The phosphatase C(X)5R motif is required for catalytic activity of the Saccharomyces cerevisiae Acr2p arsenate reductase

Arsenate reductase of Rufibacter tibetensis is a metallophosphoesterase evolved to catalyze redox reactions

An arsenate reductase (Car1) from the Bacteroidetes species Rufibacter tibetensis 1351T was isolated from the Tibetan Plateau. The strain exhibits resistance to arsenite [As(III)] and arsenate [As(V)] and reduces As(V) to As(III). Here we shed light on the mechanism of enzymatic reduction by Car1. AlphaFold2 structure prediction, active site energy minimization, and steady-state kinetics of wild-type and mutant enzymes give insight into the catalytic mechanism. Car1 is structurally related to calcineurin-like metallophosphoesterases (MPPs). It functions as a binuclear metal hydrolase with limited phosphatase activity, particularly relying on the divalent metal Ni2+. As an As(V) reductase, it displays metal promiscuity and is coupled to the thioredoxin redox cycle, requiring the participation of two cysteine residues, Cys74 and Cys76. These findings suggest that Car1 evolved from a common ancestor of extant phosphatases by incorporating a redox function into an existing MPP catalytic site. Its proposed mechanism of arsenate reduction involves Cys74 initiating a nucleophilic attack on arsenate, leading to the formation of a covalent intermediate. Next, a nucleophilic attack of Cys76 leads to the release of As(III) and the formation of a surface-exposed Cys74-Cys76 disulfide, ready for reduction by thioredoxin.

Interplay between myotubularins and Ca2+ homeostasis

The myotubularin family, encompassing myotubularin 1 (MTM1) and 14 myotubularin-related proteins (MTMRs), represents a conserved group of phosphatases featuring a protein tyrosine phosphatase domain. Nine members are characterized by an active phosphatase domain C(X)5R, dephosphorylating the D3 position of PtdIns(3)P and PtdIns(3,5)P2. Mutations in myotubularin genes result in human myopathies, and several neuropathies including X-linked myotubular myopathy and Charcot-Marie-Tooth type 4B. MTM1, MTMR6 and MTMR14 also contribute to Ca2+ signaling and Ca2+ homeostasis that play a key role in many MTM-dependent myopathies and neuropathies. Here we explore the evolving roles of MTM1/MTMRs, unveiling their influence on critical aspects of Ca2+ signaling pathways.

Deciphering roles of microbiota in arsenic biotransformation from the earthworm gut and skin

Biotransformation mediated by microbes can affect the biogeochemical cycle of arsenic. However, arsenic biotransformation mediated by earthworm-related microorganisms has not been well explored, especially the role played by earthworm skin microbiota. Herein, we reveal the profiles of arsenic biotransformation genes (ABGs) and elucidate the microbial communities of the earthworm gut, skin, and surrounding soil from five different soil environments in China. The relative abundance of ABGs in the earthworm skin microbiota, which were dominated by genes associated with arsenate reduction and transport, was approximately three times higher than that in the surrounding soil and earthworm gut microbiota. The composition and diversity of earthworm skin microbiota differed significantly from those of the soil and earthworm gut, comprising a core bacterial community with a relative abundance of 96% Firmicutes and a fungal community with relative abundances of 50% Ascomycota and 13% Mucoromycota. In addition, stochastic processes mainly contributed to the microbial community assembly across all samples. Moreover, fungal genera such as Vishniacozyma and Oomyces were important mediators of ABGs involved in the biogeochemical cycle of arsenic. This is the first study to investigate earthworm skin as a reservoir of microbial diversity in arsenic biotransformation. Our findings broaden the current scientific knowledge of the involvement of earthworms in the arsenic biogeochemical cycle.

New biochemistry in the Rhodanese-phosphatase superfamily: emerging roles in diverse metabolic processes, nucleic acid modifications, and biological conflicts

The protein-tyrosine/dual-specificity phosphatases and rhodanese domains constitute a sprawling superfamily of Rossmannoid domains that use a conserved active site with a cysteine to catalyze a range of phosphate-transfer, thiotransfer, selenotransfer and redox activities. While these enzymes have been extensively studied in the context of protein/lipid head group dephosphorylation and various thiotransfer reactions, their overall diversity and catalytic potential remain poorly understood. Using comparative genomics and sequence/structure analysis, we comprehensively investigate and develop a natural classification for this superfamily. As a result, we identified several novel clades, both those which retain the catalytic cysteine and those where a distinct active site has emerged in the same location (e.g. diphthine synthase-like methylases and RNA 2' OH ribosyl phosphate transferases). We also present evidence that the superfamily has a wider range of catalytic capabilities than previously known, including a set of parallel activities operating on various sugar/sugar alcohol groups in the context of NAD+-derivatives and RNA termini, and potential phosphate transfer activities involving sugars and nucleotides. We show that such activities are particularly expanded in the RapZ-C-DUF488-DUF4326 clade, defined here for the first time. Some enzymes from this clade are predicted to catalyze novel DNA-end processing activities as part of nucleic-acid-modifying systems that are likely to function in biological conflicts between viruses and their hosts.

Arsenic-resistance mechanisms in bacterium Leclercia adecarboxylata strain As3-1: Biochemical and genomic analyses

Microbial arsenic transformation is important in As biogeochemical cycles in the environment. In this study, a new As-resistant bacterial strain Leclercia adecarboxylata As3-1 was isolated and its associated mechanisms in As resistance and detoxification were evaluated based on genome sequencing and gene annotations. After subjecting strain As3-1 to medium containing arsenate (AsV), AsV reduction occurred and an AsV-enhanced bacterial growth was observed. Strain As3-1 lacked arsenite (AsIII) oxidation ability and displayed lower AsIII resistance than AsV, probably due to its higher AsIII accumulation. Polymerase chain reaction and phylogenetic analysis showed that strain As3-1 harbored a typical AsV reductase gene (arsC) on the plasmids. Genome sequencing and gene annotations identified four operons phoUpstBACS, arsHRBC, arsCRDABC and ttrRSBCA, with 8 additional genes outside the operons that might have involved in As resistance and detoxification in strain As3-1. These included 5 arsC genes explaining why strain As3-1 tolerated high AsV concentrations. Besides ArsC, TtrB, TtrC and TtrA proteins could also be involved in AsV reduction and consequent energy acquisition for bacterial growth. Our data provided a new example of diverse As-regulating systems and AsV-enhanced growth without ArrA in bacteria. The information helps to understand the role of As in selecting microbial systems that can transform and utilize As.

Characterization of Arsenic Biotransformation by a Typical Bryophyte Physcomitrella patens

Arsenic (As) is a ubiquitous environmental toxin that has created catastrophic human health and environmental problems around world. Physcomitrella patens is a potential model plant for the study of environmental monitoring, which exists in all kinds of ecosystems. In this study, arsenic metabolism was investigated by this moss. When supplied with different levels of arsenate (50, 100, 200 µmol/L) for a 4-week period, the total arsenic concentrations were up to 231.4-565.4 mg/kg DW in this moss. Arsenite concentration increased with increasing external arsenate concentrations, the proportion was up to 25.1-36.8% of the total As. An arsenate reductase, PpACR2, was identified and functionally characterized. Heterologous expression of PpACR2 in an As(V)-sensitive strain WC3110 (ΔarsC) of Escherichia coli conferred As(V) resistance. Purified PpACR2 protein exhibited the arsenate reductase activity. Given its powerful As accumulation ability, the bryophyte could be exploited in bioremediation of As-contaminated environments.

The arsenic hyperaccumulating Pteris vittata expresses two arsenate reductases

Enzymatic reduction of arsenate to arsenite is the first known step in arsenate metabolism in all organisms. Although the presence of one mRNA arsenate reductase (PvACR2) has been characterized in gametophytes of P. vittata, no arsenate reductase protein has been directly observed in this arsenic hyperaccumulating fern, yet. In order to assess the possible presence of arsenate reductase in P. vittata, two recombinant proteins, ACR2-His6 and Trx-His6-S-Pv2.5-8 were prepared in Escherichia coli, purified and used to produce polyclonal antibodies. The presence of these two enzymes was evaluated by qRT-PCR, immunoblotting and direct MS analysis. Enzymatic activity was detected in crude extracts. For the first time we detected and identified two arsenate reductase proteins (PvACR2 and Pv2.5-8) in sporophytes and gametophytes of P. vittata. Despite an increase of the mRNA levels for both proteins in roots, no difference was observed at the protein level after arsenic treatment. Overall, our data demonstrate the constitutive protein expression of PvACR2 and Pv2.5-8 in P. vittata tissues and propose their specific role in the complex metabolic network of arsenic reduction.

Integrated phytobial remediation for sustainable management of arsenic in soil and water

Arsenic (As), cited as the most hazardous substance by the U.S. Agency for Toxic Substance and Disease Registry (ATSDR, 2005), is an ubiquitous metalloid which when ingested for prolonged periods cause extensive health effects leading to ultimate untimely death. Plants and microbes can help mitigate soil and groundwater As problem since they have evolved elaborate detoxification machineries against this toxic metalloid as a result of their coexistence with this since the origin of life on earth. Utilization of the phytoremediation and bioremediation potential of the plants and microbes, respectively, is now regarded as two innovative tools that encompass biology, geology, biotechnology and allied sciences with cutting edge applications for sustainable mitigation of As epidemic. Discovery of As hyperaccumulating plants that uptake and concentrate large amounts of this toxic metalloid in their shoots or roots offered new hope to As phytoremediation, solar power based nature's own green remediation. This review focuses on how phytoremediation and bioremediation can be merged together to form an integrated phytobial remediation which could synergistically achieve the goal of large scale removal of As from soil, sediment and groundwater and overcome the drawbacks of the either processes alone. The review also points to the feasibility of the introduction of transgenic plants and microbes that bring new hope for more efficient treatment of As. The review identifies one critical research gap on the importance of remediation of As contaminated groundwater not only for drinking purpose but also for irrigation purpose and stresses that more research should be conducted on the use of constructed wetland, one of the most suitable areas of application of phytobial remediation. Finally the review has narrowed down on different phytoinvestigation and phytodisposal methods, which constitute the most essential and the most difficult part of pilot scale and field scale applications of phytoremediation programs.

Possible roles of plant sulfurtransferases in detoxification of cyanide, reactive oxygen species, selected heavy metals and arsenate

Plants and animals have evolved various potential mechanisms to surmount the adverse effects of heavy metal toxicity. Plants possess low molecular weight compounds containing sulfhydryl groups (-SH) that actively react with toxic metals. For instance, glutathione (γ-Glu-Cys-Gly) is a sulfur-containing tripeptide thiol and a substrate of cysteine-rich phytochelatins (γ-Glu-Cys)2-11-Gly (PCs). Phytochelatins react with heavy metal ions by glutathione S-transferase in the cytosol and afterwards they are sequestered into the vacuole for degradation. Furthermore, heavy metals induce reactive oxygen species (ROS), which directly or indirectly influence metabolic processes. Reduced glutathione (GSH) attributes as an antioxidant and participates to control ROS during stress. Maintenance of the GSH/GSSG ratio is important for cellular redox balance, which is crucial for the survival of the plants. In this context, sulfurtransferases (Str), also called rhodaneses, comprise a group of enzymes widely distributed in all phyla, paving the way for the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors, at least in vitro. The best characterized in vitro reaction is the transfer of a sulfane sulfur atom from thiosulfate to cyanide, leading to the formation of sulfite and thiocyanate. Plants as well as other organisms have multi-protein families (MPF) of Str. Despite the presence of Str activities in many living organisms, their physiological role has not been clarified unambiguously. In mammals, these proteins are involved in the elimination of cyanide released from cyanogenic compounds. However, their ubiquity suggests additional physiological functions. Furthermore, it is speculated that a member of the Str family acts as arsenate reductase (AR) and is involved in arsenate detoxification. In summary, the role of Str in detoxification processes is still not well understood but seems to be a major function in the organism.

Molecular characterization of Alr1105 a novel arsenate reductase of the diazotrophic cyanobacterium Anabaena sp. PCC7120 and decoding its role in abiotic stress management in Escherichia coli

This paper constitutes the first report on the Alr1105 of Anabaena sp. PCC7120 which functions as arsenate reductase and phosphatase and offers tolerance against oxidative and other abiotic stresses in the alr1105 transformed Escherichia coli. The bonafide of 40.8 kDa recombinant GST+Alr1105 fusion protein was confirmed by immunoblotting. The purified Alr1105 protein (mw 14.8 kDa) possessed strong arsenate reductase (Km 16.0 ± 1.2 mM and Vmax 5.6 ± 0.31 μmol min⁻¹ mg protein⁻¹) and phosphatase activity (Km 27.38 ± 3.1 mM and Vmax 0.077 ± 0.005 μmol min⁻¹ mg protein⁻¹) at an optimum temperature 37 °C and 6.5 pH. Native Alr1105 was found as a monomeric protein in contrast to its homologous Synechocystis ArsC protein. Expression of Alr1105 enhanced the arsenic tolerance in the arsenate reductase mutant E. coli WC3110 (∆arsC) and rendered better growth than the wild type W3110 up to 40 mM As (V). Notwithstanding above, the recombinant E. coli strain when exposed to CdCl₂, ZnSO₄, NiCl₂, CoCl₂, CuCl₂, heat, UV-B and carbofuron showed increase in growth over the wild type and mutant E. coli transformed with the empty vector. Furthermore, an enhanced growth of the recombinant E. coli in the presence of oxidative stress producing chemicals (MV, PMS and H₂O₂), suggested its protective role against these stresses. Appreciable expression of alr1105 gene as measured by qRT-PCR at different time points under selected stresses reconfirmed its role in stress tolerance. Thus the Alr1105 of Anabaena sp. PCC7120 functions as an arsenate reductase and possess novel properties different from the arsenate reductases known so far.

Corynebacterium glutamicum survives arsenic stress with arsenate reductases coupled to two distinct redox mechanisms

Arsenate reductases (ArsCs) evolved independently as a defence mechanism against toxic arsenate. In the genome of Corynebacterium glutamicum, there are two arsenic resistance operons (ars1 and ars2) and four potential genes coding for arsenate reductases (Cg_ArsC1, Cg_ArsC2, Cg_ArsC1' and Cg_ArsC4). Using knockout mutants, in vitro reconstitution of redox pathways, arsenic measurements and enzyme kinetics, we show that a single organism has two different classes of arsenate reductases. Cg_ArsC1 and Cg_ArsC2 are single-cysteine monomeric enzymes coupled to the mycothiol/mycoredoxin redox pathway using a mycothiol transferase mechanism. In contrast, Cg_ArsC1' is a three-cysteine containing homodimer that uses a reduction mechanism linked to the thioredoxin pathway with a k(cat)/K(M) value which is 10(3) times higher than the one of Cg_ArsC1 or Cg_ArsC2. Cg_ArsC1' is constitutively expressed at low levels using its own promoter site. It reduces arsenate to arsenite that can then induce the expression of Cg_ArsC1 and Cg_ArsC2. We also solved the X-ray structures of Cg_ArsC1' and Cg_ArsC2. Both enzymes have a typical low-molecular-weight protein tyrosine phosphatases-I fold with a conserved oxyanion binding site. Moreover, Cg_ArsC1' is unique in bearing an N-terminal three-helical bundle that interacts with the active site of the other chain in the dimeric interface.

How Saccharomyces cerevisiae copes with toxic metals and metalloids

Toxic metals and metalloids are widespread in nature and can locally reach fairly high concentrations. To ensure cellular protection and survival in such environments, all organisms possess systems to evade toxicity and acquire tolerance. This review provides an overview of the molecular mechanisms that contribute to metal toxicity, detoxification and tolerance acquisition in budding yeast Saccharomyces cerevisiae. We mainly focus on the metals/metalloids arsenic, cadmium, antimony, mercury, chromium and selenium, and emphasize recent findings on sensing and signalling mechanisms and on the regulation of tolerance and detoxification systems that safeguard cellular and genetic integrity.

Contribution of Yap1 towards Saccharomyces cerevisiae adaptation to arsenic-mediated oxidative stress

In the budding yeast Saccharomyces cerevisiae, arsenic detoxification involves the activation of Yap8, a member of the Yap (yeast AP-1-like) family of transcription factors, which in turn regulates ACR2 and ACR3, genes encoding an arsenate reductase and a plasma-membrane arsenite-efflux protein respectively. In addition, Yap1 is involved in the arsenic adaptation process through regulation of the expression of the vacuolar pump encoded by YCF1 (yeast cadmium factor 1 gene) and also contributing to the regulation of ACR genes. Here we show that Yap1 is also involved in the removal of ROS (reactive oxygen species) generated by arsenic compounds. Data on lipid peroxidation and intracellular oxidation indicate that deletion of YAP1 and YAP8 triggers cellular oxidation mediated by inorganic arsenic. In spite of the increased amounts of As(III) absorbed by the yap8 mutant, the enhanced transcriptional activation of the antioxidant genes such as GSH1 (γ- glutamylcysteine synthetase gene), SOD1 (superoxide dismutase 1 gene) and TRX2 (thioredoxin 2 gene) may prevent protein oxidation. In contrast, the yap1 mutant exhibits high contents of protein carbonyl groups and the GSSG/GSH ratio is severely disturbed on exposure to arsenic compounds in these cells. These results point to an additional level of Yap1 contribution to arsenic stress responses by preventing oxidative damage in cells exposed to these compounds. Transcriptional profiling revealed that genes of the functional categories related to sulphur and methionine metabolism and to the maintenance of cell redox homoeostasis are activated to mediate adaptation of the wild-type strain to 2 mM arsenate treatment.

Molecular mechanisms of antimony resistance in Leishmania

Leishmaniasis causes significant morbidity and mortality worldwide. The disease is endemic in developing countries of tropical regions, and in recent years economic globalization and increased travel have extended its reach to people in developed countries. In the absence of effective vaccines and vector-control measures, the main line of defence against the disease is chemotherapy. Organic pentavalent antimonials [Sb(V)] have been the first-line drugs for the treatment of leishmaniasis for the last six decades, and clinical resistance to these drugs has emerged as a primary obstacle to successful treatment and control. A multiplicity of resistance mechanisms have been described in resistantLeishmaniamutants developedin vitroby stepwise increases of the concentration of either antimony [Sb(III)] or the related metal arsenic [As(III)], the most prevalent mechanism being upregulated Sb(III) detoxification and sequestration. With the availability of resistant field isolates, it has now become possible to elucidate mechanisms of clinical resistance. The present review describes the mechanisms of antimony resistance inLeishmaniaand highlights the links between previous hypotheses and current developments in field studies. Unravelling the molecular mechanisms of clinical resistance could allow the prevention and circumvention of resistance, as well as rational drug design for the treatment of drug-resistantLeishmania.

A CDC25 homologue from rice functions as an arsenate reductase

Enzymatic reduction of arsenate to arsenite is the first step in arsenate metabolism in all organisms studied. The rice genome contains two ACR2-like genes, OsACR2.1 and OsACR2.2, which may be involved in regulating arsenic metabolism in rice. Here, we cloned both OsACR2 genes and expressed them in an Escherichia coli strain in which the arsC gene was deleted and in a yeast (Saccharomyces cerevisiae) strain with a disrupted ACR2 gene. OsACR2.1 complemented the arsenate hypersensitive phenotype of E. coli and yeast. OsACR2.2 showed much less ability to complement. The gene products were purified and demonstrated to reduce arsenate to arsenite in vitro, and both exhibited phosphatase activity. In agreement with the complementation results, OsACR2.1 exhibited higher reductase activity than OsACR2.2. Mutagenesis of cysteine residues in the putative active site HC(X)(5)R motif led to nearly complete loss of both phosphatase and arsenate reductase activities. In planta expression of OsACR2.1 increased dramatically after exposure to arsenate. OsACR2.2 was observed only in roots following arsenate exposure, and its expression was less than OsACR2.1.

Arsenate Reduction: Thiol Cascade Chemistry with Convergent Evolution

The frequent abundance of arsenic in the environment has guided the evolution of enzymes for the reduction of arsenate. The arsenate reductases (ArsC) from different sources have unrelated sequences and structural folds, and can be divided into different classes on the basis of their structures, reduction mechanisms and the locations of catalytic cysteine residues. The thioredoxin-coupled arsenate reductase class is represented by Staphylococcus aureus pI258 ArsC and Bacillus subtilis ArsC. The ArsC from Escherichia coli plasmid R773 and the eukaryotic ACR2p reductase from Saccharomyces cerevisiae represent two distinct glutaredoxin-linked ArsC classes. All are small cytoplasmic redox enzymes that reduce arsenate to arsenite by the sequential involvement of three different thiolate nucleophiles that function as a redox cascade. In contrast, the ArrAB complex is a bacterial heterodimeric periplasmic or a surface-anchored arsenate reductase that functions as a terminal electron acceptor and transfers electrons from the membrane respiratory chain to arsenate. Finally, the less well documented arsenate reductase activity of the monomeric arsenic(III) methylase, which is an S-adenosylmethionine (AdoMet)-dependent methyltransferase. After each oxidative methylation cycle and before the next methylation step, As(V) is reduced to As(III). Methylation by this enzyme is also considered an arsenic-resistance mechanism for bacteria, fungi and mammals.

Bifunctional role of the leishmanial antimonate reductase LmACR2 as a protein tyrosine phosphatase

LmACR2 is the first identified antimonate reductase responsible for the reduction of pentavalent antimony in pentostam to the active trivalent form of the drug in Leishmania. LmACR2 is a homologue of the yeast arsenate reductase Acr2p and Cdc25 phosphatases and has the HC[X]5R phosphatase motif. Purified LmACR2 exhibited phosphatase activity in vitro and was able to dephosphorylate a phosphotyrosine residue from a synthetic peptide. This phosphatase activity was inhibited by classical inhibitors such as orthovanadate. LmACR2-catalyzed phosphatase activity was inhibited by either antimonate or arsenate. Site-directed mutagenesis experiments showed that the H74C[X]5R81 motif was involved in catalysis. This is the first report of a metalloid reductase with a bifunctional role in protein tyrosine phosphatase activity. Leishmania is never exposed to metalloids during its life cycle. It is therefore unlikely that it would evolve an enzyme exclusively for drug activation. We propose that the physiological function of LmACR2 is to dephosphorylate phosphotyrosine residues in leishmanial proteins.

A novel arsenate reductase from the arsenic hyperaccumulating fern Pteris vittata

Pteris vittata sporophytes hyperaccumulate arsenic to 1% to 2% of their dry weight. Like the sporophyte, the gametophyte was found to reduce arsenate [As(V)] to arsenite [As(III)] and store arsenic as free As(III). Here, we report the isolation of an arsenate reductase gene (PvACR2) from gametophytes that can suppress the arsenate sensitivity and arsenic hyperaccumulation phenotypes of yeast (Saccharomyces cerevisiae) lacking the arsenate reductase gene ScACR2. Recombinant PvACR2 protein has in vitro arsenate reductase activity similar to ScACR2. While PvACR2 and ScACR2 have sequence similarities to the CDC25 protein tyrosine phosphatases, they lack phosphatase activity. In contrast, Arath;CDC25, an Arabidopsis (Arabidopsis thaliana) homolog of PvACR2 was found to have both arsenate reductase and phosphatase activities. To our knowledge, PvACR2 is the first reported plant arsenate reductase that lacks phosphatase activity. CDC25 protein tyrosine phosphatases and arsenate reductases have a conserved HCX5R motif that defines the active site. PvACR2 is unique in that the arginine of this motif, previously shown to be essential for phosphatase and reductase activity, is replaced with a serine. Steady-state levels of PvACR2 expression in gametophytes were found to be similar in the absence and presence of arsenate, while total arsenate reductase activity in P. vittata gametophytes was found to be constitutive and unaffected by arsenate, consistent with other known metal hyperaccumulation mechanisms in plants. The unusual active site of PvACR2 and the arsenate reductase activities of cell-free extracts correlate with the ability of P. vittata to hyperaccumulate arsenite, suggesting that PvACR2 may play an important role in this process.

Interplay between ion binding and catalysis in the thioredoxin-coupled arsenate reductase family

In the thioredoxin (Trx)-coupled arsenate reductase family, arsenate reductase from Staphylococcus aureus plasmid pI258 (Sa_ArsC) and from Bacillus subtilis (Bs_ArsC) are structurally related detoxification enzymes. Catalysis of the reduction of arsenate to arsenite involves a P-loop (Cys10Thr11Gly12Asn13Ser14Cys15Arg16) structural motif and a disulphide cascade between three conserved cysteine residues (Cys10, Cys82 and Cys89). For its activity, Sa_ArsC benefits from the binding of tetrahedral oxyanions in the P-loop active site and from the binding of potassium in a specific cation-binding site. In contrast, the steady-state kinetic parameters of Bs_ArsC are not affected by sulphate or potassium. The commonly occurring mutation of a histidine (H62), located about 6 A from the potassium-binding site in Sa_ArsC, to a glutamine uncouples the kinetic dependency on potassium. In addition, the binding affinity for potassium is affected by the presence of a lysine (K33) or an aspartic acid (D33) in combination with two negative charges (D30 and E31) on the surface of Trx-coupled arsenate reductases. In the P-loop of the Trx-coupled arsenate reductase family, the peptide bond between Gly12 and Asn13 can adopt two distinct conformations. The unique geometry of the P-loop with Asn13 in beta conformation, which is not observed in structurally related LMW PTPases, is stabilized by tetrahedral oxyanions and decreases the pK(a) value of Cys10 and Cys82. Tetrahedral oxyanions stabilize the P-loop in its catalytically most active form, which might explain the observed increase in k(cat) value for Sa_ArsC. Therefore, a subtle interplay of potassium and sulphate dictates the kinetics of Trx-coupled arsenate reductases.

Enhanced arsenate reduction by a CDC25-like tyrosine phosphatase explains increased phytochelatin accumulation in arsenate-tolerant Holcus lanatus

Decreased arsenate [As(V)] uptake is the major mechanism of naturally selected As(V) hypertolerance in plants. However, As(V)-hypertolerant ecotypes also show enhanced rates of phytochelatin (PC) accumulation, suggesting that improved sequestration might additionally contribute to the hypertolerance phenotype. Here, we show that enhanced PC-based sequestration in As(V)-hypertolerant Holcus lanatus is not due to an enhanced capacity for PC synthesis as such, but to increased As(V) reductase activity. Vacuolar transport of arsenite-thiol complexes was equal in both ecotypes. Based on homology with the yeast As(V) reductase, Acr2p, we identified a Cdc25-like plant candidate, HlAsr, and confirmed the As(V) reductase activity of both HlAsr and the homologous protein from Arabidopsis thaliana. The gene appeared to be As(V)-inducible and its expression was enhanced in the As(V)-hypertolerant H. lanatus ecotype, compared with the non-tolerant ecotype. Homologous ectopic overexpression of the AtASR cDNA in A. thaliana produced a dual phenotype. It improved tolerance to mildly toxic levels of As(V) exposure, but caused hypersensitivity to more toxic levels. Arabidopsis asr T-DNA mutants showed increased As(V) sensitivity at low exposure levels and enhanced arsenic retention in the root. It is argued that, next to decreased uptake, enhanced expression of HlASR might act as an additional determinant of As(V) hypertolerance and As transport in H. lanatus.

Yield and arsenate uptake of arbuscular mycorrhizal tomato colonized by Glomus mosseae BEG167 in As spiked soil under glasshouse conditions

A glasshouse pot experiment was conducted to study the effect of arbuscular mycorrhizal (AM) colonization by Glomus mosseae BEG167 on the yield and arsenate uptake of tomato plants in soil experimentally contaminated with five As levels (0, 25, 50, 75 and 150 mg kg(-1)). Mycorrhizal colonization (50-70% of root length) was little affected by As application and declined only in soil amended with 150 mg As kg(-1). Mycorrhizal colonization increased plant biomass at As application rates of 25, 50 and 75 mg kg(-1). Shoot As concentration increased with increasing As addition up to 50 mg kg(-1) but decreased with mycorrhizal colonization at As addition rates of 75 and 150 mg kg(-1). Shoot As uptake increased with mycorrhizal colonization at most As addition levels studied, but tended to decrease with addition of 150 mg As kg(-1). Total P uptake by mycorrhizal plants was elevated at As rates of 25, 50 and 75 mg kg(-1), and more P was allocated to the roots of mycorrhizal plants. Mycorrhizal plants had higher shoot and root P/As ratios at higher As application rates than did non-mycorrhizal controls. The soil of inoculated treatments had higher available As than uninoculated controls, and higher pH values at As addition levels of 25, 50 and 75 mg kg(-1). Mycorrhizal colonization may have increased plant resistance to potential As toxicity at the highest level of As contamination studied. Mycorrhizal tomato plants may have potential for phytoextraction of As from moderately contaminated soils or phytostabilization of more highly polluted sites.

Reduction of anti-leishmanial pentavalent antimonial drugs by a parasite-specific thiol-dependent reductase, TDR1

The reason why Leishmania parasites are susceptible to organic antimonial drugs, the standard chemotherapeutic agents for over 50 years, apparently lies in the fact that the mammalian stage of the parasite reduces the pentavalent form of the administered drug to a trivalent form that causes parasite death. We have identified and characterized a parasite-specific enzyme that can catalyse the reduction of pentavalent antimonials and may therefore be central to the anti-parasite activity of the drug. The unusual protein, a trimer of two-domain monomers in which each domain has some similarity to the Omega class glutathione S-transferases, is a thiol-dependent reductase (designated TDR1) that converts pentavalent antimonials into trivalent antimonials using glutathione as the reductant. The higher abundance of the enzyme in the mammalian stage of the parasite could explain why this parasite form is more susceptible to the drug.

Leishmania major LmACR2 is a pentavalent antimony reductase that confers sensitivity to the drug pentostam

Arsenicals and antimonials are first line drugs for the treatment of trypanosomal and leishmanial diseases. To create the active form of the drug, Sb(V) must be reduced to Sb(III). Because arsenic and antimony are related metalloids, and arsenical resistant Leishmania strains are frequently cross-resistant to antimonials, we considered the possibility that Sb(V) is reduced by a leishmanial As(V) reductase. The sequence for the arsenate reductase of Saccharomyces cerevisiae, ScAcr2p, was used to clone the gene for a homologue, LmACR2, from Leishmania major. LmACR2 was able to complement the arsenate-sensitive phenotype of an arsC deletion strain of Escherichia coli or an ScACR2 deletion strain of Saccharomyces cerevisiae. Transfection of Leishmania infantum with LmACR2 augmented Pentostam sensitivity in intracellular amastigotes. LmACR2 was purified and shown to reduce both As(V) and Sb(V). This is the first report of an enzyme that confers Pentostam sensitivity in intracellular amastigotes of Leishmania. We propose that LmACR2 is responsible for reduction of the pentavalent antimony in Pentostam to the active trivalent form of the drug in Leishmania.

Yap8p activation inSaccharomyces cerevisiaeunder arsenic conditions

Yap8p, a member of the Saccharomyces cerevisiae Yap family, is activated in response to arsenic. Both the mechanisms by which this activation takes place and its regulation have not yet been identified. In this report, we show that Yap8p is not activated at the transcriptional level but, rather, its nuclear transport is actively regulated and dependent on the exportin chromosome region maintenance protein. In addition, it is shown that Cys(132), Cys(137)and Cys(274) are essential for Yap8p localization and transactivation function both of which are required for its biological activity.

An arsenate reductase from Synechocystis sp. strain PCC 6803 exhibits a novel combination of catalytic characteristics

The deduced protein product of open reading frame slr0946 from Synechocystis sp. strain PCC 6803, SynArsC, contains the conserved sequence features of the enzyme superfamily that includes the low-molecular-weight protein-tyrosine phosphatases and the Staphylococcus aureus pI258 ArsC arsenate reductase. The recombinant protein product of slr0946, rSynArsC, exhibited vigorous arsenate reductase activity (V(max) = 3.1 micro mol/min. mg), as well as weak phosphatase activity toward p-nitrophenyl phosphate (V(max) = 0.08 micro mol/min. mg) indicative of its phosphohydrolytic ancestry. pI258 ArsC from S. aureus is the prototype of one of three distinct families of detoxifying arsenate reductases. The prototypes of the others are Acr2p from Saccharomyces cerevisiae and R773 ArsC from Escherichia coli. All three have converged upon catalytic mechanisms involving an arsenocysteine intermediate. While SynArsC is homologous to pI258 ArsC, its catalytic mechanism exhibited a unique combination of features. rSynArsC employed glutathione and glutaredoxin as the source of reducing equivalents, like Acr2p and R773 ArsC, rather than thioredoxin, as does the S. aureus enzyme. As postulated for Acr2p and R773 ArsC, rSynArsC formed a covalent complex with glutathione in an arsenate-dependent manner. rSynArsC contains three essential cysteine residues like pI258 ArsC, whereas the yeast and E. coli enzymes require only one cysteine for catalysis. As in the S. aureus enzyme, these "extra" cysteines apparently shuttle a disulfide bond to the enzyme's surface to render it accessible for reduction. SynArsC and pI258 ArsC thus appear to represent alternative branches in the evolution of their shared phosphohydrolytic ancestor into an agent of arsenic detoxification.

Arsenic sensing and resistance system in the cyanobacterium Synechocystis sp. strain PCC 6803

Arsenic is one of the most important global environmental pollutants. Here we show that the cyanobacterium Synechocystis sp. strain PCC 6803 contains an arsenic and antimony resistance operon consisting of three genes: arsB, encoding a putative arsenite and antimonite carrier, arsH, encoding a protein of unknown function, and arsC, encoding a putative arsenate reductase. While arsB mutant strains were sensitive to arsenite, arsenate, and antimonite, arsC mutants were sensitive only to arsenate. The arsH mutant strain showed no obvious phenotype under the conditions tested. In vivo the arsBHC operon was derepressed by oxyanions of arsenic and antimony (oxidation state, +3) and, to a lesser extent, by bismuth (oxidation state, +3) and arsenate (oxidation state, +5). In the absence of these effectors, the operon was repressed by a transcription repressor of the ArsR/SmtB family, encoded by an unlinked gene termed arsR. Thus, arsR null mutants showed constitutive derepression of the arsBHC operon. Expression of the arsR gene was not altered by the presence of arsenic or antimony compounds. Purified recombinant ArsR protein binds to the arsBHC promoter-operator region in the absence of metals and dissociates from the DNA in the presence of Sb(III) or As(III) but not in the presence of As(V), suggesting that trivalent metalloids are the true inducers of the system. DNase I footprinting experiments indicate that ArsR binds to two 17-bp direct repeats, with each one consisting of two inverted repeats, in the region from nucleotides -34 to + 17 of the arsBHC promoter-operator.

Ibp1p, a novel Cdc25-related phosphatase, suppresses Schizosaccharomyces pombe hsk1 ( cdc7)

We report the identification of a novel Cdc25-like protein phosphatase, Ibp1, in the fission yeast Schizosaccharomyces pombe. Ibp1 is closely related to the catalytic subunit of the Cdc25 dual-specificity phosphatases and has phosphatase activity in vitro. Over-production of catalytically active Ibp1 robustly suppresses a mutation in the replication initiation kinase Hsk1p, a member of the Cdc7 family of protein kinases and weakly suppresses mutation of Rad4/Cut5, a DNA polymerase epsilon-associated factor. Ibp1 is not required for viability, suggesting it may be a non-essential regulator of DNA replication or chromosome structure during S phase.

Directed evolution of a yeast arsenate reductase into a protein-tyrosine phosphatase

Arsenic, which is ubiquitous in the environment and comes from both geochemical and anthropogenic sources, has become a worldwide public health problem. Every organism studied has intrinsic or acquired mechanisms for arsenic detoxification. In Saccharomyces cerevisiae arsenate is detoxified by Acr2p, an arsenate reductase. Acr2p is not a phosphatase but is a homologue of CDC25 phosphatases. It has the HCX5R phosphatase motif but not the glycine-rich phosphate binding motif (GXGXXG) that is found in protein-tyrosine phosphatases. Here we show that creation of a phosphate binding motif through the introduction of glycines at positions 79, 81, and 84 in Acr2p resulted in a gain of phosphotyrosine phosphatase activity and a loss of arsenate reductase activity. Arsenate likely achieved geochemical abundance only after the atmosphere became oxidizing, creating pressure for the evolution of an arsenate reductase from a protein-tyrosine phosphatase. The ease by which an arsenate reductase can be converted into a protein-tyrosine phosphatase supports this hypothesis.

Role of yeast glutaredoxins as glutathione S-transferases

The yeast Saccharomyces cerevisiae contains two glutaredoxins, encoded by GRX1 and GRX2, that are required for resistance to reactive oxygen species. We recently reported that Grx1 is active as a glutathione peroxidase and can directly reduce hydroperoxides (Collinson, E. J., Wheeler, G. L., Garrido, E. O., Avery, A. M., Avery, S. V., and Grant, C. M. (2002) J. Biol. Chem. 277, 16712-16717). We now show that Grx2 is also a general hydroperoxidase, and kinetic data indicate that both enzymes have a similar pattern of activity, which is highest with hydrogen peroxide, followed by cumene hydroperoxide and tert-butyl hydroperoxide. Furthermore, both Grx1 and Grx2 are shown be active as glutathione S-transferases (GSTs), and their activity with model substrates such as 1-chloro-2,4-dinitrobenzene is similar to their activity with hydroperoxides. Analysis of the Grx1 active site residues shows that Cys-27, but not Cys-30, is required for both the peroxidase and transferase activities, indicating that these reactions proceed via a monothiol mechanism. Deletion analysis shows that Grx1 and Grx2 have an overlapping function with yeast GSTs, encoded by GTT1 and GTT2, and are responsible for the majority of cellular GST activity. In addition, multiple mutants lacking GRX1, GRX2, GTT1, and GTT2 show increased sensitivity to stress conditions, including exposure to xenobiotics, heat, and oxidants. In summary, glutaredoxins are multifunctional enzymes with oxidoreductase, peroxidase, and GST activity, and are therefore ideally suited to detoxify the wide range of xenobiotics and oxidants that can be generated during diverse stress conditions.

The Caenorhabditis elegans mRNA 5'-capping enzyme. In vitro and in vivo characterization

Eukaryotic mRNA capping enzymes are bifunctional, carrying both RNA triphosphatase (RTPase) and guanylyltransferase (GTase) activities. The Caenorhabditis elegans CEL-1 capping enzyme consists of an N-terminal region with RTPase activity and a C-terminal region that resembles known GTases, However, CEL-1 has not previously been shown to have GTase activity. Cloning of the cel-1 cDNA shows that the full-length protein has 623 amino acids, including an additional 38 residues at the C termini and 12 residues at the N termini not originally predicted from the genomic sequence. Full-length CEL-1 has RTPase and GTase activities, and the cDNA can functionally replace the capping enzyme genes in Saccharomyces cerevisiae. The CEL-1 RTPase domain is related by sequence to protein-tyrosine phosphatases; therefore, mutagenesis of residues predicted to be important for RTPase activity was carried out. CEL-1 uses a mechanism similar to protein-tyrosine phosphatases, except that there was not an absolute requirement for a conserved acidic residue that acts as a proton donor. CEL-1 shows a strong preference for RNA substrates of at least three nucleotides in length. RNA-mediated interference in C. elegans embryos shows that lack of CEL-1 causes development to arrest with a phenotype similar to that seen when RNA polymerase II elongation activity is disrupted. Therefore, capping is essential for gene expression in metazoans.

Non-reciprocal regulation of the redox state of the glutathione-glutaredoxin and thioredoxin systems

Our studies in yeast show that there is an essential requirement for either an active thioredoxin or an active glutathione (GSH)-glutaredoxin system for cell viability. Glutathione reductase (Glr1) and thioredoxin reductase (Trr1) are key regulatory enzymes that determine the redox state of the GSH-glutaredoxin and thioredoxin systems, respectively. Here we show that Trr1 is required during normal cell growth, whereas there is no apparent requirement for Glr1. Analysis of the redox state of thioredoxins and glutaredoxins in glr1 and trr1 mutants reveals that thioredoxins are maintained independently of the glutathione system. In contrast, there is a strong correlation between the redox state of glutaredoxins and the oxidation state of the GSSG/2GSH redox couple. We suggest that independent redox regulation of thioredoxins enables cells to survive in conditions under which the GSH-glutaredoxin system is oxidized.

CxxS: fold-independent redox motif revealed by genome-wide searches for thiol/disulfide oxidoreductase function

Redox reactions involving thiol groups in proteins are major participants in cellular redox regulation and antioxidant defense. Although mechanistically similar, thiol-dependent redox processes are catalyzed by structurally distinct families of enzymes, which are difficult to identify by available protein function prediction programs. Herein, we identified a functional motif, CxxS (cysteine separated from serine by two other residues), that was often conserved in redox enzymes, but rarely in other proteins. Analyses of complete Escherichia coli, Campylobacter jejuni, Methanococcus jannaschii, and Saccharomyces cerevisiae genomes revealed a high proportion of proteins known to use the CxxS motif for redox function. This allowed us to make predictions in regard to redox function and identity of redox groups for several proteins whose function previously was not known. Many proteins containing the CxxS motif had a thioredoxin fold, but other structural folds were also present, and CxxS was often located in these proteins upstream of an alpha-helix. Thus, a conserved CxxS sequence followed by an alpha-helix is typically indicative of a redox function and corresponds to thiol-dependent redox sites in proteins. The data also indicate a general approach of genome-wide identification of redox proteins by searching for simple conserved motifs within secondary structure patterns.

Microbial arsenic: from geocycles to genes and enzymes

Arsenic compounds have been abundant at near toxic levels in the environment since the origin of life. In response, microbes have evolved mechanisms for arsenic resistance and enzymes that oxidize As(III) to As(V) or reduce As(V) to As(III). Formation and degradation of organoarsenicals, for example methylarsenic compounds, occur. There is a global arsenic geocycle, where microbial metabolism and mobilization (or immobilization) are important processes. Recent progress in studies of the ars operon (conferring resistance to As(III) and As(V)) in many bacterial types (and related systems in Archaea and yeast) and new understanding of arsenite oxidation and arsenate reduction by respiratory-chain-linked enzyme complexes has been substantial. The DNA sequencing and protein crystal structures have established the convergent evolution of three classes of arsenate reductases (that is classes of arsenate reductases are not of common evolutionary origin). Proposed reaction mechanisms in each case involve three cysteine thiols and S-As bond intermediates, so convergent evolution to similar mechanisms has taken place.

All intermediates of the arsenate reductase mechanism, including an intramolecular dynamic disulfide cascade

The mechanism of pI258 arsenate reductase (ArsC) catalyzed arsenate reduction, involving its P-loop structural motif and three redox active cysteines, has been unraveled. All essential intermediates are visualized with x-ray crystallography, and NMR is used to map dynamic regions in a key disulfide intermediate. Steady-state kinetics of ArsC mutants gives a view of the crucial residues for catalysis. ArsC combines a phosphatase-like nucleophilic displacement reaction with a unique intramolecular disulfide bond cascade. Within this cascade, the formation of a disulfide bond triggers a reversible "conformational switch" that transfers the oxidative equivalents to the surface of the protein, while releasing the reduced substrate.

Current awareness on yeast

In order to keep subscribers up-to-date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly-published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (3 weeks journals - search completed 5th. Dec. 2001)

Bacillus subtilis arsenate reductase is structurally and functionally similar to low molecular weight protein tyrosine phosphatases

Arsenate is an abundant oxyanion that, because of its ability to mimic the phosphate group, is toxic to cells. Arsenate reductase (EC; encoded by the arsC gene in bacteria) participates to achieve arsenate resistance in both prokaryotes and yeast by reducing arsenate to arsenite; the arsenite is then exported by a specific transporter. The crystal structure of Bacillus subtilis arsenate reductase in the reduced form with a bound sulfate ion in its active site is solved at 1.6-A resolution. Significant structural similarity is seen between arsenate reductase and bovine low molecular weight protein tyrosine phosphatase, despite very low sequence identity. The similarity is especially high between their active sites. It is further confirmed that this structural homology is relevant functionally by showing the phosphatase activity of the arsenate reductase in vitro. Thus, we can understand the arsenate reduction in the light of low molecular weight protein tyrosine phosphatase mechanism and also explain the catalytic roles of essential residues such as Cys-10, Cys-82, Cys-89, Arg-16, and Asp-105. A "triple cysteine redox relay" is proposed for the arsenate reduction mechanism.