Comparative Genomics of Trace Elements: Emerging Dynamic View of Trace Element Utilization and Function

Organoselenium Compounds in Medicinal Chemistry

The chemical and biological interest in this element and the molecules bearing selenium has been exponentially growing over the years. Selenium, formerly designated as a toxin, becomes a vital trace element for life that appears as selenocysteine and its dimeric form, selenocystine, in the active sites of selenoproteins, which catalyze a wide variety of reactions, including the detoxification of reactive oxygen species and modulation of redox activities. From the point of view of drug developments, organoselenium drugs are isosteres of sulfur-containing and oxygen-containing drugs with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. This statement is the paramount relevance considering the big number of clinically employed compounds bearing sulfur or oxygen atoms in their structures including nucleosides and carbohydrates. Thus, in this article we have focused on the relevant features of the application of selenium in medicinal chemistry. With the increasing interest in selenium chemistry, we have attempted to highlight the most significant published data on this subject, mainly concentrating the analysis on the last years. In consequence, the recent advances of relevant pharmacological organoselenium compounds are discussed.

Transcriptome Analysis Reveals the Mechanism of Exogenous Selenium in Alleviating Cadmium Stress in Purple Flowering Stalks (Brassica campestris var. purpuraria)

In China, cadmium (Cd) stress has a significant role in limiting the development and productivity of purple flowering stalks (Brassica campestris var. purpuraria). Exogenous selenium supplementation has been demonstrated in earlier research to mitigate the effects of Cd stress in a range of plant species; nevertheless, the physiological and molecular processes by which exogenous selenium increases vegetable shoots' resistance to Cd stress remain unclear. Purple flowering stalks (Brassica campestris var. purpuraria) were chosen as the study subject to examine the effects of treatment with sodium selenite (Na2SeO3) on the physiology and transcriptome alterations of cadmium stress. Purple flowering stalk leaves treated with exogenous selenium had higher glutathione content, photosynthetic capacity, and antioxidant enzyme activities compared to the leaves treated with Cd stress alone. Conversely, the contents of proline, soluble proteins, soluble sugars, malondialdehyde, and intercellular CO2 concentration tended to decrease. Transcriptome analysis revealed that 2643 differentially expressed genes (DEGs) were implicated in the response of exogenous selenium treatment to Cd stress. The metabolic pathways associated with flavonoid production, carotenoid synthesis, glutathione metabolism, and glucosinolate biosynthesis were among those enriched in these differentially expressed genes. Furthermore, we discovered DEGs connected to the production route of glucosinolates. This work sheds fresh light on how purple flowering stalks' tolerance to cadmium stress is improved by exogenous selenium.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

The ionome and proteome landscape of aging in laying hens and relation to egg white quality

The ionome is essential for maintaining body function and health status by participating in diverse key biological processes. Nevertheless, the distribution and utilization of ionome among different organs and how aging impacts the ionome leading to a decline in egg white quality remain unknown. Thus, we used inductively coupled plasma mass spectrometry (ICP-MS) to analyze 35 elements and their isotopic contents in eight organs of laying hens at 35, 72, and 100 weeks. Moreover, the magnum proteome, amino acids in egg white, and egg white quality were analyzed in laying hens at three different ages using 4D proteomics techniques, an amino acid analyzer, and an egg quality analyzer. Across the organs, we identified varying distribution patterns among macroelements (Mg24, Ca43/44, K39, and P31), transition metals (Zn64/66, Cu63/65, Fe56/57, and Mn55), and toxic elements (Pb208, Ba137, and Sr86). We observed an organ-specific aging pattern characterized by the accumulation of toxic elements (Pb208, Ba137, and Sr86) and calcification in the small intestine. Additionally, a decrease in the utilization of essential trace elements selenium (Se78/82) and manganese (Mn55) was noted in the oviduct. By analyzing ionome in tandem with egg quality, egg white amino acids, and proteome, we unveiled that the reduction of selenium and manganese concentrations in the magnum during the aging process affected amino acid metabolism, particularly tryptophan metabolism, thereby inhibiting the amino acid synthesis in the magnum. Furthermore, it accelerated the senescence of magnum cells through necroptosis activation, leading to a decline in the albumen secretion function of the magnum and subsequently reducing egg white quality. Overall, this study provides insights into the evolution of 35 elements and their isotopes across 8 organs of laying hens with age. It also reveals the elemental composition, interactions, and utilization patterns of these organs, as well as their correlation with egg white quality. The present study highlights the significance of ionome and offers a comprehensive perspective on the selection of ionome for regulating the aging of laying hens.

pH-Responsive Selenium Nanoplatform for Highly Efficient Cancer Starvation Therapy by Atorvastatin Delivery

Recently, starvation-inducing nutrient deprivation has been regarded as a promising strategy for tumor suppression. As a first-line lipid-lowering drug, atorvastatin (ATV) significantly reduces caloric intake, suggesting its potential in starvation therapy for suppressing tumors. Accordingly, we developed a novel starvation therapy agent (HA-Se-ATV) in this study to suppress tumor growth by using hyaluronic acid (HA)-conjugated chitosan polymer-coated nano-selenium (Se) for loading ATV. HA-Se-ATV targets cancer cells, following which it effectively accumulates in the tumor tissue. The HA-Se-ATV nanoplatform was then activated by inducing a weakly acidic tumor microenvironment and subsequently releasing ATV. ATV and Se synergistically downregulate the levels of cellular adenosine triphosphate while inhibiting the expression of thioredoxin reductase 1. Consequently, the starvation-stress reaction of cancer cells is significantly elevated, leading to cancer cell death. Furthermore, the in vivo results indicate that HA-Se-ATV effectively suppresses tumor growth with a low level of toxicity, demonstrating its great potential for clinical translation.

Illumina RNA and SMRT Sequencing Reveals the Mechanism of Uptake and Transformation of Selenium Nanoparticles in Soybean Seedlings

Selenium (Se) is an essential element for mammals, and its deficiency in the diet is a global problem. Agronomic biofortification through exogenous Se provides a valuable strategy to enhance human Se intake. Selenium nanoparticles (SeNPs) have been regarded to be higher bioavailability and less toxicity in comparison with selenite and selenate. Still, little has been known about the mechanism of their metabolism in plants. Soybean (Glycine max L.) can enrich Se, providing an ideal carrier for Se biofortification. In this study, soybean sprouts were treated with SeNPs, and a combination of next-generation sequencing (NGS) and single-molecule real-time (SMRT) sequencing was applied to clarify the underlying molecular mechanism of SeNPs metabolism. A total of 74,662 nonredundant transcripts were obtained, and 2109 transcription factors, 9687 alternative splice events, and 3309 long non-coding RNAs (lncRNAs) were predicted, respectively. KEGG enrichment analysis of the DEGs revealed that metabolic pathways, biosynthesis of secondary metabolites, and peroxisome were most enriched both in roots and leaves after exposure to SeNPs. A total of 117 transcripts were identified to be putatively involved in SeNPs transport and biotransformation in soybean. The top six hub genes and their closely coexpressed Se metabolism-related genes, such as adenylylsulfate reductase (APR3), methionine-tRNA ligase (SYM), and chloroplastic Nifs-like cysteine desulfurases (CNIF1), were screened by WGCNA and identified to play crucial roles in SeNPs accumulation and tolerance in soybean. Finally, a putative metabolism pathway of SeNPs in soybean was proposed. These findings have provided a theoretical foundation for future elucidation of the mechanism of SeNPs metabolism in plants.

Selenium: a potent regulator of ferroptosis and biomass production

Emerging evidence supports the notion that selenium (Se) plays a beneficial role in plant development for modern crop production and is considered an essential micronutrient and the predominant source of plants. However, the essential role of selenium in plant metabolism remains unclear. When used in moderate concentrations, selenium promotes plant physiological processes such as enhancing plant growth, increasing antioxidant capacity, reducing reactive oxygen species and lipid peroxidation and offering stress resistance by preventing ferroptosis cell death. Ferroptosis, a recently discovered mechanism of regulated cell death (RCD) with unique features such as iron-dependant accumulation of lipid peroxides, is distinctly different from other known forms of cell death. Glutathione peroxidase (GPX) activity plays a significant role in scavenging the toxic by-products of lipid peroxidation in plants. A low level of GPX activity in plants causes high oxidative stress, which leads to ferroptosis. An integrated view of ferroptosis and selenium in plants and the selenium-mediated nanofertilizers (SeNPs) have been discussed in more recent studies. For instance, selenium supplementation enhanced GPX4 expression and increased TFH cell (Follicular helper T) numbers and the gene transcriptional program, which prevent lipid peroxidase and protect cells from ferroptosis. However, though ferroptosis in plants is similar to that in animals, only few studies have focused on plant-specific ferroptosis; the research on ferroptosis in plants is still in its infancy. Understanding the implication of selenium with relevance to ferroptosis is indispensable for plant bioresource technology. In this review, we hypothesize that blocking ferroptosis cell death improves plant immunity and protects plants from abiotic and biotic stresses. We also examine how SeNPs can be the basis for emerging unconventional and advanced technologies for algae/bamboo biomass production. For instance, algae treated with SeNPs accumulate high lipid profile in algal cells that could thence be used for biodiesel production. We also suggest that further studies in the field of SeNPs are essential for the successful application of this technology for the large-scale production of plant biomass.

Function of Molybdenum Insertases

For most organisms molybdenum is essential for life as it is found in the active site of various vitally important molybdenum dependent enzymes (Mo-enzymes). Here, molybdenum is bound to a pterin derivative called molybdopterin (MPT), thus forming the molybdenum cofactor (Moco). Synthesis of Moco involves the consecutive action of numerous enzymatic reaction steps, whereby molybdenum insertases (Mo-insertases) catalyze the final maturation step, i.e., the metal insertion reaction yielding Moco. This final maturation step is subdivided into two partial reactions, each catalyzed by a distinctive Mo-insertase domain. Initially, MPT is adenylylated by the Mo-insertase G-domain, yielding MPT-AMP which is used as substrate by the E-domain. This domain catalyzes the insertion of molybdate into the MPT dithiolene moiety, leading to the formation of Moco-AMP. Finally, the Moco-AMP phosphoanhydride bond is cleaved by the E-domain to liberate Moco from its synthesizing enzyme. Thus formed, Moco is physiologically active and may be incorporated into the different Mo-enzymes or bind to carrier proteins instead.

Structural Bioinformatics and Deep Learning of Metalloproteins: Recent Advances and Applications

All living organisms require metal ions for their energy production and metabolic and biosynthetic processes. Within cells, the metal ions involved in the formation of adducts interact with metabolites and macromolecules (proteins and nucleic acids). The proteins that require binding to one or more metal ions in order to be able to carry out their physiological function are called metalloproteins. About one third of all protein structures in the Protein Data Bank involve metalloproteins. Over the past few years there has been tremendous progress in the number of computational tools and techniques making use of 3D structural information to support the investigation of metalloproteins. This trend has been boosted by the successful applications of neural networks and machine/deep learning approaches in molecular and structural biology at large. In this review, we discuss recent advances in the development and availability of resources dealing with metalloproteins from a structure-based perspective. We start by addressing tools for the prediction of metal-binding sites (MBSs) using structural information on apo-proteins. Then, we provide an overview of the methods for and lessons learned from the structural comparison of MBSs in a fold-independent manner. We then move to describing databases of metalloprotein/MBS structures. Finally, we summarizing recent ML/DL applications enhancing the functional interpretation of metalloprotein structures.

Mechanisms Affecting the Biosynthesis and Incorporation Rate of Selenocysteine

Selenocysteine (Sec) is the 21st non-standard proteinogenic amino acid. Due to the particularity of the codon encoding Sec, the selenoprotein synthesis needs to be completed by unique mechanisms in specific biological systems. In this paper, the underlying mechanisms for the biosynthesis and incorporation of Sec into selenoprotein were comprehensively reviewed on five aspects: (i) the specific biosynthesis mechanism of Sec and the role of its internal influencing factors (SelA, SelB, SelC, SelD, SPS2 and PSTK); (ii) the elements (SECIS, PSL, SPUR and RF) on mRNA and their functional mechanisms; (iii) the specificity (either translation termination or translation into Sec) of UGA; (iv) the structure–activity relationship and action mechanism of SelA, SelB, SelC and SelD; and (v) the operating mechanism of two key enzyme systems for inorganic selenium source flow before Sec synthesis. Lastly, the size of the translation initiation interval, other action modes of SECIS and effects of REPS (Repetitive Extragenic Palindromic Sequences) that affect the incorporation efficiency of Sec was also discussed to provide scientific basis for the large-scale industrial fermentation for the production of selenoprotein.

Some Biogeochemical Characteristics of the Trace Element Bioaccumulation in the Benthic Fauna of the Piip Volcano (The Southwestern Bering Sea)

The Piip Volcano is a submarine volcanic edifice occupying the central part of the Volcanologists Massif in the southwestern Bering Sea, with two tops, southern and northern. The minimum depth of the northern top is located at 368 m, and of the southern at 464 m. Active hydrothermal venting occurring at both summits of the volcano supports diverse biological communities, including animals specific for chemosynthetic habitats. In benthic organisms inhabiting the northern and southern tops of the Piip Volcano, for the first time, we examined distribution patterns of the following trace elements: titanium, vanadium, chromium, manganese, iron, nickel, copper, zinc, arsenic, selenium, zirconium, molybdenum, silver, cadmium, antimony, barium, tungsten, lead, bismuth, and uranium. The element contents were quantified by the ICP-MS. Total carbon (TC) and total inorganic carbon (TIC) were determined using a Shimadzu TOC-L-CPN and mineral composition of sediment was determined using the XRD. In the water of the biotope from the northern top, concentrations of Mn, Zn, Ag, Cd, Sb, W, Pb were 2–6 times, and Ba was 50 times higher than those from the southern top. This was attributed to the lower temperature of fluids emanating at the southern top. An abundant population of Calyptogena pacifica (Bivalvia: Vesicomyidae: Pliocardiinae) was found only at the southern top. The main target of most trace elements, such as Fe, V, Cr, Co, Ni, Zn, As, Mo, Ag, Cd, W, Pb, Bi, and U were the soft parts of Calyptogena pacifica (with high TOC content, on average 53.1% in gills and 49.6% in the rest of the body). Gills were characterized by particular high contents (>100 µg g−1 dry w.) of Zn, Cd, Fe, Ni, Cu, and Pb, which can form sulphides or be associated with them. Shells of C. pacifica, as well as Brachiopoda, were depleted in these elements, as well as tissues of the carnivores Paguridae (Crustacea) and Actiniaria (Anthozoa). In suspension feeders from both tops, the lower contents of most elements were detected. Estimation of Biological Concentration Factor (BCF) for most elements varied from 102 to 104, reaching n105 for Ni, Zn, Ag, Cd, and Pb. A significant difference in BCF values between Fe and Mn was revealed.

Comparative Phytochemical Profile and Biological Activity of Four Major Medicinal Halophytes from Qassim Flora

Four halophytic plants, Lycium shawii, Anabasis articulata, Rumex vesicarius, and Zilla spinosa, growing in the central Qassim area, Saudi Arabia, were phytochemically and biologically investigated. Their hydroalcoholic extracts’ UPLC-ESIQ-TOF analyses demonstrated the presence of 44 compounds of phenolic acids, flavonoids, saponins, carbohydrates, and fatty acids chemical classes. Among all the plants’ extracts, L. shawii showed the highest quantities of total phenolics, and flavonoids contents (52.72 and 13.01 mg/gm of the gallic acid and quercetin equivalents, respectively), along with the antioxidant activity in the TAA (total antioxidant activity), FRAP (ferric reducing antioxidant power), and DPPH-SA (2,2-diphenyl-1-picryl-hydrazyl-scavenging activity) assays with 25.6, 56.68, and 19.76 mg/gm, respectively, as Trolox equivalents. The hydroalcoholic extract of the L. shawii also demonstrated the best chelating activity at 21.84 mg/gm EDTA equivalents. Among all the four halophytes, the hydroalcoholic extract of L. shawii exhibited the highest antiproliferative activity against MCF7 and K562 cell lines with IC₅₀ values at 194.5 µg/mL and 464.9 µg/mL, respectively. The hydroalcoholic extract of A. articulata demonstrated better cytotoxic activity amongst all the tested plants’ extracts against the human pancreatic cancer cell lines (PANC1) with an IC₅₀ value of 998.5 µg/mL. The L. shawii induced apoptosis in the MCF7 cell lines, and the percentage of the necrotic cells changed to 28.1% and 36.5% for the IC₅₀ and double-IC₅₀ values at 22.9% compared with the untreated groups. The hydroalcoholic extract of L. shawii showed substantial antibacterial activity against Bacillus cereus ATCC 10876 with a MIC value of 12.5 mg/mL. By contrast, the A. articulata and Z. spinosa exhibited antifungal activities against Aspergillus niger ATCC 6275 with MIC values at 12.5 and 50 mg/mL, respectively. These findings suggested that the L. shawii is a potential halophyte with remarkable biological properties, attributed to its contents of phenolics and flavonoid classes of compounds in its extract.

Selenium in Human Health and Gut Microflora: Bioavailability of Selenocompounds and Relationship With Diseases

This review covers current knowledge of selenium in the dietary intake, its bioavailability, metabolism, functions, biomarkers, supplementation and toxicity, as well as its relationship with diseases and gut microbiota specifically on the symbiotic relationship between gut microflora and selenium status. Selenium is essential for the maintenance of the immune system, conversion of thyroid hormones, protection against the harmful action of heavy metals and xenobiotics as well as for the reduction of the risk of chronic diseases. Selenium is able to balance the microbial flora avoiding health damage associated with dysbiosis. Experimental studies have shown that inorganic and organic selenocompounds are metabolized to selenomethionine and incorporated by bacteria from the gut microflora, therefore highlighting their role in improving the bioavailability of selenocompounds. Dietary selenium can affect the gut microbial colonization, which in turn influences the host's selenium status and expression of selenoproteoma. Selenium deficiency may result in a phenotype of gut microbiota that is more susceptible to cancer, thyroid dysfunctions, inflammatory bowel disease, and cardiovascular disorders. Although the host and gut microbiota benefit each other from their symbiotic relationship, they may become competitors if the supply of micronutrients is limited. Intestinal bacteria can remove selenium from the host resulting in two to three times lower levels of host's selenoproteins under selenium-limiting conditions. There are still gaps in whether these consequences are unfavorable to humans and animals or whether the daily intake of selenium is also adapted to meet the needs of the bacteria.

The physiology and evolution of microbial selenium metabolism

Selenium is an essential trace element whose compounds are widely metabolized by organisms from all three domains of life. Moreover, phylogenetic evidence indicates that selenium species, along with iron, molybdenum, tungsten, and nickel, were metabolized by the last universal common ancestor of all cellular lineages, primarily for the synthesis of the 21st amino acid selenocysteine. Thus, selenium metabolism is both environmentally ubiquitous and a physiological adaptation of primordial life. Selenium metabolic reactions comprise reductive transformations both for assimilation into macromolecules and dissimilatory reduction of selenium oxyanions and elemental selenium during anaerobic respiration. This review offers a comprehensive overview of the physiology and evolution of both assimilatory and dissimilatory selenium metabolism in bacteria and archaea, highlighting mechanisms of selenium respiration. This includes a thorough discussion of our current knowledge of the physiology of selenocysteine synthesis and incorporation into proteins in bacteria obtained from structural biology. Additionally, this is the first comprehensive discussion in a review of the incorporation of selenium into the tRNA nucleoside 5-methylaminomethyl-2-selenouridine and as an inorganic cofactor in certain molybdenum hydroxylase enzymes. Throughout, conserved mechanisms and derived features of selenium metabolism in both domains are emphasized and discussed within the context of the global selenium biogeochemical cycle.

Effects of foliar selenium application on growth and rhizospheric soil micro-ecological environment of Atractylodes macrocephala Koidz

Atractylodes macrocephala (A. macrocephala), a famous medicinal herb in China, is widely cultivated and consumed in China with various beneficial effects. Numerous studies have shown that selenium (Se) plays an important role in promoting plant growth, although Se has not been considered an essential element for higher plants. The objectives of this research were to determine the effects of foliar Se application (0, 2.5, 5.0, 10.0 and 20.0 mg m^-2 Se in sodium selenite, sprayed monthly from May to August) on the growth and rhizospheric soil micro-ecological environment of A. macrocephala, and explore the possible mechanisms underlying plant response to foliar Se application through a field experiment. The results were: The foliar application of 5.0 mg m^-2 Se significantly increased the survival rate of A. macrocephala compared to the control. The yield of A. macrocephala was increased when the Se level maintained belowed 10.0 mg m^-2 but decreased when Se level reached 20.0 mg m^-2. The Se content in the rhizome of A. macrocephala showed a significant positive correlation with the Se level, while the insect attack rate was significantly negatively correlated with the Se level. However, foliar Se application hardly affected the concentration of bioactive compound atractylenolide in the rhizome of A. macrocephala. Notably, the application of foliar Se changed the content of partial soil nutrients, microbial diversity and composition in the rhizosphere soil of A. macrocephala. Bacterial diversity was positively correlated with A. macrocephala growth whereas fungal diversity was negatively correlated, suggesting that microbial diversity in the rhizosphere soils is closely related to plant growth. Moreover, correlation analysis showed that available potassium, Burkholderia and Cupriavidus in rhizospheric soil might be critical factors for promoting the growth of A. macrocephala. Overall, the foliar application of Se at moderate concentration was beneficial for the growth of A. macrocephala, and 5.0-10.0 mg m^-2 Se level was the optimum. Our findings revealed novel insights into the response of A. macrocephala to foliar Se application from plant growth, rhizospheric soil nutrient and microbial community composition .

Selenium transport and metabolism in plants: Phytoremediation and biofortification implications

The aim of this review is to synthesize current knowledge of selenium (Se) transport and metabolism in plants, with a focus on implications for biofortification and phytoremediation. Selenium is a necessary human micronutrient, and around a billion people worldwide may be Se deficient. This can be ameliorated by Se biofortification of staple crops. Selenium is also a potential toxin at higher concentrations, and multiple environmental disasters over the past 50 years have been caused by Se pollution from agricultural and industrial sources. Phytoremediation by plants able to take up large amounts of Se is an important tool to combat pollution issues. Both biofortification and phytoremediation applications require a thorough understanding of how Se is taken up and metabolized by plants. Selenium uptake and translocation in plants are largely accomplished via sulfur (S) transport proteins. Current understanding of these transporters is reviewed here, and transporters that may be manipulated to improve Se uptake are discussed. Plant Se metabolism also largely follows the S metabolic pathway. This pathway is reviewed here, with special focus on genes that have been, or may be manipulated to reduce the accumulation of toxic metabolites or enhance the accumulation of nontoxic metabolites. Finally, unique aspects of Se transport and metabolism in Se hyperaccumulators are reviewed. Hyperaccumulators, which can accumulate Se at up to 1000 times higher concentrations than normal plants, present interesting specialized systems of Se transport and metabolism. Selenium hyperaccumulation mechanisms and potential applications of these mechanisms to biofortification and phytoremediation are presented.

Molybdenum cofactor biology, evolution and deficiency

The molybdenum cofactor (Moco) represents an ancient metal‑sulfur cofactor, which participates as catalyst in carbon, nitrogen and sulfur cycles, both on individual and global scale. Given the diversity of biological processes dependent on Moco and their evolutionary age, Moco is traced back to the last universal common ancestor (LUCA), while Moco biosynthetic genes underwent significant changes through evolution and acquired additional functions. In this review, focused on eukaryotic Moco biology, we elucidate the benefits of gene fusions on Moco biosynthesis and beyond. While originally the gene fusions were driven by biosynthetic advantages such as coordinated expression of functionally related proteins and product/substrate channeling, they also served as origin for the development of novel functions. Today, Moco biosynthetic genes are involved in a multitude of cellular processes and loss of the according gene products result in severe disorders, both related to Moco biosynthesis and secondary enzyme functions.

Developments in the study and applications of bacterial transformations of selenium species

Microbial bio-transformations of the essential trace element selenium are now recognized to occur among a wide variety of microorganisms. These transformations are used to convert this element into its assimilated form of selenocysteine, which is at the active center of a number of key enzymes, and to produce selenium nanoparticles, quantum dots, metal selenides, and methylated selenium species that are indispensable for biotechnological and bioremediation applications. The focus of this review is to present the state-of-the-art of all aspects of the investigations into the bacterial transformations of selenium species, and to consider the characterization and biotechnological uses of these transformations and their products.

Bioinformatics of Metalloproteins and Metalloproteomes

Trace metals are inorganic elements that are required for all organisms in very low quantities. They serve as cofactors and activators of metalloproteins involved in a variety of key cellular processes. While substantial effort has been made in experimental characterization of metalloproteins and their functions, the application of bioinformatics in the research of metalloproteins and metalloproteomes is still limited. In the last few years, computational prediction and comparative genomics of metalloprotein genes have arisen, which provide significant insights into their distribution, function, and evolution in nature. This review aims to offer an overview of recent advances in bioinformatic analysis of metalloproteins, mainly focusing on metalloprotein prediction and the use of different metals across the tree of life. We describe current computational approaches for the identification of metalloprotein genes and metal-binding sites/patterns in proteins, and then introduce a set of related databases. Furthermore, we discuss the latest research progress in comparative genomics of several important metals in both prokaryotes and eukaryotes, which demonstrates divergent and dynamic evolutionary patterns of different metalloprotein families and metalloproteomes. Overall, bioinformatic studies of metalloproteins provide a foundation for systematic understanding of trace metal utilization in all three domains of life.

The Effect of Spring Water Geochemistry on Copper Proteins in Tengchong Hot Springs, China

Copper (Cu) is an essential trace metal cofactor for a variety of proteins; however, excess Cu is toxic to most organisms. Cu homeostasis is maintained by a complex machinery of Cu binding proteins that control the uptake, transport, sequestration, and efflux of Cu ions. Despite the importance of Cu binding proteins in electron transfer, substrate oxidation, superoxide dismutation, and denitrification, little information exists about microbial Cu utilization in extreme environments, where the geochemical conditions may affect Cu bioavailability. Using metagenomic data from 9 hot springs in Tengchong, China, which range in temperature from 42°C to 96°C and in pH from 2.3 to 9, the effects of pH, temperature, and spring geochemistry on the distribution of Cu binding domains of proteins and oxidoreductases were studied. Dissolved Cu and Cu binding domains were detected across all temperature and pH gradients. Cu binding domains of cytochrome c oxidase subunits, heavy-metal-associated domains, and nitrous oxide reductase were detected at all sites. DoxB, a quinol oxidase, and other quinol oxidase subunits were the dominant Cu binding oxidoreductase subunits present at low-pH and high-temperature sites, whereas cbb ₃-type cytochrome c oxidase subunits were dominant at high-pH and high-temperature sites. Additionally, aa ₃-type cytochrome c oxidase was more prominent than cbb ₃-type cytochrome c oxidase under circumneutral-pH conditions. This suggests that the type of cytochrome c oxidase pathway and the Cu proteins employed by microbes to carry out important functions such as energy acquisition and efflux of excess Cu are affected by the physicochemical conditions of the springs.IMPORTANCE Copper is present in a variety of proteins and is required to carry out essential functions by all organisms. However, in hot spring environments, copper availability may be limited due to the high temperatures and the wide range in pH. The significance of our research is in relating the physicochemical environment to the distribution of copper proteins across hot spring environments, which provides increased understanding of primary functions and adaptions in these environments.

Comparative genomic analysis of selenium utilization traits in different marine environments

Selenium (Se) is an essential trace element for many organisms, which is required in the biosynthesis of proteins with selenocysteine, tRNAs with selenouridine, and certain enzymes with Se as a cofactor. Recent large-scale metagenomics projects provide a unique opportunity for studying the global trends of Se utilization in marine environments. Here, we analyzed samples from different marine microbial communities, revealed by the Tara Oceans project, to characterize the Se utilization traits. We found that the selenophosphate synthetase gene, which defines the overall Se utilization, and Se utilization traits are present in all samples. Regions with samples rich and poor in Se utilization traits were categorized. From the analysis of environmental factors, the mesopelagic zone and high temperature (> 15°C) of water are favorable, while geographical location has little influence on Se utilization. All Se utilization traits showed a relatively independent occurrence. The taxonomic classification of Se traits shows that most of the sequences corresponding to Se utilization traits belong to the phylum Proteobacteria. Overall, our study provides useful insights into the general features of Se utilization in ocean samples and may help to understand the evolutionary dynamics of Se utilization in different marine environments.

On the Ecology of Selenium Accumulation in Plants

Plants accumulate and tolerate Se to varying degrees, up to 15,000 mg Se/kg dry weight for Se hyperaccumulators. Plant Se accumulation may exert positive or negative effects on other species in the community. The movement of plant Se into ecological partners may benefit them at low concentrations, but cause toxicity at high concentrations. Thus, Se accumulation can protect plants against Se-sensitive herbivores and pathogens (elemental defense) and reduce surrounding vegetation cover via high-Se litter deposition (elemental allelopathy). While hyperaccumulators negatively impact Se-sensitive ecological partners, they offer a niche for Se-tolerant partners, including beneficial microbial and pollinator symbionts as well as detrimental herbivores, pathogens, and competing plant species. These ecological effects of plant Se accumulation may facilitate the evolution of Se resistance in symbionts. Conversely, Se hyperaccumulation may evolve driven by increasing Se resistance in herbivores, pathogens, or plant neighbors; Se resistance also evolves in mutualist symbionts, minimizing the plant's ecological cost. Interesting topics to address in future research are whether the ecological impacts of plant Se accumulation may affect species composition across trophic levels (favoring Se resistant taxa), and to what extent Se hyperaccumulators form a portal for Se into the local food chain and are important for Se cycling in the local ecosystem.

Comparative genomics and metagenomics of the metallomes

Recent achievements and advances in comparative genomic and metagenomic analyses of trace metals were reviewed and discussed.

Fungal Endophyte Alternaria tenuissima Can Affect Growth and Selenium Accumulation in Its Hyperaccumulator Host Astragalus bisulcatus

Endophytes can enhance plant stress tolerance by promoting growth and affecting elemental accumulation, which may be useful in phytoremediation. In earlier studies, up to 35% elemental selenium (Se0) was found in Se hyperaccumulator Astragalus bisulcatus. Since Se0 can be produced by microbes, the plant Se0 was hypothesized to be microbe-derived. Here we characterize a fungal endophyte of A. bisulcatus named A2. It is common in seeds from natural seleniferous habitat containing 1,000-10,000 mg kg-1 Se. We identified A2 as Alternaria tenuissima via 18S rRNA sequence analysis and morphological characterization. X-ray microprobe analysis of A. bisulcatus seeds that did or did not harbor Alternaria, showed that both contained >90% organic seleno-compounds with C-Se-C configuration, likely methylselenocysteine and glutamyl-methylselenocysteine. The seed Se was concentrated in the embryo, not the seed coat. X-ray microprobe analysis of A2 in pure culture showed the fungus produced Se0 when supplied with selenite, but accumulated mainly organic C-Se-C compounds when supplied with selenate. A2 was completely resistant to selenate up to 300 mg L-1, moderately resistant to selenite (50% inhibition at ∼50 mg Se L-1), but relatively sensitive to methylselenocysteine and to Se extracted from A. bisulcatus (50% inhibition at 25 mg Se L-1). Four-week old A. bisulcatus seedlings derived from surface-sterilized seeds containing endophytic Alternaria were up to threefold larger than seeds obtained from seeds not showing evidence of fungal colonization. When supplied with Se, the Alternaria-colonized seedlings had lower shoot Se and sulfur levels than seedlings from uncolonized seeds. In conclusion, A. tenuissima may contribute to the Se0 observed earlier in A. bisulcatus, and affect host growth and Se accumulation. A2 is sensitive to the Se levels found in its host's tissues, but may avoid Se toxicity by occupying low-Se areas (seed coat, apoplast) and converting plant Se to non-toxic Se0. These findings illustrate the potential for hyperaccumulator endophytes to affect plant properties relevant for phytoremediation. Facultative endophytes may also be applicable in bioremediation and biofortification, owing to their capacity to turn toxic inorganic forms of Se into non-toxic or even beneficial, organic forms with anticarcinogenic properties.

Mechanisms of selenium hyperaccumulation in plants: A survey of molecular, biochemical and ecological cues

BACKGROUND

Selenium (Se) is a micronutrient required for many life forms, but toxic at higher concentration. Plants do not have a Se requirement, but can benefit from Se via enhanced antioxidant activity. Some plant species can accumulate Se to concentrations above 0.1% of dry weight and seem to possess mechanisms that distinguish Se from its analog sulfur (S). Research on these so-called Se hyperaccumulators aims to identify key genes for this remarkable trait and to understand ecological implications.

SCOPE OF REVIEW

This review gives a broad overview of the current knowledge about Se uptake and metabolism in plants, with a special emphasis on hypothesized mechanisms of Se hyperaccumulation. The role of Se in plant defense responses and the associated ecological implications are discussed.

MAJOR CONCLUSIONS

Hyperaccumulators have enhanced expression of S transport and assimilation genes, and may possess transporters with higher specificity for selenate over sulfate. Genes involved in antioxidant reactions and biotic stress resistance are also upregulated. Key regulators in these processes appear to be the growth regulators jasmonic acid, salicylic acid and ethylene. Hyperaccumulation may have evolved owing to associated ecological benefits, particularly protection against pathogens and herbivores, and as a form of elemental allelopathy.

GENERAL SIGNIFICANCE

Understanding plant Se uptake and metabolism in hyperaccumulators has broad relevance for the environment, agriculture and human and animal nutrition and may help generate crops with selenate-specific uptake and high capacity to convert selenate to less toxic, anticarcinogenic, organic Se compounds.

The fascinating facets of plant selenium accumulation - biochemistry, physiology, evolution and ecology

Contents 1582 I. 1582 II. 1583 III. 1588 IV. 1590 V. 1592 1592 References 1592 SUMMARY: The importance of selenium (Se) for medicine, industry and the environment is increasingly apparent. Se is essential for many species, including humans, but toxic at elevated concentrations. Plant Se accumulation and volatilization may be applied in crop biofortification and phytoremediation. Topics covered here include beneficial and toxic effects of Se on plants, mechanisms of Se accumulation and tolerance in plants and algae, Se hyperaccumulation, and ecological and evolutionary aspects of these processes. Plant species differ in the concentration and forms of Se accumulated, Se partitioning at the whole-plant and tissue levels, and the capacity to distinguish Se from sulfur. Mechanisms of Se hyperaccumulation and its adaptive significance appear to involve constitutive up-regulation of sulfate/selenate uptake and assimilation, associated with elevated concentrations of defense-related hormones. Hyperaccumulation has evolved independently in at least three plant families, probably as an elemental defense mechanism and perhaps mediating elemental allelopathy. Elevated plant Se protects plants from generalist herbivores and pathogens, but also gives rise to the evolution of Se-resistant specialists. Plant Se accumulation affects ecological interactions with herbivores, pollinators, neighboring plants, and microbes. Hyperaccumulation tends to negatively affect Se-sensitive ecological partners while facilitating Se-resistant partners, potentially affecting species composition and Se cycling in seleniferous ecosystems.

Mechanisms of Selenium Enrichment and Measurement in Brassicaceous Vegetables, and Their Application to Human Health

Selenium (Se) is an essential micronutrient for human health. Se deficiency affects hundreds of millions of people worldwide, particularly in developing countries, and there is increasing awareness that suboptimal supply of Se can also negatively affect human health. Selenium enters the diet primarily through the ingestion of plant and animal products. Although, plants are not dependent on Se they take it up from the soil through the sulphur (S) uptake and assimilation pathways. Therefore, geographic differences in the availability of soil Se and agricultural practices have a profound influence on the Se content of many foods, and there are increasing efforts to biofortify crop plants with Se. Plants from the Brassicales are of particular interest as they accumulate and synthesize Se into forms with additional health benefits, such as methylselenocysteine (MeSeCys). The Brassicaceae are also well-known to produce the glucosinolates; S-containing compounds with demonstrated human health value. Furthermore, the recent discovery of the selenoglucosinolates in the Brassicaceae raises questions regarding their potential bioefficacy. In this review we focus on Se uptake and metabolism in the Brassicaceae in the context of human health, particularly cancer prevention and immunity. We investigate the close relationship between Se and S metabolism in this plant family, with particular emphasis on the selenoglucosinolates, and consider the methodologies available for identifying and quantifying further novel Se-containing compounds in plants. Finally, we summarize the research of multiple groups investigating biofortification of the Brassicaceae and discuss which approaches might be most successful for supplying Se deficient populations in the future.

Redox Pioneer: Professor Vadim N. Gladyshev

Professor Vadim N. Gladyshev is recognized here as a Redox Pioneer, because he has published an article on antioxidant/redox biology that has been cited more than 1000 times and 29 articles that have been cited more than 100 times. Gladyshev is world renowned for his characterization of the human selenoproteome encoded by 25 genes, identification of the majority of known selenoprotein genes in the three domains of life, and discoveries related to thiol oxidoreductases and mechanisms of redox control. Gladyshev's first faculty position was in the Department of Biochemistry, the University of Nebraska. There, he was a Charles Bessey Professor and Director of the Redox Biology Center. He then moved to the Department of Medicine at Brigham and Women's Hospital, Harvard Medical School, where he is Professor of Medicine and Director of the Center for Redox Medicine. His discoveries in redox biology relate to selenoenzymes, such as methionine sulfoxide reductases and thioredoxin reductases, and various thiol oxidoreductases. He is responsible for the genome-wide identification of catalytic redox-active cysteines and for advancing our understanding of the general use of cysteines by proteins. In addition, Gladyshev has characterized hydrogen peroxide metabolism and signaling and regulation of protein function by methionine-R-sulfoxidation. He has also made important contributions in the areas of aging and lifespan control and pioneered applications of comparative genomics in redox biology, selenium biology, and aging. Gladyshev's discoveries have had a profound impact on redox biology and the role of redox control in health and disease. He is a true Redox Pioneer. Antioxid. Redox Signal. 25, 1-9.

Is matching ruthenium with dithiocarbamato ligands a potent chemotherapeutic weapon in oncology?

In the last years, several metal-based compounds have been designed and biologically investigated worldwide in order to obtain chemotherapeutics with a better toxicological profile and comparable or higher antiblastic activity than the clinically-established platinum-based drugs. In this context, researchers have addressed their attention to alternative nonplatinum derivatives able to maximize the anticancer activity of the new drugs and to minimize the side effects. Among them, a number of ruthenium complexes have been developed, including the compounds NAMI-A and KP1019, now in clinical trials. Here, we report the results collected so far for a particular class of ruthenium complexes - the ruthenium(II/III)-dithiocarbamates - which proved more potent than cisplatin in vitro, even at nanomolar concentrations, against a wide panel of human tumor cell lines.

Comparative genomics reveals new evolutionary and ecological patterns of selenium utilization in bacteria

Selenium (Se) is an important micronutrient for many organisms, which is required for the biosynthesis of selenocysteine, selenouridine and Se-containing cofactor. Several key genes involved in different Se utilization traits have been characterized; however, systematic studies on the evolution and ecological niches of Se utilization are very limited. Here, we analyzed more than 5200 sequenced organisms to examine the occurrence patterns of all Se traits in bacteria. A global species map of all Se utilization pathways has been generated, which demonstrates the most detailed understanding of Se utilization in bacteria so far. In addition, the selenophosphate synthetase gene, which is used to define the overall Se utilization, was also detected in some organisms that do not have any of the known Se traits, implying the presence of a novel Se form in this domain. Phylogenetic analyses of components of different Se utilization traits revealed new horizontal gene transfer events for each of them. Moreover, by characterizing the selenoproteomes of all organisms, we found a new selenoprotein-rich phylum and additional selenoprotein-rich species. Finally, the relationship between ecological environments and Se utilization was investigated and further verified by metagenomic analysis of environmental samples, which indicates new macroevolutionary trends of each Se utilization trait in bacteria. Our data provide insights into the general features of Se utilization in bacteria and should be useful for a further understanding of the evolutionary dynamics of Se utilization in nature.

Organization of the Mammalian Ionome According to Organ Origin, Lineage Specialization, and Longevity

Trace elements are essential to all mammals, but their distribution and utilization across species and organs remains unclear. Here, we examined 18 elements in the brain, heart, kidney, and liver of 26 mammalian species and report the elemental composition of these organs, the patterns of utilization across the species, and their correlation with body mass and longevity. Across the organs, we observed distinct distribution patterns for abundant elements, transition metals, and toxic elements. Some elements showed lineage-specific patterns, including reduced selenium utilization in African mole rats, and positive correlation between the number of selenocysteine residues in selenoprotein P and the selenium levels in liver and kidney across mammals. Body mass was linked positively to zinc levels, whereas species lifespan correlated positively with cadmium and negatively with selenium. This study provides insights into the variation of mammalian ionome by organ physiology, lineage specialization, body mass, and longevity.

Gene-specific regulation of hepatic selenoprotein expression by interleukin-6

Sepsis is a severe inflammatory disease resulting in excessive production of pro-inflammatory cytokines including interleukin-6 (IL-6), causing oxidative stress, tissue damage and organ dysfunction. Health benefits have been observed upon selenium (Se) supplementation in severe sepsis. Selenium is incorporated into selenoproteins implicated in anti-oxidative defence, thyroid hormone metabolism and immunoregulation. Selenium metabolism is controlled by hepatocytes synthesizing and secreting the Se transporter selenoprotein P (SePP). The circulating SePP declines in sepsis causing low serum Se levels. Dysregulation of the hepatic selenoenzyme deiodinase type 1 (DIO1) potentially contributes to the low T3 (thyroid hormone) syndrome observed in severe diseases. We hypothesized that IL-6 affects hepatic selenoprotein biosynthesis directly. Testing human hepatocytes in culture, IL-6 reduced the concentrations of SePP mRNA and secreted SePP in a dose-dependent manner. In parallel, expression of DIO1 declined at the mRNA, protein and enzyme activity level. The effects of IL-6 on glutathione peroxidase (GPX) expression were isozyme-specific; GPX1 remained unaffected, while transcript concentrations of GPX2 increased and those of GPX4 decreased. This pattern of IL-6-dependent effects was mirrored in reporter gene experiments with SePP, DIO1, GPX1, and GPX2 promoter constructs pointing to direct transcriptional effects of IL-6. The redirection of hepatic selenoprotein biosynthesis by IL-6 may represent a central regulatory circuit responsible for the decline of serum Se and low T3 concentrations in sepsis. Accordingly, therapeutic IL-6 targeting may be effective for improving the Se and thyroid hormone status, adjuvant Se supplementation success and survival in sepsis.

Selenium cycling across soil-plant-atmosphere interfaces: a critical review

Selenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

Comparative genomics reveals new candidate genes involved in selenium metabolism in prokaryotes

Selenium (Se) is an important micronutrient that mainly occurs in proteins in the form of selenocysteine and in tRNAs in the form of selenouridine. In the past 20 years, several genes involved in Se utilization have been characterized in both prokaryotes and eukaryotes. However, Se homeostasis and the associated regulatory network are not fully understood. In this study, we conducted comparative genomics and phylogenetic analyses to examine the occurrence of all known Se utilization traits in prokaryotes. Our results revealed a highly mosaic pattern of species that use Se (in different forms) in spite that most organisms do not use this element. Further investigation of genomic context of known Se-related genes in different organisms suggested novel candidate genes that may participate in Se metabolism in bacteria and/or archaea. Among them, a membrane protein, YedE, which contains ten transmembrane domains and shows distant similarity to a sulfur transporter, is exclusively found in Se-utilizing organisms, suggesting that it may be involved in Se transport. A LysR-like transcription factor subfamily might be important for the regulation of Sec biosynthesis and/or other Se-related genes. In addition, a small protein family DUF3343 is widespread in Se-utilizing organisms, which probably serves as an important chaperone for Se trafficking within the cells. Finally, we proposed a simple model of Se homeostasis based on our findings. Our study reveals new candidate genes involved in Se metabolism in prokaryotes and should be useful for a further understanding of the complex metabolism and the roles of Se in biology.

Selenium-fortified wheat: potential of microbes for biofortification of selenium and other essential nutrients

Selenium (Se) is an essential micronutrient for humans and animals, and Se deficiency is a worldwide problem. Plants are a main dietary source of Se for humans and livestock. In this study we investigated the effect of two selenium-tolerant bacterial strains Bacillus cereus-YAP6 and Bacillus licheniformis-YAP7, on the growth and Se uptake by wheat plants. The bacteria-inoculated plants exhibited a significant increase in spike length, shoot length and dry biomass. Inoculated Se-treated plants also showed increased stem Se, S, Ca and Fe concentrations, by up to 375%, 40%, 55%, and 104%, respectively, and increased kernel Se, S, Ca and Fe concentrations by up to 154%, 85%, 60%, and 240%, respectively, compared to un-inoculated Se-treated plants. In conclusion, inoculation with strains YAP6 andYAP7 is a good Se biofortification strategy for wheat. Both strains showed resistance to other toxic elements, i.e., As, Cd, Co, Cr, Cu, Mn and Zn. Optimal growth temperature and pH for both strains were 37°C and pH7, respectively, but both strains can grow very well at different temperatures (28-45°C) and at alkaline pH. Both strains have high Se reduction potential: strains YAP6 and YAP7 converted 92% and 32% of selenite into elemental Se within 48 h, respectively.

Do selenium hyperaccumulators affect selenium speciation in neighboring plants and soil? An X-Ray Microprobe Analysis

Neighbors of Se hyperaccumulators Stanleya pinnata and Astragalus bisulcatus were found earlier to have elevated Se levels. Here we investigate whether Se hyperaccumulators affect Se localization and speciation in surrounding soil and neighboring plants. X-ray fluorescence mapping and X-ray absorption near-edge structure spectroscopy were used to analyze Se localization and speciation in leaves of Artemisia ludoviciana, Symphyotrichum ericoides and Chenopodium album growing next to Se hyperaccumulators or non-accumulators at a seleniferous site. Regardless of neighbors, A. ludoviciana, S. ericoides and C. album accumulated predominantly (73-92%) reduced selenocompounds with XANES spectra similar to the C-Se-C compounds selenomethionine and methyl-selenocysteine. Preliminary data indicate that the largest Se fraction (65-75%), both in soil next to hyperaccumulator S. pinnata and next to nonaccumulator species was reduced Se with spectra similar to C-Se-C standards. These same C-Se-C forms are found in hyperaccumulators. Thus, hyperaccumulator litter may be a source of organic soil Se, but soil microorganisms may also contribute. These findings are relevant for phytoremediation and biofortification since organic Se is more readily accumulated by plants, and more effective for dietary Se supplementation.

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

Analysis of selenium accumulation, speciation and tolerance of potential selenium hyperaccumulator Symphyotrichum ericoides

Symphyotrichum ericoides was shown earlier to contain hyperaccumulator levels of selenium (Se) in the field (>1000 mg kg(-1) dry weight (DW)), but only when growing next to other Se hyperaccumulators. It was also twofold larger next to hyperaccumulators and suffered less herbivory. This raised two questions: whether S. ericoides is capable of hyperaccumulation without neighbor assistance, and whether its Se-derived benefit is merely ecological or also physiological. Here, in a comparative greenhouse study, Se accumulation and tolerance of S. ericoides were analyzed in parallel with hyperaccumulator Astragalus bisulcatus, Se accumulator Brassica juncea and related Asteraceae Machaeranthera tanacetifolia. Symphyotrichum ericoides and M. tanacetifolia accumulated Se up to 3000 and 1500 mg Se kg(-1) DW, respectively. They were completely tolerant to these Se levels and even grew 1.5- to 2.5-fold larger with Se. Symphyotrichum ericoides showed very high leaf Se/sulfur (S) and shoot/root Se concentration ratios, similar to A. bisulcatus and higher than M. tanacetifolia and B. juncea. Se X-ray absorption near-edge structure spectroscopy showed that S. ericoides accumulated Se predominantly (86%) as C-Se-C compounds indistinguishable from methyl-selenocysteine, which may explain its Se tolerance. Machaeranthera tanacetifolia accumulated 55% of its Se as C-Se-C compounds; the remainder was inorganic Se. Thus, in this greenhouse study S. ericoides displayed all of the characteristics of a hyperaccumulator. The larger size of S. ericoides when growing next to hyperaccumulators may be explained by a physiological benefit, in addition to the ecological benefit demonstrated earlier.

Planar substrate-binding site dictates the specificity of ECF-type nickel/cobalt transporters

The energy-coupling factor (ECF) transporters are multi-subunit protein complexes that mediate uptake of transition-metal ions and vitamins in about 50% of the prokaryotes, including bacteria and archaea. Biological and structural studies have been focused on ECF transporters for vitamins, but the molecular mechanism by which ECF systems transport metal ions from the environment remains unknown. Here we report the first crystal structure of a NikM, TtNikM2, the substrate-binding component (S component) of an ECF-type nickel transporter from Thermoanaerobacter tengcongensis. In contrast to the structures of the vitamin-specific S proteins with six transmembrane segments (TSs), TtNikM2 possesses an additional TS at its N-terminal region, resulting in an extracellular N-terminus. The highly conserved N-terminal loop inserts into the center of TtNikM2 and occludes a region corresponding to the substrate-binding sites of the vitamin-specific S components. Nickel binds to NikM via its coordination to four nitrogen atoms, which are derived from Met1, His2 and His67 residues. These nitrogen atoms form an approximately square-planar geometry, similar to that of the metal ion-binding sites in the amino-terminal Cu(2+)- and Ni(2+)-binding (ATCUN) motif. Replacements of residues in NikM contributing to nickel coordination compromised the Ni-transport activity. Furthermore, systematic quantum chemical investigation indicated that this geometry enables NikM to also selectively recognize Co(2+). Indeed, the structure of TtNikM2 containing a bound Co(2+) ion has almost no conformational change compared to the structure that contains a nickel ion. Together, our data reveal an evolutionarily conserved mechanism underlying the metal selectivity of EcfS proteins, and provide insights into the ion-translocation process mediated by ECF transporters.

Copper and iron homeostasis in plants: the challenges of oxidative stress

SIGNIFICANCE

Photosynthesis, the process that drives life on earth, relies on transition metal (e.g., Fe and Cu) containing proteins that participate in electron transfer in the chloroplast. However, the light reactions also generate high levels of reactive oxygen species (ROS), which makes metal use in plants a challenge.

RECENT ADVANCES

Sophisticated regulatory networks govern Fe and Cu homeostasis in response to metal ion availability according to cellular needs and priorities. Molecular remodeling in response to Fe or Cu limitation leads to its economy to benefit photosynthesis. Fe toxicity is prevented by ferritin, a chloroplastic Fe-storage protein in plants. Recent studies on ferritin function and regulation revealed the interplay between iron homeostasis and the redox balance in the chloroplast.

CRITICAL ISSUES

Although the connections between metal excess and ROS in the chloroplast are established at the molecular level, the mechanistic details and physiological significance remain to be defined. The causality/effect relationship between transition metals, redox signals, and responses is difficult to establish.

FUTURE DIRECTIONS

Integrated approaches have led to a comprehensive understanding of Cu homeostasis in plants. However, the biological functions of several major families of Cu proteins remain unclear. The cellular priorities for Fe use under deficiency remain largely to be determined. A number of transcription factors that function to regulate Cu and Fe homeostasis under deficiency have been characterized, but we have not identified regulators that mediate responses to excess. Importantly, details of metal sensing mechanisms and cross talk to ROS-sensing mechanisms are so far poorly documented in plants.