Generating a Metal-responsive Transcriptional Regulator to Test What Confers Metal Sensing in Cells

The Dynamic Plasticity of P. aeruginosa CueR Copper Transcription Factor upon Cofactor and DNA Binding

Bacteria use specialized proteins, like transcription factors, to rapidly control metal ion balance. CueR is a Gram-negative bacterial copper regulator. The structure of E. coli CueR complexed with Cu(I) and DNA was published, since then many studies have shed light on its function. However, P. aeruginosa CueR, which shows high sequence similarity to E. coli CueR, has been less studied. Here, we applied room-temperature electron paramagnetic resonance (EPR) measurements to explore changes in dynamics of P. aeruginosa CueR in dependency of copper concentrations and interaction with two different DNA promoter regions. We showed that P. aeruginosa CueR is less dynamic than the E. coli CueR protein and exhibits much higher sensitivity to DNA binding as compared to its E. coli CueR homolog. Moreover, a difference in dynamical behavior was observed when P. aeruginosa CueR binds to the copZ2 DNA promoter sequence compared to the mexPQ-opmE promoter sequence. Such dynamical differences may affect the expression levels of CopZ2 and MexPQ-OpmE proteins in P. aeruginosa. Overall, such comparative measurements of protein-DNA complexes derived from different bacterial systems reveal insights about how structural and dynamical differences between two highly homologous proteins lead to quite different DNA sequence-recognition and mechanistic properties.

Activation of zinc uptake regulator by zinc binding to three regulatory sites

Zur is a Fur-family metalloregulator that is widely used to control zinc homeostasis in bacteria. In Streptomyces coelicolor, Zur (ScZur) acts as both a repressor for zinc uptake (znuA) gene and an activator for zinc exporter (zitB) gene. Previous structural studies revealed three zinc ions specifically bound per ScZur monomer; a structural one to allow dimeric architecture and two regulatory ones for DNA-binding activity. In this study, we present evidence that Zur contains a fourth specific zinc-binding site with a key histidine residue (H36), widely conserved among actinobacteria, for regulatory function. Biochemical, genetic, and calorimetric data revealed that H36 is critical for hexameric binding of Zur to the zitB zurbox and further binding to its upstream region required for full activation. A comprehensive thermodynamic model demonstrated that the DNA-binding affinity of Zur to both znuA and zitB zurboxes is remarkably enhanced upon saturation of all three regulatory zinc sites. The model also predicts that the strong coupling between zinc binding and DNA binding equilibria of Zur drives a biphasic activation of the zitB gene in response to a wide concentration change of zinc. Similar mechanisms may be pertinent to other metalloproteins, expanding their response spectrum through binding multiple regulatory metals.

Characterization of the Zinc Uptake Repressor (Zur) from Acinetobacter baumannii

Bacterial cells tightly regulate the intracellular concentrations of essential transition metal ions by deploying a panel of metal-regulated transcriptional repressors and activators that bind to operator-promoter regions upstream of regulated genes. Like other zinc uptake regulator (Zur) proteins, Acinetobacter baumannii Zur represses transcription of its regulon when ZnII is replete and binds more weakly to DNA when ZnII is limiting. Previous studies established that Zur proteins are homodimeric and harbor at least two metal sites per protomer or four per dimer. CdII X-ray absorption spectroscopy (XAS) of the Cd2Zn2 AbZur metalloderivative with CdII bound to the allosteric sites reveals a S(N/O)3 first coordination shell. Site-directed mutagenesis suggests that H89 and C100 from the N-terminal DNA binding domain and H107 and E122 from the C-terminal dimerization domain comprise the regulatory metal site. KZn for this allosteric site is 6.0 (±2.2) × 1012 M-1 with a functional "division of labor" among the four metal ligands. N-terminal domain ligands H89 and C100 contribute far more to KZn than H107 and E122, while C100S AbZur uniquely fails to bind to DNA tightly as measured by an in vitro transcription assay. The heterotropic allosteric coupling free energy, ΔGc, is negative, consistent with a higher KZn for the AbZur-DNA complex and defining a bioavailable ZnII set-point of ≈6 × 10-14 M. Small-angle X-ray scattering (SAXS) experiments reveal that only the wild-type Zn homodimer undergoes allosteric switching, while the C100S AbZur fails to switch. These data collectively suggest that switching to a high affinity DNA-binding conformation involves a rotation/translation of one protomer relative to the other in a way that is dependent on the integrity of C100. We place these findings in the context of other Zur proteins and Fur family repressors more broadly.

SINDy for delay-differential equations: application to model bacterial zinc response

We extend the data-driven method of sparse identification of nonlinear dynamics (SINDy) developed by Brunton et al. , Proc. Natl Acad. Sci. USA 113 (2016) to the case of delay differential equations (DDEs). This is achieved in a bilevel optimization procedure by first applying SINDy for fixed delay and then subsequently optimizing the error of the reconstructed SINDy model over delay times. We test the SINDy-delay method on a noisy short dataset from a toy DDE and show excellent agreement. We then apply the method to experimental data of gene expressions in the bacterium Pseudomonas aeruginosa subject to the influence of zinc. The derived SINDy model suggests that the increase in zinc concentration mainly affects the time delay and not the strengths of the interactions between the different agents controlling the zinc export mechanism.

Zinc homeostasis in Pseudomonas

In the genus Pseudomonas, zinc homeostasis is mediated by a complete set of import and export systems, whose expression is precisely controlled by three transcriptional regulators: Zur, CzcR and CadR. In this review, we describe in detail our current knowledge of these systems, their regulation, and the biological significance of zinc homeostasis, taking Pseudomonas aeruginosa as our paradigm. Moreover, significant parts of this overview are dedicated to highlight interactions and cross-regulations between zinc and copper import/export systems, and to shed light, through a review of the literature and comparative genomics, on differences in gene complement and function across the whole Pseudomonas genus. The impact and importance of zinc homeostasis in Pseudomonas and beyond will be discussed throughout this review.

Graphical abstract

Principles and practice of determining metal-protein affinities

Metal ions play many critical roles in biology, as structural and catalytic cofactors, and as cell regulatory and signalling elements. The metal–protein affinity, expressed conveniently by the metal dissociation constant, K_D, describes the thermodynamic strength of a metal–protein interaction and is a key parameter that can be used, for example, to understand how proteins may acquire metals in a cell and to identify dynamic elements (e.g. cofactor binding, changing metal availabilities) which regulate protein metalation in vivo. Here, we outline the fundamental principles and practical considerations that are key to the reliable quantification of metal–protein affinities. We review a selection of spectroscopic probes which can be used to determine protein affinities for essential biological transition metals (including Mn(II), Fe(II), Co(II), Ni(II), Cu(I), Cu(II) and Zn(II)) and, using selected examples, demonstrate how rational probe selection combined with prudent experimental design can be applied to determine accurate K_D values.

Advances in Understanding of the Copper Homeostasis in Pseudomonas aeruginosa

Thirty-five thousand people die as a result of more than 2.8 million antibiotic-resistant infections in the United States of America per year. Pseudomonas aeruginosa (P. aeruginosa) is classified a serious threat, the second-highest threat category of the U.S. Department of Health and Human Services. Among others, the World Health Organization (WHO) encourages the discovery and development of novel antibiotic classes with new targets and mechanisms of action without cross-resistance to existing classes. To find potential new target sites in pathogenic bacteria, such as P. aeruginosa, it is inevitable to fully understand the molecular mechanism of homeostasis, metabolism, regulation, growth, and resistances thereof. P. aeruginosa maintains a sophisticated copper defense cascade comprising three stages, resembling those of public safety organizations. These stages include copper scavenging, first responder, and second responder. Similar mechanisms are found in numerous pathogens. Here we compare the copper-dependent transcription regulators cueR and copRS of Escherichia coli (E. coli) and P. aeruginosa. Further, phylogenetic analysis and structural modelling of mexPQ-opmE reveal that this efflux pump is unlikely to be involved in the copper export of P. aeruginosa. Altogether, we present current understandings of the copper homeostasis in P. aeruginosa and potential new target sites for antimicrobial agents or a combinatorial drug regimen in the fight against multidrug resistant pathogens.

The requirement for cobalt in vitamin B12: A paradigm for protein metalation

Vitamin B12, cobalamin, is a cobalt-containing ring-contracted modified tetrapyrrole that represents one of the most complex small molecules made by nature. In prokaryotes it is utilised as a cofactor, coenzyme, light sensor and gene regulator yet has a restricted role in assisting only two enzymes within specific eukaryotes including mammals. This deployment disparity is reflected in another unique attribute of vitamin B12 in that its biosynthesis is limited to only certain prokaryotes, with synthesisers pivotal in establishing mutualistic microbial communities. The core component of cobalamin is the corrin macrocycle that acts as the main ligand for the cobalt. Within this review we investigate why cobalt is paired specifically with the corrin ring, how cobalt is inserted during the biosynthetic process, how cobalt is made available within the cell and explore the cellular control of cobalt and cobalamin levels. The partitioning of cobalt for cobalamin biosynthesis exemplifies how cells assist metalation.

Essential trace elemental levels (zinc, iron and copper) in the biological samples of smoker referent and pulmonary tuberculosis patients

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Smoking is linked with tuberculosis recurrence.

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Pakistan has not only high TB number of TB cases (fifth) but also high level of multidrug-resistant TB (fourth) in the world.

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Cu/Zn ratio was also considerably greater in all biological samples of TB patients as compared to the control group.

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The analysis the levels of the elements, {Fe, Cu, Zn and Cu/Zn ratio} may help the studies on the development of TB disease.

Tuberculosis is one of the major causes of illnesses and deaths throughout world particularly in Asia. Smoking is linked with tuberculosis recurrence and its mortality and may influence bacteriological conversion, clinical symptoms and treatment outcome. The aim of current study was to estimate association among essential trace elements {zinc (Zn), iron (Fe) and copper (Cu)} in human biological samples particularly blood, serum, scalp hair, saliva, sputum, and nasal fluid of smoking and nonsmoking pulmonary tuberculosis patients (n = 165, age ranged 16–35 years) residents of Hyderabad, Pakistan. The biological samples of age matched healthy controls were chosen as referents of both genders (n = 171) for the comparison purpose. The human biological samples were wet digested in microwave oven by 65 % HNO₃ and 30 % H₂O₂ with (2:1) ratio. The concentrations of elements in acid digested samples were determined by atomic absorption spectrometry. The average zinc and iron concentration was lower, while level of copper was higher in the biological samples of pulmonary Tuberculosis patients as compared to referent subjects (p < .001). It was also concluded as a result of Zn and Fe deficiency combined with high contact of copper due to smoking of tobacco can be synergistic with the risk factors related with pulmonary tuberculosis.

The interplay of the metallosensor CueR with two distinct CopZ chaperones defines copper homeostasis in Pseudomonas aeruginosa

Copper homeostasis in pathogenic bacteria is critical for cuproprotein assembly and virulence. However, in vivo biochemical analyses of these processes are challenging, which has prevented defining and quantifying the homeostatic interplay between Cu⁺-sensing transcriptional regulators, chaperones, and sequestering molecules. The cytoplasm of Pseudomonas aeruginosa contains a Cu⁺-sensing transcriptional regulator, CueR, and two homologous metal chaperones, CopZ1 and CopZ2, forming a unique system for studying Cu⁺ homeostasis. We found here that both chaperones exchange Cu⁺, albeit at a slow rate, reaching equilibrium after 3 h, a time much longer than P. aeruginosa duplication time. Therefore, they appeared as two separate cellular Cu⁺ pools. Although both chaperones transferred Cu⁺ to CueR in vitro, experiments in vivo indicated that CopZ1 metallates CueR, eliciting the translation of Cu⁺ efflux transporters involved in metal tolerance. Although this observation was consistent with the relative Cu⁺ affinities of the three proteins (CopZ1 < CueR < CopZ2), in vitro and in silico analyses also indicated a stronger interaction between CopZ1 and CueR that was independent of Cu⁺ In contrast, CopZ2 function was defined by its distinctly high abundance during Cu²⁺ stress. Under resting conditions, CopZ2 remained largely in its apo form. Metal stress quickly induced CopZ2 expression, and its holo form predominated, reaching levels commensurate with the cytoplasmic Cu⁺ levels. In summary, these results show that CopZ1 acts as chaperone delivering Cu⁺ to the CueR sensor, whereas CopZ2 functions as a fast-response Cu⁺-sequestering storage protein. We propose that equivalent proteins likely play similar roles in most bacterial systems.

Bacterial sensors define intracellular free energies for correct enzyme metalation

There is a challenge for metalloenzymes to acquire their correct metals because some inorganic elements form more stable complexes with proteins than do others. These preferences can be overcome provided some metals are more available than others. However, while the total amount of cellular metal can be readily measured, the available levels of each metal have been more difficult to define. Metal-sensing transcriptional regulators are tuned to the intracellular availabilities of their cognate ions. Here we have determined the standard free energy for metal complex formation to which each sensor, in a set of bacterial metal sensors, is attuned: The less competitive the metal, the less favorable the free energy and hence greater availability to which the cognate allosteric mechanism is tuned. Comparing these free energies with values derived from the metal affinities of a metalloprotein reveals the mechanism of correct metalation exemplified here by a cobalt-chelatase for vitamin B₁₂.

Bacterial zinc uptake regulator proteins and their regulons

All organisms must regulate the cellular uptake, efflux, and intracellular trafficking of essential elements, including d-block metal ions. In bacteria, such regulation is achieved by the action of metal-responsive transcriptional regulators. Among several families of zinc-responsive transcription factors, the ‘zinc uptake regulator’ Zur is the most widespread. Zur normally represses transcription in its zinc-bound form, in which DNA-binding affinity is enhanced allosterically. Experimental and bioinformatic searches for Zur-regulated genes have revealed that in many cases, Zur proteins govern zinc homeostasis in a much more profound way than merely through the expression of uptake systems. Zur regulons also comprise biosynthetic clusters for metallophore synthesis, ribosomal proteins, enzymes, and virulence factors. In recognition of the importance of zinc homeostasis at the host–pathogen interface, studying Zur regulons of pathogenic bacteria is a particularly active current research area.

Fine control of metal concentrations is necessary for cells to discern zinc from cobalt

Bacteria possess transcription factors whose DNA-binding activity is altered upon binding to specific metals, but metal binding is not specific in vitro. Here we show that tight regulation of buffered intracellular metal concentrations is a prerequisite for metal specificity of Zur, ZntR, RcnR and FrmR in Salmonella Typhimurium. In cells, at non-inhibitory elevated concentrations, Zur and ZntR, only respond to Zn(II), RcnR to cobalt and FrmR to formaldehyde. However, in vitro all these sensors bind non-cognate metals, which alters DNA binding. We model the responses of these sensors to intracellular-buffered concentrations of Co(II) and Zn(II) based upon determined abundances, metal affinities and DNA affinities of each apo- and metalated sensor. The cognate sensors are modelled to respond at the lowest concentrations of their cognate metal, explaining specificity. However, other sensors are modelled to respond at concentrations only slightly higher, and cobalt or Zn(II) shock triggers mal-responses that match these predictions. Thus, perfect metal specificity is fine-tuned to a narrow range of buffered intracellular metal concentrations.

Sort-Seq Approach to Engineering a Formaldehyde-Inducible Promoter for Dynamically Regulated Escherichia coli Growth on Methanol

Tight and tunable control of gene expression is a highly desirable goal in synthetic biology for constructing predictable gene circuits and achieving preferred phenotypes. Elucidating the sequence–function relationship of promoters is crucial for manipulating gene expression at the transcriptional level, particularly for inducible systems dependent on transcriptional regulators. Sort-seq methods employing fluorescence-activated cell sorting (FACS) and high-throughput sequencing allow for the quantitative analysis of sequence–function relationships in a robust and rapid way. Here we utilized a massively parallel sort-seq approach to analyze the formaldehyde-inducible Escherichia coli promoter (P_frm) with single-nucleotide resolution. A library of mutated formaldehyde-inducible promoters was cloned upstream of gfp on a plasmid. The library was partitioned into bins via FACS on the basis of green fluorescent protein (GFP) expression level, and mutated promoters falling into each expression bin were identified with high-throughput sequencing. The resulting analysis identified two 19 base pair repressor binding sites, one upstream of the −35 RNA polymerase (RNAP) binding site and one overlapping with the −10 site, and assessed the relative importance of each position and base therein. Key mutations were identified for tuning expression levels and were used to engineer formaldehyde-inducible promoters with predictable activities. Engineered variants demonstrated up to 14-fold lower basal expression, 13-fold higher induced expression, and a 3.6-fold stronger response as indicated by relative dynamic range. Finally, an engineered formaldehyde-inducible promoter was employed to drive the expression of heterologous methanol assimilation genes and achieved increased biomass levels on methanol, a non-native substrate of E. coli.

Metallochaperones and metalloregulation in bacteria

Bacterial transition metal homoeostasis or simply 'metallostasis' describes the process by which cells control the intracellular availability of functionally required metal cofactors, from manganese (Mn) to zinc (Zn), avoiding both metal deprivation and toxicity. Metallostasis is an emerging aspect of the vertebrate host-pathogen interface that is defined by a 'tug-of-war' for biologically essential metals and provides the motivation for much recent work in this area. The host employs a number of strategies to starve the microbial pathogen of essential metals, while for others attempts to limit bacterial infections by leveraging highly competitive metals. Bacteria must be capable of adapting to these efforts to remodel the transition metal landscape and employ highly specialized metal sensing transcriptional regulators, termed metalloregulatory proteins,and metallochaperones, that allocate metals to specific destinations, to mediate this adaptive response. In this essay, we discuss recent progress in our understanding of the structural mechanisms and metal specificity of this adaptive response, focusing on energy-requiring metallochaperones that play roles in the metallocofactor active site assembly in metalloenzymes and metallosensors, which govern the systems-level response to metal limitation and intoxication.

A tight tunable range for Ni(II) sensing and buffering in cells

The metal affinities of metal-sensing transcriptional regulators co-vary with cellular metal concentrations over more than 12 orders of magnitude. To understand the cause of this relationship, we determined the structure of the Ni(II) sensor InrS and then created cyanobacteria (Synechocystis PCC 6803) in which transcription of genes encoding a Ni(II) exporter and a Ni(II) importer were controlled by InrS variants with weaker Ni(II) affinities. Variant strains were sensitive to elevated nickel and contained more nickel, but the increase was small compared with the change in Ni(II) affinity. All of the variant sensors retained the allosteric mechanism that inhibits DNA binding following metal binding, but a response to nickel in vivo was observed only when the sensitivity was set to respond in a relatively narrow (less than two orders of magnitude) range of nickel concentrations. Thus, the Ni(II) affinity of InrS is attuned to cellular metal concentrations rather than the converse.

Essential and toxic metals in serum of individuals with active pulmonary tuberculosis in an endemic region

Trace elements play an important role in tuberculosis infection because their deficiencies can be associated with impaired immunity. Blood samples were collected from a total of 320 active pulmonary tuberculosis patients and healthy individuals. The serum concentrations of Zinc, Iron, Copper, Calcium, lead, Arsenic and Selenium were analyzed by atomic absorption spectrometry. The levels of trace elements were measured after 2, 4 and 6 months of anti-TB treatment initiation in TB infected groups. Compared to the control group, the concentrations of Zinc, Selenium, and Iron were significantly lower (P < 0.001) in tuberculosis patients; however, that of Arsenic, Lead, and copper was significantly higher (P < 0.001) in the serum of patients. Cu/Zn and Cu/Se ratios were also significantly higher (P < 0.001) in TB patients compared to the control group. In addition, serum concentration calcium was similar in both TB patients and healthy controls. Our results indicated that trace elements concentrations in tuberculosis patients are related to each element role in immune system. Wherever the element is essential for the pathogenesis of bacteria, its concentration will remain low; and contrariwise, when the element is toxic for the bacteria, its level will be regulated up to provide a perfect condition for bacterial growth.

The mechanism of a formaldehyde-sensing transcriptional regulator

Most organisms are exposed to the genotoxic chemical formaldehyde, either from endogenous or environmental sources. Therefore, biology has evolved systems to perceive and detoxify formaldehyde. The frmRA(B) operon that is present in many bacteria represents one such system. The FrmR protein is a transcriptional repressor that is specifically inactivated in the presence of formaldehyde, permitting expression of the formaldehyde detoxification machinery (FrmA and FrmB, when the latter is present). The X-ray structure of the formaldehyde-treated Escherichia coli FrmR (EcFrmR) protein reveals the formation of methylene bridges that link adjacent Pro2 and Cys35 residues in the EcFrmR tetramer. Methylene bridge formation has profound effects on the pattern of surface charge of EcFrmR and combined with biochemical/biophysical data suggests a mechanistic model for formaldehyde-sensing and derepression of frmRA(B) expression in numerous bacterial species.

The Effectors and Sensory Sites of Formaldehyde-responsive Regulator FrmR and Metal-sensing Variant

The DUF156 family of DNA-binding transcriptional regulators includes metal sensors that respond to cobalt and/or nickel (RcnR, InrS) or copper (CsoR) plus CstR, which responds to persulfide, and formaldehyde-responsive FrmR. Unexpectedly, the allosteric mechanism of FrmR from Salmonella enterica serovar Typhimurium is triggered by metals in vitro, and variant FrmR(E64H) gains responsiveness to Zn(II) and cobalt in vivo Here we establish that the allosteric mechanism of FrmR is triggered directly by formaldehyde in vitro Sensitivity to formaldehyde requires a cysteine (Cys(35) in FrmR) conserved in all DUF156 proteins. A crystal structure of metal- and formaldehyde-sensing FrmR(E64H) reveals that an FrmR-specific amino-terminal Pro(2) is proximal to Cys(35), and these residues form the deduced formaldehyde-sensing site. Evidence is presented that implies that residues spatially close to the conserved cysteine tune the sensitivities of DUF156 proteins above or below critical thresholds for different effectors, generating the semblance of specificity within cells. Relative to FrmR, RcnR is less responsive to formaldehyde in vitro, and RcnR does not sense formaldehyde in vivo, but reciprocal mutations FrmR(P2S) and RcnR(S2P), respectively, impair and enhance formaldehyde reactivity in vitro Formaldehyde detoxification by FrmA requires S-(hydroxymethyl)glutathione, yet glutathione inhibits formaldehyde detection by FrmR in vivo and in vitro Quantifying the number of FrmR molecules per cell and modeling formaldehyde modification as a function of [formaldehyde] demonstrates that FrmR reactivity is optimized such that FrmR is modified and frmRA is derepressed at lower [formaldehyde] than required to generate S-(hydroxymethyl)glutathione. Expression of FrmA is thereby coordinated with the accumulation of its substrate.

Bacterial Strategies to Maintain Zinc Metallostasis at the Host-Pathogen Interface

Among the biologically required first row, late d-block metals from Mn^II to Zn^II, the catalytic and structural reach of Zn^II ensures that this essential micronutrient touches nearly every major metabolic process or pathway in the cell. Zn is also toxic in excess, primarily because it is a highly competitive divalent metal and will displace more weakly bound transition metals in the active sites of metalloenzymes if left unregulated. The vertebrate innate immune system uses several strategies to exploit this "Achilles heel" of microbial physiology, but bacterial evolution has responded in kind. This review highlights recent insights into transcriptional, transport, and trafficking mechanisms that pathogens use to "win the fight" over zinc and thrive in an otherwise hostile environment.

Formaldehyde Stress Responses in Bacterial Pathogens

Formaldehyde is the simplest of all aldehydes and is highly cytotoxic. Its use and associated dangers from environmental exposure have been well documented. Detoxification systems for formaldehyde are found throughout the biological world and they are especially important in methylotrophic bacteria, which generate this compound as part of their metabolism of methanol. Formaldehyde metabolizing systems can be divided into those dependent upon pterin cofactors, sugar phosphates and those dependent upon glutathione. The more prevalent thiol-dependent formaldehyde detoxification system is found in many bacterial pathogens, almost all of which do not metabolize methane or methanol. This review describes the endogenous and exogenous sources of formaldehyde, its toxic effects and mechanisms of detoxification. The methods of formaldehyde sensing are also described with a focus on the formaldehyde responsive transcription factors HxlR, FrmR, and NmlR. Finally, the physiological relevance of detoxification systems for formaldehyde in bacterial pathogens is discussed.