The Role of Intermetal Competition and Mis-Metalation in Metal Toxicity

Metals in Motion: Understanding Labile Metal Pools in Bacteria

Metal ions are essential for all life. In microbial cells, potassium (K+) is the most abundant cation and plays a key role in maintaining osmotic balance. Magnesium (Mg2+) is the dominant divalent cation and is required for nucleic acid structure and as an enzyme cofactor. Microbes typically require the transition metals manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn), although the precise set of metal ions needed to sustain life is variable. Intracellular metal pools can be conceptualized as a chemically complex mixture of rapidly exchanging (labile) ions, complemented by those reservoirs that exchange slowly relative to cell metabolism (sequestered). Labile metal pools are buffered by transient interactions with anionic metabolites and macromolecules, with the ribosome playing a major role. Sequestered metal pools include many metalloproteins, cofactors, and storage depots, with some pools redeployed upon metal depletion. Here, I review the size, composition, and dynamics of intracellular metal pools and highlight the major gaps in understanding.

Metalation of Extracytoplasmic Proteins and Bacterial Cell Envelope Homeostasis

Cell physiology requires innumerable metalloenzymes supported by the selective import of metal ions. Within the crowded cytosol, most enzymes acquire their cognate cofactors from a buffered labile pool. Metalation of membrane-bound and secreted exoenzymes is more problematic since metal concentrations are highly variable outside the cell. Here, we focus on metalloenzymes involved in cell envelope homeostasis. Peptidoglycan synthesis often relies on Zn-dependent hydrolases, and metal-dependent β-lactamases play important roles in antibiotic resistance. In gram-positive bacteria, lipoteichoic acid synthesis requires Mn, with TerC family Mn exporters in a supporting role. For some exoenzymes, metalation occurs in the cytosol, and metalated enzymes are exported through the TAT secretion system. For others, metalation is facilitated by metal exporters, metallochaperones, or partner proteins that enhance metal affinity. To help ensure function, some metalloenzymes can function with multiple metals. Thus, cells employ a diversity of strategies to ensure metalation of enzymes functioning outside the cytosol.

Investigation of Zur-regulated metal transport systems reveals an unexpected role of pyochelin in zinc homeostasis

Limiting the availability of transition metals at infection sites serves as a critical defense mechanism employed by the innate immune system to combat microbial infections. Pseudomonas aeruginosa exhibits a remarkable ability to thrive in zinc-deficient environments, facilitated by intricate cellular responses governed by numerous genes regulated by the zinc-responsive transcription factor Zur. Many of these genes have unknown functions, including those within the predicted PA2911-PA2914 and PA4063-PA4066 operons. A structural bioinformatics investigation revealed that PA2911-PA2914 comprises a TonB-dependent outer membrane receptor and inner membrane ABC-permeases responsible for importing metal-chelating molecules, whereas PA4063-PA4066 contains genes encoding a MacB transporter, likely involved in the export of large molecules. Molecular genetics and biochemical experiments, feeding assays, and intracellular metal content measurements support the hypothesis that PA2911-PA2914 and PA4063-PA4066 are engaged in the import and export of the pyochelin-cobalt complex, respectively. Notably, cobalt can reduce zinc demand and promote the growth of P. aeruginosa strains unable to import zinc, highlighting pyochelin-mediated cobalt import as a novel bacterial strategy to counteract zinc deficiency. These results unveil an unexpected role for pyochelin in zinc homeostasis and challenge the traditional view of this metallophore exclusively as an iron transporter.

IMPORTANCE

The mechanisms underlying the remarkable ability of Pseudomonas aeruginosa to resist the zinc sequestration mechanisms implemented by the vertebrate innate immune system to control bacterial infections are still far from being fully understood. This study reveals that the Zur-regulated gene clusters PA2911-2914 and PA4063-PA4066 encode systems for the import and export of cobalt-bound pyochelin, respectively. This proves to be a useful strategy to counteract conditions of severe zinc deficiency since cobalt can replace zinc in many proteins. The discovery that pyochelin may contribute to cellular responses to zinc deficiency leads to a reevaluation of the paradigm that pyochelin is a siderophore involved exclusively in iron acquisition and suggests that this molecule has a broader role in modulating the homeostasis of multiple metals.

Oceanobacillus picturae alleviates cadmium stress and promotes growth in soybean seedlings

Cadmium (Cd) is a heavy metal that significantly impacts human health and the environment. Microorganisms play a crucial role in reducing heavy metal stress in plants; however, the mechanisms by which microorganisms enhance plant tolerance to Cd stress and the interplay between plants and microorganisms under such stress remain unclear. In this study, Oceanobacillus picturae (O. picturae) was isolated for interaction with soybean seedlings under Cd stress. Results indicated that Cd treatment alone markedly inhibited soybean seedling growth. Conversely, inoculation with O. picturae significantly improved growth indices such as plant height, root length, and fresh weight, while also promoting recovery in soil physiological indicators and pH. Metabolomic and transcriptomic analyses identified 157 genes related to aspartic acid, cysteine, and flavonoid biosynthesis pathways. Sixty-three microbial species were significantly associated with metabolites in these pathways, including pathogenic, adversity-resistant, and bioconductive bacteria. This research experimentally demonstrates, for the first time, the growth-promoting effect of the O. picturae strain on soybean seedlings under non-stress conditions. It also highlights its role in enhancing root growth and reducing Cd accumulation in the roots under Cd stress. Additionally, through the utilization of untargeted metabolomics, metagenomics, and transcriptomics for a multi-omics analysis, we investigated the impact of O. picturae on the soil microbiome and its correlation with differential gene expression in plants. This innovative approach unveils the molecular mechanisms underlying O. picturae's promotion of root growth and adaptation to Cd stress.

The Bacillus subtilis yqgC-sodA operon protects magnesium-dependent enzymes by supporting manganese efflux

Microbes encounter a myriad of stresses during their life cycle. Dysregulation of metal ion homeostasis is increasingly recognized as a key factor in host-microbe interactions. Bacterial metal ion homeostasis is tightly regulated by dedicated metalloregulators that control uptake, sequestration, trafficking, and efflux. Here, we demonstrate that deletion of the Bacillus subtilis yqgC-sodA (YS) complex operon, but not deletion of the individual genes, causes hypersensitivity to manganese (Mn). YqgC is an integral membrane protein of unknown function, and SodA is a Mn-dependent superoxide dismutase (MnSOD). The YS strain has reduced expression of two Mn efflux proteins, MneP and MneS, consistent with the observed Mn sensitivity. The YS strain accumulated high levels of Mn, had increased reactive radical species (RRS), and had broad metabolic alterations that can be partially explained by the inhibition of Mg-dependent enzymes. Although the YS operon deletion strain and an efflux-deficient mneP mneS double mutant both accumulate Mn and have similar metabolic perturbations, they also display phenotypic differences. Several mutations that suppressed Mn intoxication of the mneP mneS efflux mutant did not benefit the YS mutant. Further, Mn intoxication in the YS mutant, but not the mneP mneS strain, was alleviated by expression of Mg-dependent, chorismate-utilizing enzymes of the menaquinone, siderophore, and tryptophan (MST) family. Therefore, despite their phenotypic similarities, the Mn sensitivity in the mneP mneS and the YS deletion mutants results from distinct enzymatic vulnerabilities.IMPORTANCEBacteria require multiple trace metal ions for survival. Metal homeostasis relies on the tightly regulated expression of metal uptake, storage, and efflux proteins. Metal intoxication occurs when metal homeostasis is perturbed and often results from enzyme mis-metalation. In Bacillus subtilis, Mn-dependent superoxide dismutase (MnSOD) is the most abundant Mn-containing protein and is important for oxidative stress resistance. Here, we report novel roles for MnSOD and a co-regulated membrane protein, YqgC, in Mn homeostasis. Loss of both MnSOD and YqgC (but not the individual proteins) prevents the efficient expression of Mn efflux proteins and leads to a large-scale perturbation of the metabolome due to inhibition of Mg-dependent enzymes, including key chorismate-utilizing MST (menaquinone, siderophore, and tryptophan) family enzymes.

Conversion of metal-enriched magnetite mine tailings into suitable soil for vegetation by phytoremediation process with Bougainvillaea glabra under the influence of Thiobacillus ferroxidance

Magnetite mining is a significant contributor to land deterioration as well as HM-based soil contamination. The characteristics of magnetite mine tailing were examined in the present study, in addition to the positive and sustainable restoration strategy with Bougainvillaea glabra under the influence of Thiobacillus ferroxidance. The traits of test soil analysis findings demonstrated that the majority of the parameters exceeded the allowable limits (For instance: HMs such as Cr, Cu, Zn, Pb, Fe, and Co were found to be 208 ± 2.3, 131.43 ± 1.6, 185.41 ± 3.3, 312 ± 5.11, 956 ± 5.3, and 26.89 ± 2.43 mg kg-1 respectively). T. ferroxidance exhibited impressive HMs tolerance for as much as 800 g mL-1 concentrations of Cr, Cu, Zn, Pb, Fe, and Co. To prevent HMs toxic effects, the HMs contents in test soil were decreased by diluting with normal soil in the ratios of Ex-3 and Ex-2. A typical greenhouse study was carried out to assess the phytoremediation ability of B. glabra across six setups for experiments (Ex-1 to Ex-6). According to the findings of this research, the HMs tolerant T. ferroxidance from Ex-3 and Ex-2 had an outstanding impact on the growth, biomolecules level (such as chlorophylls: 65.84 & 41.1 mg g-1, proteins: 165.1 & 151.1 mg g-1, as well as carbohydrates: 227.4 & 159.3 mg g-1) as well as phytoremediation potential of B. glabra on magnetite mine soil. These findings indicated that a mixture of B. glabra as well as T. ferroxidance might serve as a valuable sustainable agent for removing HMs from contaminated soil.

Microbes vary strategically in their metalation of mononuclear enzymes

Studies have determined that nonredox enzymes that are cofactored with Fe(II) are the most oxidant-sensitive targets inside Escherichia coli. These enzymes use Fe(II) cofactors to bind and activate substrates. Because of their solvent exposure, the metal can be accessed and oxidized by reactive oxygen species, thereby inactivating the enzyme. Because these enzymes participate in key physiological processes, the consequences of stress can be severe. Accordingly, when E. coli senses elevated levels of H2O2, it induces both a miniferritin and a manganese importer, enabling the replacement of the iron atom in these enzymes with manganese. Manganese does not react with H2O2 and thereby preserves enzyme activity. In this study, we examined several diverse microbes to identify the metal that they customarily integrate into ribulose-5-phosphate 3-epimerase, a representative of this enzyme family. The anaerobe Bacteroides thetaiotaomicron, like E. coli, uses iron. In contrast, Bacillus subtilis and Lactococcus lactis use manganese, and Saccharomyces cerevisiae uses zinc. The latter organisms are therefore well suited to the oxidizing environments in which they dwell. Similar results were obtained with peptide deformylase, another essential enzyme of the mononuclear class. Strikingly, heterologous expression experiments show that it is the metal pool within the organism, rather than features of the protein itself, that determine which metal is incorporated. Further, regardless of the source organism, each enzyme exhibits highest turnover with iron and lowest turnover with zinc. We infer that the intrinsic catalytic properties of the metal cannot easily be retuned by evolution of the polypeptide.

The Bacillus subtilis yqgC-sodA operon protects magnesium-dependent enzymes by supporting manganese efflux

Microbes encounter a myriad of stresses during their life cycle. Dysregulation of metal ion homeostasis is increasingly recognized as a key factor in host-microbe interactions. Bacterial metal ion homeostasis is tightly regulated by dedicated metalloregulators that control uptake, sequestration, trafficking, and efflux. Here, we demonstrate that deletion of the Bacillus subtilis yqgC-sodA (YS) complex operon, but not deletion of the individual genes, causes hypersensitivity to manganese (Mn). YqgC is an integral membrane protein of unknown function and SodA is a Mn-dependent superoxide dismutase (MnSOD). The YS strain has reduced expression of two Mn efflux proteins, MneP and MneS, consistent with the observed Mn sensitivity. The YS strain accumulated high levels of Mn, had increased reactive radical species (RRS), and had broad metabolic alterations that can be partially explained by the inhibition of Mg-dependent enzymes. Although the YS operon deletion strain and an efflux-deficient mneP mneS double mutant both accumulate Mn and have similar metabolic perturbations they also display phenotypic differences. Several mutations that suppressed Mn intoxication of the mneP mneS efflux mutant did not benefit the YS mutant. Further, Mn intoxication in the YS mutant, but not the mneP mneS strain, was alleviated by expression of Mg-dependent, chorismate-utilizing enzymes of the menaquinone, siderophore, and tryptophan (MST) family. Therefore, despite their phenotypic similarities, the Mn sensitivity in the mneP mneS and the yqgC-sodA deletion mutants results from distinct enzymatic vulnerabilities.

Recent advances in copper sulfide nanoparticles for phototherapy of bacterial infections and cancer

Copper sulfide nanoparticles (CuS NPs) have attracted growing interest in biomedical research due to their remarkable properties, such as their high photothermal and thermodynamic capabilities, which are ideal for anticancer and antibacterial applications. This comprehensive review focuses on the current state of antitumor and antibacterial applications of CuS NPs. The initial section provides an overview of the various approaches to synthesizing CuS NPs, highlighting the size, shape and composition of CuS NPs fabricated using different methods. In this review, the mechanisms underlying the antitumor and antibacterial activities of CuS NPs in medical applications are discussed and the clinical challenges associated with the use of CuS NPs are also addressed.

TerC proteins function during protein secretion to metalate exoenzymes

Cytosolic metalloenzymes acquire metals from buffered intracellular pools. How exported metalloenzymes are appropriately metalated is less clear. We provide evidence that TerC family proteins function in metalation of enzymes during export through the general secretion (Sec-dependent) pathway. Bacillus subtilis strains lacking MeeF(YceF) and MeeY(YkoY) have a reduced capacity for protein export and a greatly reduced level of manganese (Mn) in the secreted proteome. MeeF and MeeY copurify with proteins of the general secretory pathway, and in their absence the FtsH membrane protease is essential for viability. MeeF and MeeY are also required for efficient function of the Mn2+-dependent lipoteichoic acid synthase (LtaS), a membrane-localized enzyme with an extracytoplasmic active site. Thus, MeeF and MeeY, representative of the widely conserved TerC family of membrane transporters, function in the co-translocational metalation of Mn2+-dependent membrane and extracellular enzymes.

Temporal modelling of the biofilm lifecycle (TMBL) establishes kinetic analysis of plate-based bacterial biofilm dynamics

Bacterial biofilms are critical to pathogenesis and infection. They are associated with rising rates of antimicrobial resistance. Biofilms are correlated with worse clinical outcomes, making them important to infectious diseases research. There is a gap in knowledge surrounding biofilm kinetics and dynamics which makes biofilm research difficult to translate from bench to bedside. To address this gap, this work employs a well-characterized crystal violet biomass accrual and planktonic cell density assay across a clinically relevant time course and expands statistical analysis to include kinetic information in a protocol termed the TMBL (Temporal Mapping of the Biofilm Lifecycle) assay. TMBL's statistical framework quantitatively compares biofilm communities across time, species, and media conditions in a 96-well format. Measurements from TMBL can reliably be condensed into response features that inform the time-dependent behavior of adherent biomass and planktonic cell populations. Staphylococcus aureus and Pseudomonas aeruginosa biofilms were grown in conditions of metal starvation in nutrient-variable media to demonstrate the rigor and translational potential of this strategy. Significant differences in single-species biofilm formation are seen in metal-deplete conditions as compared to their controls which is consistent with the consensus literature on nutritional immunity that metal availability drives transcriptomic and metabolomic changes in numerous pathogens. Taken together, these results suggest that kinetic analysis of biofilm by TMBL represents a statistically and biologically rigorous approach to studying the biofilm lifecycle as a time-dependent process. In addition to current methods to study the impact of microbe and environmental factors on the biofilm lifecycle, this kinetic assay can inform biological discovery in biofilm formation and maintenance.

Discriminating Susceptibility of Xanthine Oxidoreductase Family to Metals

The xanthine oxidoreductase (XOR) family are metal-containing enzymes that use the molybdenum cofactor (Moco), 2Fe-2S clusters, and flavin adenine dinucleotide (FAD) for their catalytic activity. This large molybdoenzyme family includes xanthine, aldehyde, and CO dehydrogenases. XORs are widely distributed from bacteria to humans due to their key roles in the catabolism of purines, aldehydes, drugs, and xenobiotics, as well as interconversions between CO and CO2. Assessing the effect of excess metals on the Rubrivivax gelatinosus bacterium, we found that exposure to copper (Cu) or cadmium (Cd) caused a dramatic decrease in the activity of a high-molecular-weight soluble complex exhibiting nitroblue tetrazolium reductase activity. Mass spectrometry and genetic analyses showed that the complex corresponds to a putative CO dehydrogenase (pCOD). Using mutants that accumulate either Cu+ or Cd2+ in the cytoplasm, we show that Cu+ or Cd2+ is a potent inhibitor of XORs (pCOD and the xanthine dehydrogenase [XDH]) in vivo. This is the first in vivo demonstration that Cu+ affects Moco-containing enzymes. The specific inhibitory effect of these compounds on the XOR activity is further supported in vitro by direct addition of competing metals to protein extracts. Moreover, emphasis is given on the inhibitory effect of Cu on bovine XOR, showing that the XOR family could be a common target of Cu. Given the conservation of XOR structure and function across the tree of life, we anticipate that our findings could be transferable to other XORs and organisms. IMPORTANCE The high toxicity of Cu, Cd, Pb, As, and other metals arises from their ability to cross membranes and target metalloenzymes in the cytoplasm. Identifying these targets provides insights into the toxicity mechanisms. The vulnerability of metalloenzymes arises from the accessibility of their cofactors to ions. Accordingly, many enzymes whose cofactors are solvent exposed are likely to be targets of competing metals. Here, we describe for the first time, with in vivo and in vitro experiments, a direct effect of excess Cu on the xanthine oxidoreductase family (XOR/XDH/pCOD). We show that toxic metal affects these Moco enzymes, and we suggest that access to the Moco center by Cu ions could explain the Cu inhibition of XORs in living organisms. Human XOR activity is associated with hyperuricemia, xanthinuria, gout arthritis, and other diseases. Our findings in vivo highlight XOR as a Cu target and thus support the potential use of Cu in metal-based therapeutics against these diseases.

Copper-binding proteins genes set predicting the overall survival and immune infiltration in hepatocellular carcinoma by bioinformatic analysis

Abnormal Copper (Cu) accumulation shared a close association with hepatocellular carcinoma (HCC), but the regulatory role of Copper-binding proteins in HCC remains largely unknown. The aim of study was to identify the potential regulatory role of Cu-binding proteins, including copper homeostasis maintainer and the downstream effectors of Cu, in the progression of HCC. We conducted a comprehensive bioinformatic analysis of Cu-binding proteins in HCC using data from TCGA and ICGC database. Univariate cox regression analysis was conducted, and four prognostic Cu-binding proteins was identified to be differentially expressed between the normal liver tissues and HCC tissues. In addition, the Cu-binding proteins-based predictive signature (CuPscore) model was generated using the least absolute shrinkage and selection operator (LASSO) cox regression model. Here, we identified the crucial prognostic value of CuPscore in HCC. The pathological stage and CuPscore were independent risk factors for the prognosis of HCC patients. Pathological stage and CuPscore-based nomogram model exhibited great performance in predicting the prognosis of HCC patients. We also observed that the CuPscore shared a close association with several immunomodulatory molecules and the proportion of several tumor infiltrating immune cells, suggesting a potential value of CuPscore in predicting the response to immunotherapy in HCC. Our results demonstrated the prognostic value of Cu-binding proteins and its correlation with immune microenvironment in HCC, providing a therapeutic basis for the precision medicine strategy through targeting Cu-binding proteins in HCC.

TerC Proteins Function During Protein Secretion to Metalate Exoenzymes

Cytosolic metalloenzymes acquire metals from buffered intracellular pools. How exported metalloenzymes are appropriately metalated is less clear. We provide evidence that TerC family proteins function in metalation of enzymes during export through the general secretion (Sec-dependent) pathway. Bacillus subtilis strains lacking MeeF(YceF) and MeeY(YkoY) have a reduced capacity for protein export and a greatly reduced level of manganese (Mn) in the secreted proteome. MeeF and MeeY copurify with proteins of the general secretory pathway, and in their absence the FtsH membrane protease is essential for viability. MeeF and MeeY are also required for efficient function of the Mn 2+ -dependent lipoteichoic acid synthase (LtaS), a membrane-localized enzyme with an extracytoplasmic active site. Thus, MeeF and MeeY, representative of the widely conserved TerC family of membrane transporters, function in the co-translocational metalation of Mn 2+ -dependent membrane and extracellular enzymes.

An ancient metalloenzyme evolves through metal preference modulation

Evolution creates functional diversity of proteins, the essential building blocks of all biological systems. However, studies of natural proteins sampled across the tree of life and evaluated in a single experimental system are lacking. Almost half of enzymes require metals, and metalloproteins tend to optimally utilize the physicochemical properties of a specific metal co-factor. Life must adapt to changes in metal bioavailability, including those during the transition from anoxic to oxic Earth or pathogens' exposure to nutritional immunity. These changes can challenge the ability of metalloenzymes to maintain activity, presumptively driving their evolution. Here we studied metal-preference evolution within the natural diversity of the iron/manganese superoxide dismutase (SodFM) family of reactive oxygen species scavengers. We identified and experimentally verified residues with conserved roles in determining metal preference that, when combined with an understanding of the protein's evolutionary history, improved prediction of metal utilization across the five SodFM subfamilies defined herein. By combining phylogenetics, biochemistry and structural biology, we demonstrate that SodFM metal utilization can be evolutionarily fine tuned by sliding along a scale between perfect manganese and iron specificities. Over the history of life, SodFM metal preference has been modulated multiple independent times within different evolutionary and ecological contexts, and can be changed within short evolutionary timeframes.

Adaptive Mechanisms of Shewanella xiamenensis DCB 2-1 Metallophilicity

The dose-dependent effects of single metals (Zn, Ni, and Cu) and their combinations at steady time-actions on the cell viability of the bacteria Shewanella xiamenensis DCB 2-1, isolated from a radionuclide-contaminated area, have been estimated. The accumulation of metals by Shewanella xiamenensis DCB 2-1 in single and multi-metal systems was assessed using the inductively coupled plasma atomic emission spectroscopy. To estimate the response of the bacteria's antioxidant defense system, doses of 20 and 50 mg/L of single studied metals and 20 mg/L of each metal in their combinations (non-toxic doses, determined by the colony-forming viability assay) were used. Emphasis was given to catalase and superoxide dismutase since they form the primary line of defense against heavy metal action and their regulatory circuit of activity is crucial. The effect of metal ions on total thiol content, an indicator of cellular redox homeostasis, in bacterial cells was evaluated. Genome sequencing of Shewanella xiamenensis DCB 2-1 reveals genes responsible for heavy metal tolerance and detoxification, thereby improving understanding of the potential of the bacterial strain for bioremediation.

The elements of life: A biocentric tour of the periodic table

Living systems are built from a small subset of the atomic elements, including the bulk macronutrients (C,H,N,O,P,S) and ions (Mg,K,Na,Ca) together with a small but variable set of trace elements (micronutrients). Here, we provide a global survey of how chemical elements contribute to life. We define five classes of elements: those that are (i) essential for all life, (ii) essential for many organisms in all three domains of life, (iii) essential or beneficial for many organisms in at least one domain, (iv) beneficial to at least some species, and (v) of no known beneficial use. The ability of cells to sustain life when individual elements are absent or limiting relies on complex physiological and evolutionary mechanisms (elemental economy). This survey of elemental use across the tree of life is encapsulated in a web-based, interactive periodic table that summarizes the roles chemical elements in biology and highlights corresponding mechanisms of elemental economy.

Zinc nutrition and dietary zinc supplements

As the second most abundant trace element in the human body, zinc nutrition is constantly a hot topic. More than one-third population is suffering zinc deficiency, which results in various types of diseases or nutritional deficiencies. Traditional ways of zinc supplementation seem with low absorption rates and significant side effects. Zinc supplements with dietary components are easily accessible and improve zinc utilization rate significantly. Also, mechanisms of maintaining zinc homeostasis are of broad interest. The present review focuses on zinc nutrition in human health in inductive methods. Mainly elaborate on different diseases relating to zinc disorder, highlighting the impact on the immune system and the recent COVID-19. Then raise food-derived zinc-binding compounds, including protein, peptide, polysaccharide, and polyphenol, and also analyze their possibilities to serve as zinc complementary. Finally, illustrate the way to maintain zinc homeostasis and the corresponding mechanisms. The review provides data information for maintaining zinc homeostasis with the food-derived matrix.

Interaction between selenium and essential micronutrient elements in plants: A systematic review

Nutrient imbalance (i.e., deficiency and toxicity) of microelements is an outstanding environmental issue that influences each aspect of ecosystems. Although the crucial roles of microelements in entire lifecycle of plants have been widely acknowledged, the effective control of microelements is still neglected due to the narrow safe margins. Selenium (Se) is an essential element for humans and animals. Although it is not believed to be indispensable for plants, many literatures have reported the significance of Se in terms of the uptake, accumulation, and detoxification of essential microelements in plants. However, most papers only concerned on the antagonistic effect of Se on metal elements in plants and ignored the underlying mechanisms. There is still a lack of systematic review articles to summarize the comprehensive knowledge on the connections between Se and microelements in plants. In this review, we conclude the bidirectional effects of Se on micronutrients in plants, including iron, zinc, copper, manganese, nickel, molybdenum, sodium, chlorine, and boron. The regulatory mechanisms of Se on these micronutrients are also analyzed. Moreover, we further emphasize the role of Se in alleviating element toxicity and adjusting the concentration of micronutrients in plants by altering the soil conditions (e.g., adsorption, pH, and organic matter), promoting microbial activity, participating in vital physiological and metabolic processes, generating element competition, stimulating metal chelation, organelle compartmentalization, and sequestration, improving the antioxidant defense system, and controlling related genes involved in transportation and tolerance. Based on the current understanding of the interaction between Se and these essential elements, future directions for research are suggested.

Impairment of the Zn/Cd detoxification systems affects the ability of Salmonella to colonize Arabidopsis thaliana

Salmonella capacity to colonize different environments depends on its ability to respond efficiently to fluctuations in micronutrient availability. Among micronutrients, Zn, besides playing an essential role in bacterial physiology, is a key element whose concentration can influence bacterial survival in a particular niche. Plant colonization by Salmonella enterica was described for several years, and some molecular determinants involved in this host-pathogen interaction have started to be characterized. However, it is still unclear if Zn plays a role in the outcome of this interaction, as well established for animal hosts that employ nutritional immunity strategies to counteract pathogens infections. In this study, we have investigated the involvement of Salmonella Typhimurium main effectors of zinc homeostasis in plant colonization, using Arabidopsis thaliana as a model host. The results show that to colonize plant tissues, Salmonella takes advantage of its ability to export excess metal through the efflux pumps ZntA and ZitB. In fact, the deletion of these Zn/Cd detoxification systems can affect bacterial persistence in the shoots, depending on metal availability in the plant tissues. The importance of Salmonella ability to export excess metal was enhanced in the colonization of plants grown in high Zn conditions. On the contrary, the bacterial disadvantage related to Zn detoxification impairment can be abrogated if the plant cannot efficiently translocate Zn to the shoots. Overall, our work highlights the role of Zn in Salmonella-plant interaction and suggests that modulation of plant metal content through biofortification may be an efficient strategy to control pathogen colonization.

Possible functions of CobW domain-containing (CBWD) genes in dinoflagellates using Karlodinium veneficum as a representative

Since > 91% of dinoflagellates are proven auxotrophs of vitamin B₁₂ and the cobalamin synthetase W (CobW) is a key gene involved in vitamin B₁₂ synthesis pathway, a number of CobW domain-containing (CBWD) genes in dinoflagellates (DinoCBWDs) were surprisedly found from our transcriptomic and meta-transcriptomic studies. A total of 88 DinoCBWD genes were identified from the genomes and transcriptomes of four dinoflagellates, with five being cloned for full-lengths and characterized using the cosmopolitan and ecologically-important dinoflagellates Karlodinium veneficum and Scrippsiella trochoidea (synonym of Scrippsiella acuminata). DinoCBWDs were verified being irrelevant to vitamin B₁₂ biosynthesis due to their transcriptions irresponsive to vitamin B₁₂ levels and their phylogenetic positions. A comprehensive phylogenetic analysis demonstrated 75 out of the 88 DinoCBWD genes identified belong to three subfamilies of COG0523 protein family, of which most prokaryotic members are reported to be metallochaperones and the eukaryotic members are ubiquitously found but mostly unknown for their functions. Our results from K. veneficum demonstrated DinoCBWDs are associated with metal homeostasis and other divergent functions, with four KvCBWDs involving in zinc homeostasis and KvCBWD1 likely functioning as Fe-type nitrile hydratase activator. In addition, conserved motif analysis revealed the structural foundation of KvCBWD proteins that are consistent with previously described CBWD proteins with GTPase activity and metal binding. Our results provide a stepping-stone toward better understanding the functions of DinoCBWDs and the COG0523 family.

Calcium Determines Lactiplantibacillus plantarum Intraspecies Competitive Fitness

The importance of individual nutrients for microbial strain robustness and coexistence in habitats containing different members of the same species is not well understood. To address this for Lactiplantibacillus plantarum in food fermentations, we performed comparative genomics and examined the nutritive requirements and competitive fitness for L. plantarum strains B1.1 and B1.3 isolated from a single sample of teff injera fermentation batter. Compared to B1.1 and other L. plantarum strains, B1.3 has a smaller genome, limited biosynthetic capacities, and large mobilome. Despite these differences, B1.3 was equally competitive with B1.1 in a suspension of teff flour. In commercially sourced, nutrient-replete MRS (cMRS) medium, strain B1.3 reached 3-fold-higher numbers than B1.1 within 2 days of passage. Because B1.3 growth and competitive fitness were poor in mMRS medium (here called mMRS), a modified MRS medium lacking beef extract, we used mMRS to identify nutrients needed for robust B1.3 growth. No improvement was observed when mMRS was supplemented with nucleotides, amino acids, vitamins, or monovalent metals. Remarkably, the addition of divalent metal salts increased the growth rate and cell yields of B1.3 in mMRS. Metal requirements were confirmed by inductively coupled plasma mass spectrometry, showing that total B1.3 intracellular metal concentrations were significantly (up to 2.7-fold) reduced compared to B1.1. Supplemental CaCl₂ conferred the greatest effect, resulting in equal growth between B1.1 and B1.3 over five successive passages in mMRS. Moreover, calcium supplementation reversed a B1.3 strain-specific, stationary-phase, flocculation phenotype. These findings show how L. plantarum calcium requirements affect competitive fitness at the strain level.

IMPORTANCE Ecological theory states that the struggle for existence is stronger between closely related species. Contrary to this assertion, fermented foods frequently sustain conspecific individuals, in spite of their high levels of phylogenetic relatedness. Therefore, we investigated two isolates of Lactiplantibacillus plantarum, B1.1 and B1.3, randomly selected from a single batch of teff injera batter. These strains spanned the known genomic and phenotypic range of the L. plantarum species, and in laboratory culture medium used for strain screening, B1.3 exhibited poor growth and was outcompeted by the more robust strain B1.1. Nonetheless, B1.1 and B1.3 were equally competitive in teff flour. This result shows how L. plantarum has adapted for coexistence in that environment. The capacity for the single macronutrient calcium to restore B1.3 competitive fitness in laboratory culture medium suggests that L. plantarum intraspecies diversity found in food systems is fine-tuned to nutrient requirements at the strain level.

Biochemical and Metabolomic Responses of Antarctic Bacterium Planococcus sp. O5 Induced by Copper Ion

Heavy metal pollution in the Antarctic has gone beyond our imagination. Copper toxicity is a selective pressure on Planococcus sp. O5. We observed relatively broad tolerance in the polar bacterium. The heavy metal resistance pattern is Pb2+ > Cu2+ > Cd2+ > Hg2+ > Zn2+. In the study, we combined biochemical and metabolomics approaches to investigate the Cu2+ adaptation mechanisms of the Antarctic bacterium. Biochemical analysis revealed that copper treatment elevated the activity of antioxidants and enzymes, maintaining the bacterial redox state balance and normal cell division and growth. Metabolomics analysis demonstrated that fatty acids, amino acids, and carbohydrates played dominant roles in copper stress adaptation. The findings suggested that the adaptive mechanisms of strain O5 to copper stress included protein synthesis and repair, accumulation of organic permeable substances, up-regulation of energy metabolism, and the formation of fatty acids.

Zng1 is a GTP-dependent zinc transferase needed for activation of methionine aminopeptidase

The evolution of zinc (Zn) as a protein cofactor altered the functional landscape of biology, but dependency on Zn also created an Achilles' heel, necessitating adaptive mechanisms to ensure Zn availability to proteins. A debated strategy is whether metallochaperones exist to prioritize essential Zn-dependent proteins. Here, we present evidence for a conserved family of putative metal transferases in human and fungi, which interact with Zn-dependent methionine aminopeptidase type I (MetAP1/Map1p/Fma1). Deletion of the putative metal transferase in Saccharomyces cerevisiae (ZNG1; formerly YNR029c) leads to defective Map1p function and a Zn-deficiency growth defect. In vitro, Zng1p can transfer Zn2+ or Co2+ to apo-Map1p, but unlike characterized copper chaperones, transfer is dependent on GTP hydrolysis. Proteomics reveal mis-regulation of the Zap1p transcription factor regulon because of loss of ZNG1 and Map1p activity, suggesting that Zng1p is required to avoid a compounding effect of Map1p dysfunction on survival during Zn limitation.

Gene-environment interaction analysis of redox-related metals and genetic variants with plasma metabolic patterns in a general population from Spain: The Hortega Study

Background

Limited studies have evaluated the joint influence of redox-related metals and genetic variation on metabolic pathways. We analyzed the association of 11 metals with metabolic patterns, and the interacting role of candidate genetic variants, in 1145 participants from the Hortega Study, a population-based sample from Spain.

Methods

Urine antimony (Sb), arsenic, barium (Ba), cadmium (Cd), chromium (Cr), cobalt (Co), molybdenum (Mo) and vanadium (V), and plasma copper (Cu), selenium (Se) and zinc (Zn) were measured by ICP-MS and AAS, respectively. We summarized 54 plasma metabolites, measured with targeted NMR, by estimating metabolic principal components (mPC). Redox-related SNPs (N = 291) were measured by oligo-ligation assay.

Results

In our study, the association with metabolic principal component (mPC) 1 (reflecting non-essential and essential amino acids, including branched chain, and bacterial co-metabolism versus fatty acids and VLDL subclasses) was positive for Se and Zn, but inverse for Cu, arsenobetaine-corrected arsenic (As) and Sb. The association with mPC2 (reflecting essential amino acids, including aromatic, and bacterial co-metabolism) was inverse for Se, Zn and Cd. The association with mPC3 (reflecting LDL subclasses) was positive for Cu, Se and Zn, but inverse for Co. The association for mPC4 (reflecting HDL subclasses) was positive for Sb, but inverse for plasma Zn. These associations were mainly driven by Cu and Sb for mPC1; Se, Zn and Cd for mPC2; Co, Se and Zn for mPC3; and Zn for mPC4. The most SNP-metal interacting genes were NOX1, GSR, GCLC, AGT and REN. Co and Zn showed the highest number of interactions with genetic variants associated to enriched endocrine, cardiovascular and neurological pathways.

Conclusions

Exposures to Co, Cu, Se, Zn, As, Cd and Sb were associated with several metabolic patterns involved in chronic disease. Carriers of redox-related variants may have differential susceptibility to metabolic alterations associated to excessive exposure to metals.

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In a population-based sample, cobalt, copper, selenium, zinc, arsenic, cadmium and antimony exposures were related to some metabolic patterns.

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Carriers of redox-related variants displayed differential susceptibility to metabolic alterations associated to excessive metal exposures.

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Cobalt and zinc showed a number of statistical interactions with variants from genes sharing biological pathways with a role in chronic diseases.

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The metabolic impact of metals combined with variation in redox-related genes might be large in the population, given metals widespread exposure.

Transcriptomics and Functional Analysis of Copper Stress Response in the Sulfate-Reducing Bacterium Desulfovibrio alaskensis G20

Copper (Cu) is an essential micronutrient required as a co-factor in the catalytic center of many enzymes. However, excess Cu can generate pleiotropic effects in the microbial cell. In addition, leaching of Cu from pipelines results in elevated Cu concentration in the environment, which is of public health concern. Sulfate-reducing bacteria (SRB) have been demonstrated to grow in toxic levels of Cu. However, reports on Cu toxicity towards SRB have primarily focused on the degree of toxicity and subsequent elimination. Here, Cu(II) stress-related effects on a model SRB, Desulfovibrio alaskensis G20, is reported. Cu(II) stress effects were assessed as alterations in the transcriptome through RNA-Seq at varying Cu(II) concentrations (5 µM and 15 µM). In the pairwise comparison of control vs. 5 µM Cu(II), 61.43% of genes were downregulated, and 38.57% were upregulated. In control vs. 15 µM Cu(II), 49.51% of genes were downregulated, and 50.5% were upregulated. The results indicated that the expression of inorganic ion transporters and translation machinery was massively modulated. Moreover, changes in the expression of critical biological processes such as DNA transcription and signal transduction were observed at high Cu(II) concentrations. These results will help us better understand the Cu(II) stress-response mechanism and provide avenues for future research.

Effect of Long-Term and Short-Term Imbalanced Zn Manipulation on Gut Microbiota and Screening for Microbial Markers Sensitive to Zinc Status

Zinc (Zn) imbalance is a common single-nutrient disorder worldwide, but little is known about the short-term and long-term effects of imbalanced dietary zinc in the intestinal microbiome. Here, 3-week-old C57BL/6 mice were fed diets supplemented with Zn at the doses of 0 (low Zn), 30 (control Zn), 150 (high Zn), and 600 mg/kg of body weight (excess Zn) for 4 weeks (short term) and 8 weeks (long term). The gut bacterial composition at the phyla, genus, and species levels were changed as the result of the imbalanced Zn diet (e.g., Lactobacillus reuteri and Akkermansia muciniphila). Moreover, pathways including carbohydrate, glycan, and nucleotide metabolism were decreased by a short-term low-Zn diet. Valeriate production was suppressed by a long-term low-Zn diet. Pathways such as drug resistance and infectious diseases were upregulated in high- and excess-Zn diets over 4-week and 8-week intervals. Long-term zinc fortification doses, especially at the high-Zn level, suppressed the abundance of short-chain fatty acids (SCFAs)-producing genera as well as the concentrations of metabolites. Finally, Melainabacteria (phylum) and Desulfovibrio sp. strain ABHU2SB (species) were identified to be potential markers for Zn status with high accuracy (area under the curve [AUC], >0.8). Collectively, this study identified significant changes in gut microbial composition and its metabolite concentration in altered Zn-fed mice and the relevant microbial markers for Zn status. IMPORTANCE Zn insufficiency is an essential health problem in developing countries. To prevent the occurrence of zinc deficit, zinc fortification and supplementation are widely used. However, in developed countries, the amounts of Zn consumed often exceed the tolerable upper intake limit. Our results demonstrated that dietary Zn is an essential mediator of microbial community structure and that both Zn deficiency and Zn overdose can generate a dysbiosis in the gut microbiota. Moreover, specific microbial biomarkers of Zn status were identified and correlated with serum Zn level. Our study found that a short-term low-Zn diet (0 mg/kg) and a long-term high-zinc diet (150 mg/kg) had obvious negative effects in a mouse model. Thus, these results indicate that the provision and duration of supplemental Zn should be approached with caution.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

A genome-wide screen reveals the involvement of enterobactin-mediated iron acquisition in Escherichia coli survival during copper stress

Copper (Cu) is a key transition metal that is involved in many important biological processes in a cell. Cu is also utilized by the immune system to hamper pathogen growth during infection. However, genome-level knowledge on the mechanisms involved in adaptation to Cu stress is limited. Here, we report the results of a genome-wide reverse genetic screen for Cu-responsive phenotypes in Escherichia coli. Our screen has identified novel genes involved in adaptation to Cu stress in E. coli. We detected multiple genes involved in the biosynthesis and uptake of enterobactin, a siderophore utilized for high-affinity TonB-dependent acquisition of iron (Fe), as critical players in survival under Cu intoxication. We demonstrated the specificity of Cu-dependent killing by chelation of Cu and by genetic complementation of tonB. Notably, TonB is involved in protection from Cu in both laboratory and uropathogenic strains of E. coli. Cu stress leads to increased expression of the genes involved in Fe uptake, indicating that Fur regulon is derepressed during exposure to excess Cu. Trace element analyses revealed that Fe homeostasis is dysregulated during Cu stress. Taken together, our data supports a model in which lack of enterobactin-dependent Fe uptake leads to exacerbation of Cu toxicity, and elucidates the intricate connection between the homeostasis of Cu and Fe in a bacterial cell.

Dual-purpose isocyanides produced by Aspergillus fumigatus contribute to cellular copper sufficiency and exhibit antimicrobial activity

The maintenance of sufficient but nontoxic pools of metal micronutrients is accomplished through diverse homeostasis mechanisms in fungi. Siderophores play a well established role for iron homeostasis; however, no copper-binding analogs have been found in fungi. Here we demonstrate that, in Aspergillus fumigatus, xanthocillin and other isocyanides derived from the xan biosynthetic gene cluster (BGC) bind copper, impact cellular copper content, and have significant metal-dependent antimicrobial properties. xan BGC-derived isocyanides are secreted and bind copper as visualized by a chrome azurol S (CAS) assay, and inductively coupled plasma mass spectrometry analysis of A. fumigatus intracellular copper pools demonstrated a role for xan cluster metabolites in the accumulation of copper. A. fumigatus coculture with a variety of human pathogenic fungi and bacteria established copper-dependent antimicrobial properties of xan BGC metabolites, including inhibition of laccase activity. Remediation of xanthocillin-treated Pseudomonas aeruginosa growth by copper supported the copper-chelating properties of xan BGC isocyanide products. The existence of the xan BGC in several filamentous fungi suggests a heretofore unknown role of eukaryotic natural products in copper homeostasis and mediation of interactions with competing microbes.

Manganese impairs the QoxABCD terminal oxidase leading to respiration-associated toxicity

Cell physiology relies on metalloenzymes and can be easily disrupted by imbalances in metal ion pools. Bacillus subtilis requires manganese for growth and has highly regulated mechanisms for import and efflux that help maintain homeostasis. Cells defective for manganese (Mn) efflux are highly sensitive to intoxication, but the processes impaired by Mn excess are often unknown. Here, we employed a forward genetics approach to identify pathways affected by manganese intoxication. Our results highlight a central role for the membrane-localized electron transport chain in metal intoxication during aerobic growth. In the presence of elevated manganese, there is an increased generation of reactive radical species associated with dysfunction of the major terminal oxidase, the cytochrome aa₃ heme-copper menaquinol oxidase (QoxABCD). Intoxication is suppressed by diversion of menaquinol to alternative oxidases or by a mutation affecting heme A synthesis that is known to convert QoxABCD from an aa₃ to a bo₃ -type oxidase. Manganese sensitivity is also reduced by derepression of the MhqR regulon, which protects cells against reactive quinones. These results suggest that dysfunction of the cytochrome aa₃ -type quinol oxidase contributes to metal-induced intoxication.

Interplay between central carbon metabolism and metal homeostasis in mycobacteria and other human pathogens

Bacterial nutrition is a fundamental aspect of pathogenesis. While the host environment is in principle nutrient-rich, hosts have evolved strategies to interfere with nutrient acquisition by pathogens. In turn, pathogens have developed mechanisms to circumvent these restrictions. Changing the availability of bioavailable metal ions is a common strategy used by hosts to limit bacterial replication. Macrophages and neutrophils withhold iron, manganese, and zinc ions to starve bacteria. Alternatively, they can release manganese, zinc, and copper ions to intoxicate microorganisms. Metals are essential micronutrients and participate in catalysis, macromolecular structure, and signalling. This review summarises our current understanding of how central carbon metabolism in pathogens adapts to local fluctuations in free metal ion concentrations. We focus on the transcriptomics and proteomics data produced in studies of the iron-sparing response in Mycobacterium tuberculosis, the etiological agent of tuberculosis, and consequently generate a hypothetical model linking trehalose accumulation, succinate secretion and substrate-level phosphorylation in iron-starved M. tuberculosis. This review also aims to highlight a large gap in our knowledge of pathogen physiology: the interplay between metal homeostasis and central carbon metabolism, two cellular processes which are usually studied separately. Integrating metabolism and metal biology would allow the discovery of new weaknesses in bacterial physiology, leading to the development of novel and improved antibacterial therapies.

Synthetic biology approaches to copper remediation: bioleaching, accumulation and recycling

One of the current aims of synthetic biology is the development of novel microorganisms that can mine economically important elements from the environment or remediate toxic waste compounds. Copper, in particular, is a high-priority target for bioremediation owing to its extensive use in the food, metal and electronic industries and its resulting common presence as an environmental pollutant. Even though microbe-aided copper biomining is a mature technology, its application to waste treatment and remediation of contaminated sites still requires further research and development. Crucially, any engineered copper-remediating chassis must survive in copper-rich environments and adapt to copper toxicity; they also require bespoke adaptations to specifically extract copper and safely accumulate it as a human-recoverable deposit to enable biorecycling. Here, we review current strategies in copper bioremediation, biomining and biorecycling, as well as strategies that extant bacteria use to enhance copper tolerance, accumulation and mineralization in the native environment. By describing the existing toolbox of copper homeostasis proteins from naturally occurring bacteria, we show how these modular systems can be exploited through synthetic biology to enhance the properties of engineered microbes for biotechnological copper recovery applications.

Increasing the copper sensitivity of microorganisms by restricting iron supply, a strategy for bio-management practices

Pollution by copper (Cu2+ ) extensively used as antimicrobial in agriculture and farming represents a threat to the environment and human health. Finding ways to make microorganisms sensitive to lower metal concentrations could help decreasing the use of Cu2 + in agriculture. In this respect, we showed that limiting iron (Fe) uptake makes bacteria much more susceptible to Cu2 + or Cd2+ poisoning. Using efflux mutants of the purple bacterium Rubrivivax gelatinosus, we showed that Cu+ and Cd2+ resistance relies on the expression of the Fur-regulated FbpABC and Ftr iron transporters. To support this conclusion, inactivation of these Fe-importers in the Cu+ or Cd2+ -ATPase efflux mutants gave rise to hypersensitivity towards these ions. Moreover, in metal overloaded cells the expression of FbpA, the periplasmic iron-binding component of the ferric ion transport FbpABC system was induced, suggesting that cells perceived an 'iron-starvation' situation and responded to it by inducing Fe-importers. In this context, the Fe-Sod activity increased in response to Fe homoeostasis dysregulation. Similar results were obtained for Vibrio cholerae and Escherichia coli, suggesting that perturbation of Fe-homoeostasis by metal excess appeared as an adaptive response commonly used by a variety of bacteria. The presented data support a model in which metal excess induces Fe-uptake to support [4Fe-4S] synthesis and thereby induce ROS detoxification system.

Additive effects of metal excess and superoxide, a highly toxic mixture in bacteria

Heavy metal contamination is a serious environmental problem. Understanding the toxicity mechanisms may allow to lower concentration of metals in the metal-based antimicrobial treatments of crops, and reduce metal content in soil and groundwater. Here, we investigate the interplay between metal efflux systems and the superoxide dismutase (SOD) in the purple bacterium Rubrivivax gelatinosus and other bacteria through analysis of the impact of metal accumulation. Exposure of the Cd2+ -efflux mutant ΔcadA to Cd2+ caused an increase in the amount and activity of the cytosolic Fe-Sod SodB, thereby suggesting a role of SodB in the protection against Cd2+ . In support of this conclusion, inactivation of sodB gene in the ΔcadA cells alleviated detoxification of superoxide and enhanced Cd2+ toxicity. Similar findings were described in the Cu+ -efflux mutant with Cu+ . Induction of the Mn-Sod or Fe-Sod in response to metals in other bacteria, including Escherichia coli, Pseudomonas aeruginosa, Pseudomonas putida, Vibrio cholera and Bacillus subtilis, was also shown. Both excess Cd2+ or Cu+ and superoxide can damage [4Fe-4S] clusters. The additive effect of metal and superoxide on the [4Fe-4S] could therefore explain the hypersensitive phenotype in mutants lacking SOD and the efflux ATPase. These findings underscore that ROS defence system becomes decisive for bacterial survival under metal excess.

Cadmium and Copper Cross-Tolerance. Cu+ Alleviates Cd2 + Toxicity, and Both Cations Target Heme and Chlorophyll Biosynthesis Pathway in Rubrivivax gelatinosus

Cadmium, although not redox active is highly toxic. Yet, the underlying mechanisms driving toxicity are still to be characterized. In this study, we took advantage of the purple bacterium Rubrivivax gelatinosus strain with defective Cd^{2 +}-efflux system to identify targets of this metal. Exposure of the ΔcadA strain to Cd^{2 +} causes a decrease in the photosystem amount and in the activity of respiratory complexes. As in case of Cu⁺ toxicity, the data indicated that Cd^{2 +} targets the porphyrin biosynthesis pathway at the level of HemN, a S-adenosylmethionine and CxxxCxxC coordinated [4Fe-4S] containing enzyme. Cd^{2 +} exposure therefore results in a deficiency in heme and chlorophyll dependent proteins and metabolic pathways. Given the importance of porphyrin biosynthesis, HemN represents a key metal target to account for toxicity. In the environment, microorganisms are exposed to mixture of metals. Nevertheless, the biological effects of such mixtures, and the toxicity mechanisms remain poorly addressed. To highlight a potential cross-talk between Cd^{2 +} and Cu⁺ -efflux systems, we show (i) that Cd^{2 +} induces the expression of the Cd^{2 +}-efflux pump CadA and the Cu⁺ detoxification system CopA and CopI; and (ii) that Cu⁺ ions improve tolerance towards Cd^{2 +}, demonstrating thus that metal mixtures could also represent a selective advantage in the environment.

Copper stress in Staphylococcus aureus leads to adaptive changes in central carbon metabolism

Copper toxicity has been a long-term selection pressure on bacteria due to its presence in the environment and its use as an antimicrobial agent by grazing protozoa, by phagocytic cells of the immune system, and in man-made medical and commercial products. There is recent evidence that exposure to increased copper stress may have been a key driver in the evolution and spread of methicillin-resistant Staphylococcus aureus, a globally important pathogen that causes significant mortality and morbidity worldwide. Yet it is unclear how S. aureus physiology is affected by copper stress or how it adapts in order to be able to grow in the presence of excess copper. Here, we have determined quantitatively how S. aureus alters its proteome during growth under copper stress conditions, comparing this adaptive response in two different types of growth regime. We found that the adaptive response involves induction of the conserved copper detoxification system as well as induction of enzymes of central carbon metabolism, with only limited induction of proteins involved in the oxidative stress response. Further, we identified a protein that binds copper inside S. aureus cells when stressed by copper excess. This copper-binding enzyme, a glyceraldehyde-3-phosphate dehydrogenase essential for glycolysis, is inhibited by copper in vitro and inside S. aureus cells. Together, our data demonstrate that copper stress leads to the inhibition of glycolysis in S. aureus, and that the bacterium adapts to this stress by altering its central carbon utilisation pathways.

Comparative differential cuproproteomes of Rhodobacter capsulatus reveal novel copper homeostasis related proteins

Copper (Cu) is an essential, but toxic, micronutrient for living organisms and cells have developed sophisticated response mechanisms towards both the lack and the excess of Cu in their environments. In this study, we achieved a global view of Cu-responsive changes in the prokaryotic model organism Rhodobacter capsulatus using label-free quantitative differential proteomics. Semi-aerobically grown cells under heterotrophic conditions in minimal medium (∼0.3 μM Cu) were compared with cells supplemented with either 5 μM Cu or with 5 mM of the Cu-chelator bathocuproine sulfonate. Mass spectrometry based bottom-up proteomics of unfractionated cell lysates identified 2430 of the 3632 putative proteins encoded by the genome, producing a robust proteome dataset for R. capsulatus. Use of biological and technical replicates for each growth condition yielded high reproducibility and reliable quantification for 1926 of the identified proteins. Comparison of cells grown under Cu-excess or Cu-depleted conditions to those grown under minimal Cu-sufficient conditions revealed that 75 proteins exhibited statistically significant (p < 0.05) abundance changes, ranging from 2- to 300-fold. A subset of the highly Cu-responsive proteins was orthogonally probed using molecular genetics, validating that several of them were indeed involved in cellular Cu homeostasis.

Cobalt can fully recover the phenotypes related to zinc deficiency in Salmonella Typhimurium

Cobalt is an essential element for living systems, which, however, make very limited use of this metal, using it mainly in cobalamin-containing enzymes. The reduced use of cobalt compared to other transition metals is generally attributed to the potential toxicity of this element. In this work, we demonstrate that cobalt not only does not have an obvious toxic effect on Salmonella Typhimurium, but that it can efficiently compensate for zinc deficiency in a znuABC deleted strain. In fact, cobalt, but not cobalamin supplementation, rescued all major phenotypic defects of the znuABC strain, including the reduced ability to grow and swim in zinc-deficient media and the high susceptibility to hydrogen peroxide stress. Growth in a cobalt-supplemented defined medium led to the accumulation of large amounts of cobalt both in the wild type and in the znuABC strain. These data suggest that atoms of cobalt may be incorporated in bacterial proteins in place of zinc, ensuring their functionality. In support of this hypothesis we have shown that, in vivo, cobalt can accumulate in ribosomes and replace zinc in a periplasmic Cu,Zn superoxide dismutase (SodCII). Finally, we provide evidence of the ability of cobalt to modulate the intracellular concentration of zinc-regulated proteins (ZnuA, ZinT, and SodCII). Although some observations suggest that in some proteins the replacement of zinc with cobalt can lead to subtle structural changes, the data reported in this study indicate that Salmonella has the ability to use cobalt instead of zinc, without evident harmful effects for cell physiology.

Sensitivity to the two peptide bacteriocin plantaricin EF is dependent on CorC, a membrane-bound, magnesium/cobalt efflux protein

Lactic acid bacteria produce a variety of antimicrobial peptides known as bacteriocins. Most bacteriocins are understood to kill sensitive bacteria through receptor‐mediated disruptions. Here, we report on the identification of the Lactobacillus plantarum plantaricin EF (PlnEF) receptor. Spontaneous PlnEF‐resistant mutants of the PlnEF‐indicator strain L. plantarum NCIMB 700965 (LP965) were isolated and confirmed to maintain cellular ATP levels in the presence of PlnEF. Genome comparisons resulted in the identification of a single mutated gene annotated as the membrane‐bound, magnesium/cobalt efflux protein CorC. All isolates contained a valine (V) at position 334 instead of a glycine (G) in a cysteine‐β‐synthase domain at the C‐terminal region of CorC. In silico template‐based modeling of this domain indicated that the mutation resides in a loop between two β‐strands. The relationship between PlnEF, CorC, and metal homeostasis was supported by the finding that PlnEF‐resistance was lost when PlnEF was applied together with high concentrations of Mg²⁺, Co²⁺, Zn²⁺, or Cu²⁺. Lastly, PlnEF sensitivity was increased upon heterologous expression of LP965 corC but not the G334V CorC mutant in the PlnEF‐resistant strain Lactobacillus casei BL23. These results show that PlnEF kills sensitive bacteria by targeting CorC.

Functional Status of Neuronal Calcium Sensor-1 Is Modulated by Zinc Binding

Neuronal calcium sensor-1 (NCS-1) protein is abundantly expressed in the central nervous system and retinal neurons, where it regulates many vital processes such as synaptic transmission. It coordinates three calcium ions by EF-hands 2-4, thereby transducing Ca2+ signals to a wide range of protein targets, including G protein-coupled receptors and their kinases. Here, we demonstrate that NCS-1 also has Zn2+-binding sites, which affect its structural and functional properties upon filling. Fluorescence and circular dichroism experiments reveal the impact of Zn2+ binding on NCS-1 secondary and tertiary structure. According to atomic absorption spectroscopy and isothermal titration calorimetry studies, apo-NCS-1 has two high-affinity (4 × 106 M-1) and one low-affinity (2 × 105 M-1) Zn2+-binding sites, whereas Mg2+-loaded and Ca2+-loaded forms (which dominate under physiological conditions) bind two zinc ions with submicromolar affinity. Metal competition analysis and circular dichroism studies suggest that Zn2+-binding sites of apo- and Mg2+-loaded NCS-1 overlap with functional EF-hands of the protein. Consistently, high Zn2+ concentrations displace Mg2+ from the EF-hands and decrease the stoichiometry of Ca2+ binding. Meanwhile, one of the EF-hands of Zn2+-saturated NCS-1 exhibits a 14-fold higher calcium affinity, which increases the overall calcium sensitivity of the protein. Based on QM/MM molecular dynamics simulations, Zn2+ binding to Ca2+-loaded NCS-1 could occur at EF-hands 2 and 4. The high-affinity zinc binding increases the thermal stability of Ca2+-free NCS-1 and favours the interaction of its Ca2+-loaded form with target proteins, such as dopamine receptor D2R and GRK1. In contrast, low-affinity zinc binding promotes NCS-1 aggregation accompanied by the formation of twisted rope-like structures. Altogether, our findings suggest a complex interplay between magnesium, calcium and zinc binding to NCS-1, leading to the appearance of multiple conformations of the protein, in turn modulating its functional status.

Silver and Copper Acute Effects on Membrane Proteins and Impact on Photosynthetic and Respiratory Complexes in Bacteria

The use of metal ions represents a serious threat to the environment and to all living organisms because of the acute toxicity of these ions. Nowadays, silver nanoparticles are one of the most widely used nanoparticles in various industrial and health applications. The antimicrobial effect of nanoparticles is in part related to the released Ag⁺ ions and their ability to interact with bacterial membranes. Here, we identify, both in vitro and in vivo, specific targets of Ag⁺ ions within the membrane of bacteria. This include complexes involved in photosynthesis, but also complexes involved in respiration.

Silver (Ag⁺) and copper (Cu⁺) ions have been used for centuries in industry, as well as antimicrobial agents in agriculture and health care. Nowadays, Ag⁺ is also widely used in the field of nanotechnology. Yet, the underlying mechanisms driving toxicity of Ag⁺ ions in vivo are poorly characterized. It is well known that exposure to excess metal impairs the photosynthetic apparatus of plants and algae. Here, we show that the light-harvesting complex II (LH2) is the primary target of Ag⁺ and Cu⁺ exposure in the purple bacterium Rubrivivax gelatinosus. Ag⁺ and Cu⁺ specifically inactivate the 800-nm absorbing bacteriochlorophyll a (B800), while Ni²⁺ or Cd²⁺ treatment had no effect. This was further supported by analyses of CuSO₄- or AgNO₃-treated membrane proteins. Indeed, this treatment induced changes in the LH2 absorption spectrum related to the disruption of the interaction of B800 molecules with the LH2 protein. This caused the release of B800 molecules and subsequently impacted the spectral properties of the carotenoids within the 850-nm absorbing LH2. Moreover, previous studies have suggested that Ag⁺ can affect the respiratory chain in mitochondria and bacteria. Our data demonstrated that exposure to Ag⁺, both in vivo and in vitro, caused a decrease of cytochrome c oxidase and succinate dehydrogenase activities. Ag⁺ inhibition of these respiratory complexes was also observed in Escherichia coli, but not in Bacillus subtilis.

The role of zinc and nutritional immunity in Clostridium difficile infection

Clostridium difficile in one of the most commonly reported nosocomial pathogens worldwide. Beyond antibiotic use, little is known about the host, microbiota, and environmental factors that contribute to susceptibility to and severity of C. difficile infection (CDI). We recently observed that in a mouse model of CDI, excess dietary zinc (Zn) alters the gut microbiota and decreases resistance to CDI. Moreover, we determined that high levels of Zn exacerbate C. difficile-associated disease and calprotectin-mediated Zn limitation is an essential host response to infection. In this addendum, we discuss how these findings add to our understanding of CDI and consider the potential implications of excess metal intake on the microbiota and infection.

Widespread Distribution and Functional Specificity of the Copper Importer CcoA: Distinct Cu Uptake Routes for Bacterial Cytochrome c Oxidases

Cytochrome c oxidases are members of the heme-copper oxidase superfamily. These enzymes have different subunits, cofactors, and primary electron acceptors, yet they all contain identical heme-copper (Cu_B) binuclear centers within their catalytic subunits. The uptake and delivery pathways of the Cu_B atom incorporated into this active site, where oxygen is reduced to water, are not well understood. Our previous work with the facultative phototrophic bacterium Rhodobacter capsulatus indicated that the copper atom needed for the Cu_B site of cbb₃-type cytochrome c oxidase (cbb₃-Cox) is imported to the cytoplasm by a major facilitator superfamily-type transporter, CcoA. In this study, a comparative genomic analysis of CcoA orthologs in alphaproteobacterial genomes showed that CcoA is widespread among organisms and frequently co-occurs with cytochrome c oxidases. To define the specificity of CcoA activity, we investigated its function in Rhodobacter sphaeroides, a close relative of R. capsulatus that contains both cbb₃- and aa₃-Cox. Phenotypic, genetic, and biochemical characterization of mutants lacking CcoA showed that in its absence, or even in the presence of its bypass suppressors, only the production of cbb₃-Cox and not that of aa₃-Cox was affected. We therefore concluded that CcoA is dedicated solely to cbb₃-Cox biogenesis, establishing that distinct copper uptake systems provide the Cu_B atoms to the catalytic sites of these two similar cytochrome c oxidases. These findings illustrate the large variety of strategies that organisms employ to ensure homeostasis and fine control of copper trafficking and delivery to the target cuproproteins under different physiological conditions.IMPORTANCE The cbb₃- and aa₃-type cytochrome c oxidases belong to the widespread heme-copper oxidase superfamily. They are membrane-integral cuproproteins that catalyze oxygen reduction to water under hypoxic and normoxic growth conditions. These enzymes diverge in terms of subunit and cofactor composition, yet they all share a conserved heme-copper binuclear site within their catalytic subunit. In this study, we show that the copper atoms of the catalytic center of two similar cytochrome c oxidases from this superfamily are provided by different copper uptake systems during their biogenesis. This finding illustrates different strategies by which organisms fine-tune the trafficking of copper, which is an essential but toxic micronutrient.

Ferrous iron efflux systems in bacteria

Bacteria require iron for growth, with only a few reported exceptions. In many environments, iron is a limiting nutrient for growth and high affinity uptake systems play a central role in iron homeostasis. However, iron can also be detrimental to cells when it is present in excess, particularly under aerobic conditions where its participation in Fenton chemistry generates highly reactive hydroxyl radicals. Recent results have revealed a critical role for iron efflux transporters in protecting bacteria from iron intoxication. Systems that efflux iron are widely distributed amongst bacteria and fall into several categories: P1B-type ATPases, cation diffusion facilitator (CDF) proteins, major facilitator superfamily (MFS) proteins, and membrane bound ferritin-like proteins. Here, we review the emerging role of iron export in both iron homeostasis and as part of the adaptive response to oxidative stress.