydE, in Alishewanella Sp. WH16-1

Sulfate-reducing bacteria are a group of microorganisms that use sulfate as an electron acceptor. These bacteria are useful in the bioremediation of heavy metal pollution since they can reduce/precipitate metals. Previously, we identified the Alishewanella strain WH16-1 from soil of a copper and iron mine and determined that it can reduce sulfate and chromate and that it was tolerant to many heavy metals. In this study, we investigated the chromate reduction mechanism of strain WH16-1 through Tn5 transposon mutagenesis. A cytochrome bd (cytbd) Tn5 mutant was generated (Δcytbd), and a detail analysis showed that the following: (1) gene cydE (coding for a GbsR-type regulator) was co-transcribed with the two subunits coding genes of the Cytochrome bd complex (Cytbd), namely, cydA and cydB, based on RT-PCR analysis, and similar gene arrangements were also found in other Alteromonadaceae family strains; (2) the chromate resistance level was dramatically decreased and chromate reduction efficiency also decreased in strain Δcytbd compared to the wild-type and a complemented strain (Δcytbd-C); (3) Cytbd could catalyze the decomposition of H₂O₂ according to the analyses of H₂O₂ decomposition ability, cellular H₂O₂ contents, H₂O₂ inhibition zone, and H₂O₂ sensitivity tests; (4) surprisingly, chromate was not an inducer of the expression of Cytbd, but sulfate induced expression of Cytbd, and sulfate/sulfide resistance levels were also decreased in the Δcytbd strain; (5) the addition of sulfate enhanced the chromate resistance level and reduction efficiency; (6) Cytbd expression was repressed by CydE and derepressed by sulfate based on an in vivo bacterial one hybrid system and in vitro EMSA tests; and (7) DNA footprinting and short-fragment EMSA tests revealed two binding sites of CydE in its promoter region. All these results showed that Cytbd is negatively regulated by CydE and derepressed by sulfate. In addition, Cytbd contributes to the resistance of sulfate and sulfide, and sulfide could be used as a reductant to reduce chromate. Moreover, Cytbd is essential to decompose H₂O₂ to decrease cellular oxidative stress. Thus, the regulation and function of Cytbd may explain why sulfate could enhance chromate reduction.

Hydrogen Sulfide Sensing through Reactive Sulfur Species (RSS) and Nitroxyl (HNO) in Enterococcus faecalis

Recent studies of hydrogen sulfide (H2S) signaling implicate low molecular weight (LMW) thiol persulfides and other reactive sulfur species (RSS) as signaling effectors. Here, we show that a CstR protein from the human pathogen Enterococcus faecalis ( E. faecalis), previously identified in Staphylococcus aureus ( S. aureus), is an RSS-sensing repressor that transcriptionally regulates a cst-like operon in response to both exogenous sulfide stress and Angeli's salt, a precursor of nitroxyl (HNO). E. faecalis CstR reacts with coenzyme A persulfide (CoASSH) to form interprotomer disulfide and trisulfide bridges between C32 and C61', which negatively regulate DNA binding to a consensus CstR DNA operator. A Δ cstR strain exhibits deficiency in catheter colonization in a catheter-associated urinary tract infection (CAUTI) mouse model, suggesting sulfide regulation and homeostasis is critical for pathogenicity. Cellular polysulfide metabolite profiling of sodium sulfide-stressed E. faecalis confirms an increase in both inorganic polysulfides and LMW thiols and persulfides sensed by CstR. The cst-like operon encodes two authentic thiosulfate sulfurtransferases and an enzyme we characterize here as an NADH and FAD-dependent coenzyme A (CoA) persulfide reductase (CoAPR) that harbors an N-terminal CoA disulfide reductase (CDR) domain and a C-terminal rhodanese homology domain (RHD). Both cysteines in the CDR (C42) and RHD (C508) domains are required for CoAPR activity and complementation of a sulfide-induced growth phenotype of a S. aureus strain lacking cstB, encoding a nonheme FeII persulfide dioxygenase. We propose that S. aureus CstB and E. faecalis CoAPR employ orthogonal chemistries to lower CoASSH that accumulates under conditions of cellular sulfide toxicity and signaling.

Taxonomic distribution, structure/function relationship and metabolic context of the two families of sulfide dehydrogenases: SQR and FCSD

Hydrogen sulfide (H₂S) is a versatile molecule with different functions in living organisms: it can work as a metabolite of sulfur and energetic metabolism or as a signaling molecule in higher Eukaryotes. H₂S is also highly toxic since it is able to inhibit heme cooper oxygen reductases, preventing oxidative phosphorylation. Due to the fact that it can both inhibit and feed the respiratory chain, the immediate role of H₂S on energy metabolism crucially relies on its bioavailability, meaning that studying the central players involved in the H₂S homeostasis is key for understanding sulfide metabolism. Two different enzymes with sulfide oxidation activity (sulfide dehydrogenases) are known: flavocytochrome c sulfide dehydrogenase (FCSD), a sulfide:cytochrome c oxidoreductase; and sulfide:quinone oxidoreductase (SQR). In this work we performed a thorough bioinformatic study of SQRs and FCSDs and integrated all published data. We systematized several properties of these proteins: (i) nature of flavin binding, (ii) capping loops and (iii) presence of key amino acid residues. We also propose an update to the SQR classification system and discuss the role of these proteins in sulfur metabolism.

Quantification of Hydrogen Sulfide and Cysteine Excreted by Bacterial Cells

Bacteria release cysteine to moderate the size of their intracellular pools. They can also evolve hydrogen sulfide, either through dissimilatory reduction of oxidized forms of sulfur or through the deliberate or inadvertent degradation of intracellular cysteine. These processes can have important consequences upon microbial communities, because excreted cysteine autoxidizes to generate hydrogen peroxide, and hydrogen sulfide is a potentially toxic species that can block aerobic respiration by inhibiting cytochrome oxidases. Lead acetate strips can be used to obtain semiquantitative data of sulfide evolution (Oguri et al., 2012). Here we describe methods that allow more-quantitative and discriminatory measures of cysteine and hydrogen sulfide release from bacterial cells. An illustrative example is provided in which Escherichia coli rapidly evolves both cysteine and sulfide upon exposure to exogenous cystine (Chonoles Imlay et al., 2015; Korshunov et al., 2016).

Hydrogen Sulfide and Reactive Sulfur Species Impact Proteome S-Sulfhydration and Global Virulence Regulation in Staphylococcus aureus

Hydrogen sulfide (H₂S) is thought to protect bacteria from oxidative stress, but a comprehensive understanding of its function in bacteria is largely unexplored. In this study, we show that the human pathogen Staphylococcus aureus (S. aureus) harbors significant effector molecules of H₂S signaling, reactive sulfur species (RSS), as low molecular weight persulfides of bacillithiol, coenzyme A, and cysteine, and significant inorganic polysulfide species. We find that proteome S-sulfhydration, a post-translational modification (PTM) in H₂S signaling, is widespread in S. aureus. RSS levels modulate the expression of secreted virulence factors and the cytotoxicity of the secretome, consistent with an S-sulfhydration-dependent inhibition of DNA binding by MgrA, a global virulence regulator. Two previously uncharacterized thioredoxin-like proteins, denoted TrxP and TrxQ, are S-sulfhydrated in sulfide-stressed cells and are capable of reducing protein hydrodisulfides, suggesting that this PTM is potentially regulatory in S. aureus. In conclusion, our results reveal that S. aureus harbors a pool of proteome- and metabolite-derived RSS capable of impacting protein activities and gene regulation and that H₂S signaling can be sensed by global regulators to affect the expression of virulence factors.

Sulfur Cycling and the Intestinal Microbiome

In this review, we focus on the activities transpiring in the anaerobic segment of the sulfur cycle occurring in the gut environment where hydrogen sulfide is produced. While sulfate-reducing bacteria are considered as the principal agents for hydrogen sulfide production, the enzymatic desulfhydration of cysteine by heterotrophic bacteria also contributes to production of hydrogen sulfide. For sulfate-reducing bacteria respiration, molecular hydrogen and lactate are suitable as electron donors while sulfate functions as the terminal electron acceptor. Dietary components provide fiber and macromolecules that are degraded by bacterial enzymes to monomers, and these are fermented by intestinal bacteria with the production to molecular hydrogen which promotes the metabolic dominance by sulfate-reducing bacteria. Sulfate is also required by the sulfate-reducing bacteria, and this can be supplied by sulfate- and sulfonate-containing compounds that are hydrolyzed by intestinal bacterial with the release of sulfate. While hydrogen sulfide in the intestinal biosystem may be beneficial to bacteria by increasing resistance to antibiotics, and protecting them from reactive oxygen species, hydrogen sulfide at elevated concentrations may become toxic to the host.

Nitrite Derived from Endogenous Bacterial Nitric Oxide Synthase Activity Promotes Aerobic Respiration

Macrophage-derived nitric oxide (NO·) is a crucial effector against invading pathogens. Yet, paradoxically, several bacterial species, including some pathogens, are known to endogenously produce NO· via nitric oxide synthase (NOS) activity, despite its apparent cytotoxicity. Here, we reveal a conserved role for bacterial NOS in activating aerobic respiration. We demonstrate that nitrite generated from endogenous NO· decomposition stimulates quinol oxidase activity in Staphylococcus aureus and increases the rate of cellular respiration. This not only supports optimal growth of this organism but also prevents a dysbalance in central metabolism. Further, we also show that activity of the SrrAB two-component system alleviates the physiological defects of the nos mutant. Our findings suggest that NOS and SrrAB constitute two distinct but functionally redundant routes for controlling staphylococcal respiration during aerobic growth.

Despite its potential autotoxic effects, several bacterial species, including pathogenic staphylococcal species, produce NO· endogenously through nitric oxide synthase (NOS) activity. Therefore, how endogenous NO· influences bacterial fitness remains unclear. Here we show that the oxidation of NO· to nitrite increases aerobic respiration and consequently optimizes central metabolism to favor growth. Importantly, we also demonstrate that cells have a “fail-safe” mechanism that can maintain respiratory activity through the SrrAB two-component signaling regulon should NOS-derived nitrite levels decrease. These findings identify NOS and SrrAB as critical determinants of staphylococcal respiratory control and highlight their potential as therapeutic targets.

Anaerobic Cysteine Degradation and Potential Metabolic Coordination in Salmonella enterica and Escherichia coli

Salmonella enterica has two CyuR-activated enzymes that degrade cysteine, i.e., the aerobic CdsH and an unidentified anaerobic enzyme; Escherichia coli has only the latter. To identify the anaerobic enzyme, transcript profiling was performed for E. coli without cyuR and with overexpressed cyuR Thirty-seven genes showed at least 5-fold changes in expression, and the cyuPA (formerly yhaOM) operon showed the greatest difference. Homology suggested that CyuP and CyuA represent a cysteine transporter and an iron-sulfur-containing cysteine desulfidase, respectively. E. coli and S. enterica ΔcyuA mutants grown with cysteine generated substantially less sulfide and had lower growth yields. Oxygen affected the CyuR-dependent genes reciprocally; cyuP-lacZ expression was greater anaerobically, whereas cdsH-lacZ expression was greater aerobically. In E. coli and S. enterica, anaerobic cyuP expression required cyuR and cysteine and was induced by l-cysteine, d-cysteine, and a few sulfur-containing compounds. Loss of either CyuA or RidA, both of which contribute to cysteine degradation to pyruvate, increased cyuP-lacZ expression, which suggests that CyuA modulates intracellular cysteine concentrations. Phylogenetic analysis showed that CyuA homologs are present in obligate and facultative anaerobes, confirming an anaerobic function, and in archaeal methanogens and bacterial acetogens, suggesting an ancient origin. Our results show that CyuA is the major anaerobic cysteine-catabolizing enzyme in both E. coli and S. enterica, and it is proposed that anaerobic cysteine catabolism can contribute to coordination of sulfur assimilation and amino acid synthesis.IMPORTANCE Sulfur-containing compounds such as cysteine and sulfide are essential and reactive metabolites. Exogenous sulfur-containing compounds can alter the thiol landscape and intracellular redox reactions and are known to affect several cellular processes, including swarming motility, antibiotic sensitivity, and biofilm formation. Cysteine inhibits several enzymes of amino acid synthesis; therefore, increasing cysteine concentrations could increase the levels of the inhibited enzymes. This inhibition implies that control of intracellular cysteine levels, which is the immediate product of sulfide assimilation, can affect several pathways and coordinate metabolism. For these and other reasons, cysteine and sulfide concentrations must be controlled, and this work shows that cysteine catabolism contributes to this control.

Escherichia coli cytochrome c peroxidase is a respiratory oxidase that enables the use of hydrogen peroxide as a terminal electron acceptor

Microbial cytochrome c peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Escherichia coli Intriguingly, ccp transcription requires both the presence of H₂O₂ and the absence of O₂ Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous H₂O₂ However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors. Indeed, Ccp enabled E. coli to grow on a nonfermentable carbon source when H₂O₂ was supplied. Salmonella behaved similarly. This role rationalizes ccp repression in oxic environments. We speculate that micromolar H₂O₂ is created both biologically and abiotically at natural oxic/anoxic interfaces. The OxyR response appears to exploit this H₂O₂ as a terminal oxidant while simultaneously defending the cell against its toxicity.

Cytochrome bd and Gaseous Ligands in Bacterial Physiology

Cytochrome bd is a unique prokaryotic respiratory terminal oxidase that does not belong to the extensively investigated family of haem-copper oxidases (HCOs). The enzyme catalyses the four-electron reduction of O₂ to 2H₂O, using quinols as physiological reducing substrates. The reaction is electrogenic and cytochrome bd therefore sustains bacterial energy metabolism by contributing to maintain the transmembrane proton motive force required for ATP synthesis. As compared to HCOs, cytochrome bd displays several distinctive features in terms of (i) metal composition (it lacks Cu and harbours a d-type haem in addition to two haems b), (ii) overall three-dimensional structure, that only recently has been solved, and arrangement of the redox cofactors, (iii) lesser energetic efficiency (it is not a proton pump), (iv) higher O₂ affinity, (v) higher resistance to inhibitors such as cyanide, nitric oxide (NO) and hydrogen sulphide (H₂S) and (vi) ability to efficiently metabolize potentially toxic reactive oxygen and nitrogen species like hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^-). Compelling evidence suggests that, beyond its bioenergetic role, cytochrome bd plays multiple functions in bacterial physiology and affords protection against oxidative and nitrosative stress. Relevant to human pathophysiology, thanks to its peculiar properties, the enzyme has been shown to promote virulence in several bacterial pathogens, being currently recognized as a target for the development of new antibiotics. This review aims to give an update on our current understanding of bd-type oxidases with a focus on their reactivity with gaseous ligands and its potential impact on bacterial physiology and human pathophysiology.

"On demand" redox buffering by H2S contributes to antibiotic resistance revealed by a bacteria-specific H2S donor

Understanding the mechanisms of antimicrobial resistance (AMR) will help launch a counter-offensive against human pathogens that threaten our ability to effectively treat common infections. Herein, we report bis(4-nitrobenzyl)sulfanes, which are activated by a bacterial enzyme to produce hydrogen sulfide (H2S) gas. We found that H2S helps maintain redox homeostasis and protects bacteria against antibiotic-triggered oxidative stress "on demand", through activation of alternate respiratory oxidases and cellular antioxidants. We discovered, a hitherto unknown role for this gas, that chemical inhibition of H2S biosynthesis reversed antibiotic resistance in multidrug-resistant (MDR) uropathogenic Escherichia coli strains of clinical origin, whereas exposure to the H2S donor restored drug tolerance. Together, our study provides a greater insight into the dynamic defence mechanisms of this gas, modes of antibiotic action as well as resistance while progressing towards new pharmacological targets to address AMR.

Metal homeostasis and resistance in bacteria

Metal ions are essential for many reactions, but excess metals can be toxic. In bacteria, metal limitation activates pathways that are involved in the import and mobilization of metals, whereas excess metals induce efflux and storage. In this Review, we highlight recent insights into metal homeostasis, including protein-based and RNA-based sensors that interact directly with metals or metal-containing cofactors. The resulting transcriptional response to metal stress takes place in a stepwise manner and is reinforced by post-transcriptional regulatory systems. Metal limitation and intoxication by the host are evolutionarily ancient strategies for limiting bacterial growth. The details of the resulting growth restriction are beginning to be understood and seem to be organism-specific.

Physiology, anaerobes, and the origin of mitosing cells 50 years on

Endosymbiotic theory posits that some organelles or structures of eukaryotic cells stem from free-living prokaryotes that became endosymbionts within a host cell. Endosymbiosis has a long and turbulent history of controversy and debate going back over 100 years. The 1967 paper by Lynn Sagan (later Lynn Margulis) forced a reluctant field to take endosymbiotic theory seriously and to incorporate it into the fabric of evolutionary thinking. Margulis envisaged three cellular partners associating in series at eukaryotic origin: the host (an engulfing bacterium), the mitochondrion (a respiring bacterium), and the flagellum (a spirochaete), with lineages descended from that flagellated eukaryote subsequently acquiring plastids from cyanobacteria, but on multiple different occasions in her 1967 account. Today, the endosymbiotic origin of mitochondria and plastids (each single events, the data now say) is uncontested textbook knowledge. The host has been more elusive, recent findings identifying it as a member of the archaea, not as a sister group of the archaea. Margulis's proposal for a spirochaete origin of flagellae was abandoned by everyone except her, because no data ever came around to support the idea. Her 1967 proposal that mitochondria and plastids arose from different endosymbionts was novel. The paper presented an appealing narrative that linked the origin of mitochondria with oxygen in Earth history: cyanobacteria make oxygen, oxygen starts accumulating in the atmosphere about 2.4 billion years ago, oxygen begets oxygen-respiring bacteria that become mitochondria via symbiosis, followed by later (numerous) multiple, independent symbioses involving cyanobacteria that brought photosynthesis to eukaryotes. With the focus on oxygen, Margulis's account of eukaryote origin was however unprepared to accommodate the discovery of mitochondria in eukaryotic anaerobes. Today's oxygen narrative has it that the oceans were anoxic up until about 580 million years ago, while the atmosphere attained modern oxygen levels only about 400 million years ago. Since eukaryotes are roughly 1.6 billion years old, much of eukaryotic evolution took place in low oxygen environments, readily explaining the persistence across eukaryotic supergroups of eukaryotic anaerobes and anaerobic mitochondria at the focus of endosymbiotic theories that came after the 1967 paper.

Hydrogen sulfide - cysteine cycle system enhances cadmium tolerance through alleviating cadmium-induced oxidative stress and ion toxicity in Arabidopsis roots

Cadmium (Cd²⁺) is a common toxic heavy metal ion. We investigated the roles of hydrogen sulfide (H₂S) and cysteine (Cys) in plant responses to Cd²⁺ stress. The expression of H₂S synthetic genes LCD and DES1 were induced by Cd²⁺ within 3 h, and endogenous H₂S was then rapidly released. H₂S promoted the expression of Cys synthesis-related genes SAT1 and OASA1, which led to endogenous Cys accumulation. The H₂S and Cys cycle system was stimulated by Cd²⁺ stress, and it maintained high levels in plant cells. H₂S inhibited the ROS burst by inducing alternative respiration capacity (AP) and antioxidase activity. H₂S weakened Cd²⁺ toxicity by inducing the metallothionein (MTs) genes expression. Cys promoted GSH accumulation and inhibited the ROS burst, and GSH induced the expression of phytochelatin (PCs) genes, counteracting Cd²⁺ toxicity. In summary, the H₂S and Cys cycle system played a key role in plant responses to Cd²⁺ stress. The Cd²⁺ tolerance was weakened when the cycle system was blocked in lcddes1-1 and oasa1 mutants. This paper is the first to describe the role of the H₂S and Cys cycle system in Cd²⁺ stress and to explore the relevant and specificity mechanisms of H₂S and Cys in mediating Cd²⁺ stress.

Cytochrome bd-Dependent Bioenergetics and Antinitrosative Defenses in Salmonella Pathogenesis

In the course of an infection, Salmonella enterica occupies diverse anatomical sites with various concentrations of oxygen (O₂) and nitric oxide (NO). These diatomic gases compete for binding to catalytic metal groups of quinol oxidases. Enterobacteriaceae express two evolutionarily distinct classes of quinol oxidases that differ in affinity for O₂ and NO as well as stoichiometry of H⁺ translocated across the cytoplasmic membrane. The investigations presented here show that the dual function of bacterial cytochrome bd in bioenergetics and antinitrosative defense enhances Salmonella virulence. The high affinity of cytochrome bd for O₂ optimizes respiratory rates in hypoxic cultures, and thus, this quinol oxidase maximizes bacterial growth under O₂-limiting conditions. Our investigations also indicate that cytochrome bd, rather than cytochrome bo, is an intrinsic component of the adaptive antinitrosative toolbox of Salmonella Accordingly, induction of cytochrome bd helps Salmonella grow and respire in the presence of inhibitory NO. The combined antinitrosative defenses of cytochrome bd and the flavohemoglobin Hmp account for a great part of the adaptations that help Salmonella recover from the antimicrobial activity of NO. Moreover, the antinitrosative defenses of cytochrome bd and flavohemoglobin Hmp synergize to promote Salmonella growth in systemic tissues. Collectively, our investigations indicate that cytochrome bd is a critical means by which Salmonella resists the nitrosative stress that is engendered in the innate response of mammalian hosts while it concomitantly allows for proper O₂ utilization in tissue hypoxia.

IMPORTANCE

It is becoming quite apparent that metabolism is critically important to the virulence potential of pathogenic microorganisms. Bacterial cells use a variety of terminal electron acceptors to power electron transport chains and metabolic processes. Of all the electron acceptors available to bacteria, utilization of O₂ yields the most energy while diversifying the type of substrates that a pathogen can use. Recent investigations have demonstrated important roles for bd-type quinol oxidases with high affinity for O₂ in bacterial pathogenesis. The investigations presented here have revealed that cytochrome bd potentiates virulence of a clinically relevant bacterial pathogen by fueling bioenergetics of prokaryotic cells while protecting the respiratory chain against NO toxicity. The adaptive antinitrosative defenses afforded by cytochrome bd synergize with other NO-detoxifying systems to preserve cellular bioenergetics, thereby promoting bacterial virulence in tissue hypoxia.

Intracellular Zn(II) Intoxication Leads to Dysregulation of the PerR Regulon Resulting in Heme Toxicity in Bacillus subtilis

Transition metal ions (Zn(II), Cu(II)/(I), Fe(III)/(II), Mn(II)) are essential for life and participate in a wide range of biological functions. Cellular Zn(II) levels must be high enough to ensure that it can perform its essential roles. Yet, since Zn(II) binds to ligands with high avidity, excess Zn(II) can lead to protein mismetallation. The major targets of mismetallation, and the underlying causes of Zn(II) intoxication, are not well understood. Here, we use a forward genetic selection to identify targets of Zn(II) toxicity. In wild-type cells, in which Zn(II) efflux prevents intoxication of the cytoplasm, extracellular Zn(II) inhibits the electron transport chain due to the inactivation of the major aerobic cytochrome oxidase. This toxicity can be ameliorated by depression of an alternate oxidase or by mutations that restrict access of Zn(II) to the cell surface. Conversely, efflux deficient cells are sensitive to low levels of Zn(II) that do not inhibit the respiratory chain. Under these conditions, intracellular Zn(II) accumulates and leads to heme toxicity. Heme accumulation results from dysregulation of the regulon controlled by PerR, a metal-dependent repressor of peroxide stress genes. When metallated with Fe(II) or Mn(II), PerR represses both heme biosynthesis (hemAXCDBL operon) and the abundant heme protein catalase (katA). Metallation of PerR with Zn(II) disrupts this coordination, resulting in depression of heme biosynthesis but continued repression of catalase. Our results support a model in which excess heme partitions to the membrane and undergoes redox cycling catalyzed by reduced menaquinone thereby resulting in oxidative stress.