Ycf1p-dependent Hg(II) detoxification in Saccharomyces cerevisiae

MTP family analysis and association study reveal the role of ZmMTP11 in lead (Pb) accumulation

The metal tolerance protein (MTP) gene family plays an essential role in the transport of heavy metals, however the function of the MTP family in transporting lead (Pb) was still unclear in plants. In this study, we identified and characterized 12 ZmMTPs in the whole genome of maize. These ZmMTP genes were divided into three subfamilies in evolution, namely Zn-CDF, Zn/Fe-CDF, Mn-CDF subfamilies, which showed diverse expression patterns in different tissues of maize. Using gene-based association analyses, we identified a Pb accumulation-related MTP member in maize, ZmMTP11, which was located in plasma membrane and had the potential of transporting Pb ion. Under the Pb treatment, ZmMTP11 showed a generally decreased expression relative to the normal conditions. Heterologous expressions of ZmMTP11 in yeast, Arabidopsis, and rice demonstrated that ZmMTP11 enhanced Pb accumulation in the cells without affecting yeast and plant growth under Pb stress. Remarkably, the increased Pb concentration in the plant roots did not cause changes in Pb content in the shoots. Our study provides new insights into the genetic improvement of heavy metal tolerance in plants and contributes to bioremediation of Pb-contaminant soils.

Multilevel Regulation of Membrane Proteins in Response to Metal and Metalloid Stress: A Lesson from Yeast

In the face of flourishing industrialization and global trade, heavy metal and metalloid contamination of the environment is a growing concern throughout the world. The widespread presence of highly toxic compounds of arsenic, antimony, and cadmium in nature poses a particular threat to human health. Prolonged exposure to these toxins has been associated with severe human diseases, including cancer, diabetes, and neurodegenerative disorders. These toxins are known to induce analogous cellular stresses, such as DNA damage, disturbance of redox homeostasis, and proteotoxicity. To overcome these threats and improve or devise treatment methods, it is crucial to understand the mechanisms of cellular detoxification in metal and metalloid stress. Membrane proteins are key cellular components involved in the uptake, vacuolar/lysosomal sequestration, and efflux of these compounds; thus, deciphering the multilevel regulation of these proteins is of the utmost importance. In this review, we summarize data on the mechanisms of arsenic, antimony, and cadmium detoxification in the context of membrane proteome. We used yeast Saccharomyces cerevisiae as a eukaryotic model to elucidate the complex mechanisms of the production, regulation, and degradation of selected membrane transporters under metal(loid)-induced stress conditions. Additionally, we present data on orthologues membrane proteins involved in metal(loid)-associated diseases in humans.

Structural Basis for Oxidized Glutathione Recognition by the Yeast Cadmium Factor 1

Transporters from the ABCC family have an essential role in detoxifying electrophilic compounds including metals, drugs, and lipids, often through conjugation with glutathione complexes. The Yeast Cadmium Factor 1 (Ycf1) transports glutathione alone as well as glutathione conjugated to toxic heavy metals including Cd2+, Hg2+, and As3+. To understand the complicated selectivity and promiscuity of heavy metal substrate binding, we determined the cryo-EM structure of Ycf1 bound to the substrate, oxidized glutathione. We systematically tested binding determinants with cellular survival assays against cadmium to determine how the substrate site accommodates different-sized metal complexes. We identify a "flex-pocket" for substrate binding that binds glutathione complexes asymmetrically and flexes to accommodate different size complexes.

The potential of microorganisms as biomonitoring and bioremediation tools for mercury-contaminated soils

Mercury (Hg) pollution is a global issue due to the high toxicity and wide dispersion of Hg around the world. Whether due to anthropogenic activities or natural processes, Hg emissions are steadily increasing, with very high levels in some regions, directly threatening human and ecosystem health. However, bacteria and fungi have evolved and adapted in response to Hg-induced stress and have developed tolerance mechanisms, notably based on the mer operon system that is involved in Hg uptake and biovolatilization via Hg reduction reactions. Other processes, such as bioaccumulation or extracellular sequestration, are involved in Hg resistance, and the study of contaminated soils has allowed the isolation of a number of microorganisms capable of these mechanisms, with strong potential for the implementation of bioremediation approaches. In addition to playing an important role in determining the fate of Hg in the biogeochemical cycle, these microorganisms can indeed be applied to reduce Hg concentrations or at least stabilize Hg for the remediation of polluted soils. Moreover, thanks to the development of biotechnological tools, bioremediation based on Hg-tolerant microorganisms can be optimized. Finally, these microorganisms are relevant candidates for biomonitoring, for example, through the engineering of biosensors, because the detection of Hg is a major issue in preserving the health of living beings.

Investigation of the role of TmMYB16/123 and their targets (TmMTP1/11) in the tolerance of Taxus media to cadmium

The toxicity and stress caused by heavy metal contamination has become an important constraint to the growth and flourishing of trees. In particular, species belonging to the genus Taxus, which are the only natural source for the anti-tumor medicine paclitaxel, are known to be highly sensitive to environmental changes. To investigate the response of Taxus spp. to heavy metal stress, we analyzed the transcriptomic profiles of Taxus media trees exposed to cadmium (Cd2+). In total, six putative genes from the metal tolerance protein (MTP) family were identified in T. media, including two Cd2+ stress inducible TMP genes (TmMTP1, TmMTP11 and Taxus media). Secondary structure analyses predicted that TmMTP1 and TmMTP11, which are members of the Zn-CDF and Mn-CDF subfamily proteins, respectively, contained six and four classic transmembrane domains, respectively. The introduction of TmMTP1/11 into the ∆ycf1 yeast cadmium-sensitive mutant strain showed that TmMTP1/11 might regulate the accumulation of Cd2+ to yeast cells. To screen the upstream regulators, partial promoter sequences of the TmMTP1/11 genes were isolated using the chromosome walking method. Several myeloblastosis (MYB) recognition elements were identified in the promoters of these genes. Furthermore, two Cd2+-induced R2R3-MYB TFs, TmMYB16 and TmMYB123, were identified. Both in vitro and in vivo assays confirmed that TmMTB16/123 play a role in Cd2+ tolerance by activating and repressing the expression of TmMTP1/11 genes. The present study elucidated new regulatory mechanisms underlying the response to Cd stress and can contribute to the breeding of Taxus species with high environmental adaptability.

Phytochelatins: Sulfur-Containing Metal(loid)-Chelating Ligands in Plants

Phytochelatins (PCs) are small cysteine-rich peptides capable of binding metal(loid)s via SH-groups. Although the biosynthesis of PCs can be induced in vivo by various metal(loid)s, PCs are mainly involved in the detoxification of cadmium and arsenic (III), as well as mercury, zinc, lead, and copper ions, which have high affinities for S-containing ligands. The present review provides a comprehensive account of the recent data on PC biosynthesis, structure, and role in metal(loid) transport and sequestration in the vacuoles of plant cells. A comparative analysis of PC accumulation in hyperaccumulator plants, which accumulate metal(loid)s in their shoots, and in the excluders, which accumulate metal(loid)s in their roots, investigates the question of whether the endogenous PC concentration determines a plant's tolerance to metal(loid)s. Summarizing the available data, it can be concluded that PCs are not involved in metal(loid) hyperaccumulation machinery, though they play a key role in metal(loid) homeostasis. Unraveling the physiological role of metal(loid)-binding ligands is a fundamental problem of modern molecular biology, plant physiology, ionomics, and toxicology, and is important for the development of technologies used in phytoremediation, biofortification, and phytomining.

The structural basis for regulation of the glutathione transporter Ycf1 by regulatory domain phosphorylation

Yeast Cadmium Factor 1 (Ycf1) sequesters heavy metals and glutathione into the vacuole to counter cell stress. Ycf1 belongs to the ATP binding cassette C-subfamily (ABCC) of transporters, many of which are regulated by phosphorylation on intrinsically-disordered domains. The regulatory mechanism of phosphorylation is still poorly understood. Here, we report two cryo-EM structures of Ycf1 at 3.4 Å and 4.0 Å resolution in inward-facing open conformations that capture previously unobserved ordered states of the intrinsically disordered regulatory domain (R-domain). R-domain phosphorylation is clearly evident and induces a topology promoting electrostatic and hydrophobic interactions with Nucleotide Binding Domain 1 (NBD1) and the Lasso motif. These interactions stay constant between the structures and are related by rigid body movements of the NBD1/R-domain complex. Biochemical data further show R-domain phosphorylation reorganizes the Ycf1 architecture and is required for maximal ATPase activity. Together, we provide insights into how R-domains control ABCC transporter activity.

Molecular Biology of Cadmium Toxicity in Saccharomyces cerevisiae

Cadmium (Cd) is a toxic heavy metal mainly originating from industrial activities and causes environmental pollution. To better understand its toxicity and pollution remediation, we must understand the effects of Cd on living beings. Saccharomyces cerevisiae (budding yeast) is an eukaryotic unicellular model organism. It has provided much scientific knowledge about cellular and molecular biology in addition to its economic benefits. Effects associated with copper and zinc, sulfur and selenium metabolism, calcium (Ca²⁺) balance/signaling, and structure of phospholipids as a result of exposure to cadmium have been evaluated. In yeast as a result of cadmium stress, "mitogen-activated protein kinase," "high osmolarity glycerol," and "cell wall integrity" pathways have been reported to activate different signaling pathways. In addition, abnormalities and changes in protein structure, ribosomes, cell cycle disruption, and reactive oxygen species (ROS) following cadmium cytotoxicity have also been detailed. Moreover, the key OLE1 gene that encodes for delta-9 FA desaturase in relation to cadmium toxicity has been discussed in more detail. Keeping all these studies in mind, an attempt has been made to evaluate published cellular and molecular toxicity data related to Cd stress, and specifically published on S. cerevisiae.

Glutathione-coordinated metal complexes as substrates for cellular transporters

Glutathione is the major thiol-containing species in both prokaryotes and eukaryotes and plays a wide variety of roles, including detoxification of metals by sequestration, reduction, and efflux. ABC transporters such as MRP1 and MRP2 detoxify the cell from certain metals by exporting the cations as a metal-glutathione complex. The ability of the bacterial Atm1 protein to efflux metal-glutathione complexes appears to have evolved over time to become the ABCB7 transporter in mammals, located in the inner mitochondrial membrane. No longer needed for the role of cellular detoxification, ABCB7 appears to be used to transport glutathione-coordinated iron-sulfur clusters from mitochondria to the cytosol.

Role of ATP-binding cassette transporters in maintaining plant homeostasis under abiotic and biotic stresses

The ATP-binding cassette (ABC) transporters belong to a large protein family predominantly present in diverse species. ABC transporters are driven by ATP hydrolysis and can act as exporters as well as importers. These proteins are localized in the membranes of chloroplasts, mitochondria, peroxisomes and vacuoles. ABC proteins are involved in regulating diverse biological processes in plants, such as growth, development, uptake of nutrients, tolerance to biotic and abiotic stresses, tolerance to metal toxicity, stomatal closure, shape and size of grains, protection of pollens, transport of phytohormones, etc. In mitochondria and chloroplast, the iron metabolism and its transport across the membrane are mediated by ABC transporters. Tonoplast-localized ABC transporters are involved in internal detoxification of metal ion; thus protecting against the DNA impairment and maintaining cell growth. ABC transporters are involved in the transport of secondary metabolites inside the cells. Microorganisms also engage a large number of ABC transporters to import and expel substrates decisive for their pathogenesis. ABC transporters also suppress the seed embryonic growth until favorable conditions come. This review aims at giving insights on ABC transporters, their evolution, structure, functions and roles in different biological processes for helping the terrestrial plants to survive under adverse environmental conditions. These specialized plant membrane transporters ensure a sustainable economic yield and high-quality products, especially under unfavorable conditions of growth. These transporters can be suitably manipulated to develop 'Plants for the Future'.

Interactions between Hg and soil microbes: microbial diversity and mechanisms, with an emphasis on fungal processes

Mercury (Hg) is a highly toxic metal with no known biological function, and it can be highly bioavailable in terrestrial ecosystems. Although fungi are important contributors to a number of soil processes including plant nutrient uptake and decomposition, little is known about the effect of Hg on fungi. Fungi accumulate the largest amount of Hg and are the organisms capable of the highest bioaccumulation of Hg. While referring to detailed mechanisms in bacteria, this mini-review emphasizes the progress made recently on this topic and represents the first step towards a better understanding of the mechanisms underlying Hg tolerance and accumulation in fungal species and hence on the role of fungi within the Hg cycle at Hg-contaminated sites. KEY POINTS: • The fungal communities are more resilient than bacterial communities to Hg exposure. • The exposure to Hg is a threat to microbial soil functions involved in both C and nutrient cycles. • Fungal (hyper)accumulation of Hg may be important for the Hg cycle in terrestrial environments. • Understanding Hg tolerance and accumulation by fungi may lead to new remediation biotechnologies.

Glutathione: subcellular distribution and membrane transport 1

Glutathione (γ-l-glutamyl-l-cysteinylglycine) is a small tripeptide found at millimolar concentrations in nearly all eukaryotes as well as many prokaryotic cells. Glutathione synthesis is restricted to the cytosol in animals and fungi and to the cytosol and plastids in plants. Nonetheless, glutathione is found in virtually all subcellular compartments. This implies that transporters must exist that facilitate glutathione transport into and out of the various subcellular compartments. Glutathione may also be exported and imported across the plasma membrane in many cells. However, in most cases, the molecular identity of these transporters remains unclear. Whilst glutathione transport is essential for the supply and replenishment of subcellular glutathione pools, recent evidence supports a more active role for glutathione transport in the regulation of subcellular glutathione redox homeostasis. However, our knowledge of glutathione redox homeostasis at the level of specific subcellular compartments remains remarkably limited and the role of glutathione transport remains largely unclear. In this review, we discuss how new tools and techniques have begun to yield insights into subcellular glutathione distribution and glutathione redox homeostasis. In particular, we discuss the known and putative glutathione transporters and examine their contribution to the regulation of subcellular glutathione redox homeostasis.

Transcriptomic responses of the basidiomycete yeast Sporobolomyces sp. to the mycotoxin patulin

BACKGROUND

Patulin is a mycotoxin produced by Penicillium expansum, the causal agent of blue mold of stored pome fruits, and several other species of filamentous fungi. This mycotoxin has genotoxic, teratogenic and immunotoxic effects in mammals, and its presence in pome fruits and derived products represents a serious health hazard. Biocontrol agents in the Pucciniomycotina, such as the yeasts Sporobolomyces sp. strain IAM 13481 and Rhodosporidium kratochvilovae strain LS11, are able to resist patulin and degrade it into the less toxic compounds desoxypatulinic acid and ascladiol.

RESULTS

In this investigation we applied a transcriptomic approach based on RNAseq to annotate the genome of Sporobolomyces sp. IAM 13481 and then study the changes of gene expression in Sporobolomyces sp. exposed to patulin. Patulin treatment leads to ROS production and oxidative stress that result in the activation of stress response mechanisms controlled by transcription factors. Upregulated Sporobolomyces genes were those involved in oxidation-reduction and transport processes, suggesting the activation of defense mechanisms to resist patulin toxicity and expel the mycotoxin out of the cells. Other upregulated genes encoded proteins involved in metabolic processes such as those of the glutathione and thioredoxin systems, which are essential to restore the cellular redox homeostasis. Conversely, patulin treatment decreased the expression of genes involved in the processes of protein synthesis and modification, such as transcription, RNA processing, translation, protein phosphorylation and biosynthesis of amino acids. Also, genes encoding proteins involved in transport of ions, cell division and cell cycle were downregulated. This indicates a reduction of metabolic activity, probably due to the high energy requirement by the cells or metabolic arrest while recovering from the insult caused by patulin toxicity.

CONCLUSIONS

Complex mechanisms are activated in a biocontrol yeast in response to patulin. The genes identified in this study can pave the way to develop i) a biodetoxification process of patulin in juices and ii) a biosensor for the rapid and cost-effective detection of this mycotoxin.

Genome-scale genetic screen of lead ion-sensitive gene deletion mutations in Saccharomyces cerevisiae

Pb (lead) is one of the most widespread and toxic heavy metal contaminants and imposes potential harm to human health. Pb ions cause cellular damage and induce loss of cell viability. However, mechanisms regulating Pb toxicity remain poorly understood. Through a genome-scale screen, we have identified 30 yeast single-gene deletion mutants that are sensitive to lead ions. These genes are involved in the metabolism, transcription, protein synthesis, cell cycle and DNA processing, protein folding, modification, destination, as well as cellular transport process. Comparative analyses to cadmium-sensitive mutations identified from previous studies indicate that overlapping genes of lead- and cadmium-sensitive mutations are involved in both the metabolism and the cellular transport process. Furthermore, eleven lead-sensitive mutants show elevated levels of lead contents in response to lead stress. Our findings provide a basis to understand molecular mechanisms underlying the detoxification of lead ions by yeast cells.

ABCC subfamily vacuolar transporters are involved in Pb (lead) detoxification in Saccharomyces cerevisiae

The present work has as objective to contribute for the elucidation of the mechanism associated with Pb detoxification, using the yeast Saccharomyces cerevisiae as a model organism. The deletion of GTT1 or GTT2 genes, coding for functional glutathione transferases (GST) enzymes in S. cerevisiae, caused an increased susceptibility to high Pb concentrations (500-1000 μmol L(-1)). These results suggest that the formation of glutathione-Pb conjugate (GS-Pb), dependent of GSTs, is important in Pb detoxification. The involvement of ATP-binding cassette (ABC) vacuolar transporters, belonging to class C subfamily (ABCC) in vacuolar compartmentalization of Pb, was evaluated. For this purpose, mutant strains disrupted in YCF1, VMR1, YBT1 or BPT 1 genes were used. All mutants tested, without vacuolar ABCC transporters, presented an increased sensitivity to 500-1000 μmol L(-1) Pb comparative to wild-type strain. Taken together, the obtained results suggest that Pb detoxification, by vacuolar compartmentalization, can occur as a result of the concerted action of GSTs and vacuolar ABCC transporters. Pb is conjugated with glutathione, catalysed by glutathione transferases and followed to the transport of GS-Pb conjugate to the vacuole by ABCC transporters.

Class C ABC transporters and Saccharomyces cerevisiae vacuole fusion

Membrane fusion is carried out by core machinery that is conserved throughout eukaryotes. This is comprised of Rab GTPases and their effectors, and SNARE proteins, which together are sufficient to drive the fusion of reconstituted proteoliposomes. However, an outer layer of factors that are specific to individual trafficking pathways in vivo regulates the spatial and temporal occurrence of fusion. The homotypic fusion of Saccharomyces cerevisiae vacuolar lysosomes utilizes a growing set of factors to regulate the fusion machinery that include members of the ATP binding cassette (ABC) transporter family. Yeast vacuoles have five class C ABC transporters that are known to transport a variety of toxins into the vacuole lumen as part of detoxifying the cell. We have found that ABCC transporters can also regulate vacuole fusion through novel mechanisms. For instance Ybt1 serves as negative regulator of fusion through its effects on vacuolar Ca²⁺ homeostasis. Additional studies showed that Ycf1 acts as a positive regulator by affecting the efficient recruitment of the SNARE Vam7. Finally, we discuss the potential interface between the translocation of lipids across the membrane bilayer, also known as lipid flipping, and the efficiency of fusion.

Yeast ABC proteins involved in multidrug resistance

Pleiotropic drug resistance is a complex phenomenon that involves many proteins that together create a network. One of the common mechanisms of multidrug resistance in eukaryotic cells is the active efflux of a broad range of xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces cerevisiae is often used as a model to study such activity because of the functional and structural similarities of its ABC transporters to mammalian ones. Numerous ABC transporters are found in humans and some are associated with the resistance of tumors to chemotherapeutics. Efflux pump modulators that change the activity of ABC proteins are the most promising candidate drugs to overcome such resistance. These modulators can be chemically synthesized or isolated from natural sources (e.g., plant alkaloids) and might also be used in the treatment of fungal infections. There are several generations of synthetic modulators that differ in specificity, toxicity and effectiveness, and are often used for other clinical effects.

The MFS-type efflux pump Flr1 induced by Yap1 promotes canthin-6-one resistance in yeast

Screening for suppressors of canthin-6-one toxicity in yeast identified Yap1, a transcription factor involved in cell response to a broad range of injuries. Although canthin-6-one did not promote a significant oxidative stress, overexpression of YAP1 gene clearly increased resistance to this drug. We demonstrated that Yap1-mediated resistance involves the plasma membrane major-facilitator-superfamily efflux pump Flr1 but not the vacuolar ATP-binding-cassette transporter Ycf1. FLR1 overexpression was sufficient to reduce sensitivity to the drug, but strictly dependent on a functional YAP1 gene.

Aerobic transformation of cadmium through metal sulfide biosynthesis in photosynthetic microorganisms

Background

Cadmium is a non-essential metal that is toxic because of its interference with essential metals such as iron, calcium and zinc causing numerous detrimental metabolic and cellular effects. The amount of this metal in the environment has increased dramatically since the advent of the industrial age as a result of mining activities, the use of fertilizers and sewage sludge in farming, and discharges from manufacturing activities. The metal bioremediation utility of phototrophic microbes has been demonstrated through their ability to detoxify Hg(II) into HgS under aerobic conditions. Metal sulfides are generally very insoluble and therefore, biologically unavailable.

Results

When Cd(II) was exposed to cells it was bioconverted into CdS by the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the cyanobacterium, Synechoccocus leopoliensis. Supplementation of the two eukaryotic algae with extra sulfate, but not sulfite or cysteine, increased their cadmium tolerances as well as their abilities to produce CdS, indicating an involvement of sulfate assimilation in the detoxification process. However, the combined activities of extracted serine acetyl-transferase (SAT) and O-acetylserine(thiol)lyase (OASTL) used to monitor sulfate assimilation, was not significantly elevated during cell treatments that favored sulfide biosynthesis. It is possible that the prolonged incubation of the experiments occurring over two days could have compensated for the low rates of sulfate assimilation. This was also the case for S. leopoliensis where sulfite and cysteine as well as sulfate supplementation enhanced CdS synthesis. In general, conditions that increased cadmium sulfide production also resulted in elevated cysteine desulfhydrase activities, strongly suggesting that cysteine is the direct source of sulfur for CdS synthesis.

Conclusions

Cadmium(II) tolerance and CdS formation were significantly enhanced by sulfate supplementation, thus indicating that algae and cyanobacteria can produce CdS in a manner similar to that of HgS. Significant increases in sulfate assimilation as measured by SAT-OASTL activity were not detected. However, the enhanced activity of cysteine desulfhydrase indicates that it is instrumental in the provision of H₂S for aerobic CdS biosynthesis.

The yeast ATP-binding cassette (ABC) transporter Ycf1p enhances the recruitment of the soluble SNARE Vam7p to vacuoles for efficient membrane fusion

The Saccharomyces cerevisiae vacuole contains five ATP-binding cassette class C (ABCC) transporters, including Ycf1p, a family member that was originally characterized as a Cd(2+) transporter. Ycf1p has also been found to physically interact with a wide array of proteins, including factors that regulate vacuole homeostasis. In this study, we examined the role of Ycf1p and other ABCC transporters in the regulation of vacuole homotypic fusion. We found that deletion of YCF1 attenuated in vitro vacuole fusion by up to 40% relative to wild-type vacuoles. Plasmid-expressed wild-type Ycf1p rescued the deletion phenotype; however, Ycf1p containing a mutation of the conserved Lys-669 to Met in the Walker A box of the first nucleotide-binding domain (Ycf1p(K669M)) was unable to complement the fusion defect of ycf1Δ vacuoles. This indicates that the ATPase activity of Ycf1p is required for its function in regulating fusion. In addition, we found that deleting YCF1 caused a striking decrease in vacuolar levels of the soluble SNARE Vam7p, whereas total cellular levels were not altered. The attenuated fusion of ycf1Δ vacuoles was rescued by the addition of recombinant Vam7p to in vitro experiments. Thus, Ycf1p contributes in the recruitment of Vam7p to the vacuole for efficient membrane fusion.

Neutralization by metal ions of the toxicity of sodium selenide

Inert metal-selenide colloids are found in animals. They are believed to afford cross-protection against the toxicities of both metals and selenocompounds. Here, the toxicities of metal salt and sodium selenide mixtures were systematically studied using the death rate of Saccharomyces cerevisiae cells as an indicator. In parallel, the abilities of these mixtures to produce colloids were assessed. Studied metal cations could be classified in three groups: (i) metal ions that protect cells against selenium toxicity and form insoluble colloids with selenide (Ag⁺, Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺ and Zn²⁺), (ii) metal ions which protect cells by producing insoluble metal-selenide complexes and by catalyzing hydrogen selenide oxidation in the presence of dioxygen (Co²⁺ and Ni²⁺) and, finally, (iii) metal ions which do not afford protection and do not interact (Ca²⁺, Mg²⁺, Mn²⁺) or weakly interact (Fe²⁺) with selenide under the assayed conditions. When occurring, the insoluble complexes formed from divalent metal ions and selenide contained equimolar amounts of metal and selenium atoms. With the monovalent silver ion, the complex contained two silver atoms per selenium atom. Next, because selenides are compounds prone to oxidation, the stabilities of the above colloids were evaluated under oxidizing conditions. 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), the reduction of which can be optically followed, was used to promote selenide oxidation. Complexes with cadmium, copper, lead, mercury or silver resisted dissolution by DTNB treatment over several hours. With nickel and cobalt, partial oxidation by DTNB occurred. On the other hand, when starting from ZnSe or FeSe complexes, full decompositions were obtained within a few tens of minutes. The above properties possibly explain why ZnSe and FeSe nanoparticles were not detected in animals exposed to selenocompounds.

Genomic analysis of Saccharomyces cerevisiae isolates that grow optimally with glucose as the sole carbon source

A population of Saccharomyces cerevisiae was cultured for approximately 450 generations in the presence of high glucose to select for genetic variants that grew optimally under these conditions. Using the parental strain BY4741 as the starting population, an evolved culture was obtained after aerobic growth in a high glucose medium for approximately 450 generations. After the evolution period, three single colony isolates were selected for analysis. Next-generation Ion Torrent sequencing was used to evaluate genetic changes. Greater than 100 deletion/insertion changes were found with approximately half of these effecting genes. Additionally, over 180 SNPs were identified with more than one-quarter of these resulting in a nonsynonymous mutation. Affymetrix DNA microarrays and RNseq analysis were used to determine differences in gene expression in the evolved strains compared to the parental strain. It was established that approximately 900 genes demonstrated significantly altered expression in the evolved strains relative to the parental strain. Many of these genes showed similar alterations in their expression in all three evolved strains. Interestingly, genes with altered expression in the three evolved strains included genes with a role in oxidative metabolism. Overall these results are consistent with the physiological observations of optimal growth with glucose as the carbon source. Namely, the decreased ethanol production suggest that the underlying metabolism switched from fermentation to respiration during the selection for optimal growth on glucose.

Hijacking membrane transporters for arsenic phytoextraction

Arsenic is a toxic metalloid and recognized carcinogen. Arsenate and arsenite are the most common arsenic species available for uptake by plants. As an inorganic phosphate (Pi) analog, arsenate is acquired by plant roots through endogenous Pi transport systems. Inside the cell, arsenate is reduced to the thiol-reactive form arsenite. Glutathione (GSH)-conjugates of arsenite may be extruded from the cell or sequestered in vacuoles by members of the ATP-binding cassette (ABC) family of transporters. In the present study we sought to enhance both plant arsenic uptake through Pi transporter overexpression, and plant arsenic tolerance through ABC transporter overexpression. We demonstrate that Arabidopsis thaliana plants overexpressing the high-affinity Pi transporter family members, AtPht1;1 or AtPht1;7, are hypersensitive to arsenate due to increased arsenate uptake. These plants do not exhibit increased sensitivity to arsenite. Co-overexpression of the yeast ABC transporter YCF1 in combination with AtPht1;1 or AtPht1;7 suppresses the arsenate-sensitive phenotype while further enhancing arsenic uptake. Taken together, our results support an arsenic transport mechanism in which arsenate uptake is increased through Pi transporter overexpression, and arsenic tolerance is enhanced through YCF1-mediated vacuolar sequestration. This work substantiates the viability of coupling enhanced uptake and vacuolar sequestration as a means for developing a prototypical engineered arsenic hyperaccumulator.

The phytochelatin transporters AtABCC1 and AtABCC2 mediate tolerance to cadmium and mercury

Heavy metals such as cadmium (Cd) and mercury (Hg) are toxic pollutants that are detrimental to living organisms. Plants employ a two-step mechanism to detoxify toxic ions. First, phytochelatins bind to the toxic ion, and then the metal-phytochelatin complex is sequestered in the vacuole. Two ABCC-type transporters, AtABCC1 and AtABCC2, that play a key role in arsenic detoxification, have recently been identified in Arabidopsis thaliana. However, it is unclear whether these transporters are also implicated in phytochelatin-dependent detoxification of other heavy metals such as Cd(II) and Hg(II). Here, we show that atabcc1 single or atabcc1 atabcc2 double knockout mutants exhibit a hypersensitive phenotype in the presence of Cd(II) and Hg(II). Microscopic analysis using a Cd-sensitive probe revealed that Cd is mostly located in the cytosol of protoplasts of the double mutant, whereas it occurs mainly in the vacuole of wild-type cells. This suggests that the two ABCC transporters are important for vacuolar sequestration of Cd. Heterologous expression of the transporters in Saccharomyces cerevisiae confirmed their role in heavy metal tolerance. Over-expression of AtABCC1 in Arabidopsis resulted in enhanced Cd(II) tolerance and accumulation. Together, these results demonstrate that AtABCC1 and AtABCC2 are important vacuolar transporters that confer tolerance to cadmium and mercury, in addition to their role in arsenic detoxification. These transporters provide useful tools for genetic engineering of plants with enhanced metal tolerance and accumulation, which are desirable characteristics for phytoremediation.

Selenodiglutathione uptake by the Saccharomyces cerevisiae vacuolar ATP-binding cassette transporter Ycf1p

The Saccharomyces cerevisiae vacuolar ATP-binding cassette transporter Ycf1p is involved in heavy metal detoxification by mediating the ATP-dependent transport of glutathione-metal conjugates to the vacuole. In the case of selenite toxicity, deletion of YCF1 was shown to confer increased resistance, rather than sensitivity, to selenite exposure [Pinson B, Sagot I & Daignan-Fornier B (2000) Mol Microbiol36, 679-687]. Here, we show that when Ycf1p is expressed from a multicopy plasmid, the toxicity of selenite is exacerbated. Using secretory vesicles isolated from a sec6-4 mutant transformed either with the plasmid harbouring YCF1 or the control plasmid, we establish that the glutathione-conjugate selenodigluthatione is a high-affinity substrate of this ATP-binding cassette transporter and that oxidized glutathione is also efficiently transported. Finally, we show that the presence of Ycf1p impairs the glutathione/oxidized glutathione ratio of cells subjected to a selenite stress. Possible mechanisms by which Ycf1p-mediated vacuolar uptake of selenodiglutathione and oxidized glutathione enhances selenite toxicity are discussed.

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

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.

Suppression of Ycf1p function by Cka1p-dependent phosphorylation is attenuated in response to salt stress

The yeast vacuolar membrane protein Ycf1p and its mammalian counterpart, MRP1, belong to the ABCC subfamily of ATP-binding cassette transporters. Genetic evidence suggests that the yeast casein kinase 2α, Cka1p, negatively regulates Ycf1p function via phosphorylation of Ser251 within the N-terminus. In this study, we provide strong evidence that Cka1p regulates Ycf1p function via phosphorylation of Ser251. We show that the CK2 holoenzyme interacts with Ycf1p. However, genetic analysis suggests that only Cka1p is required for Ser251 phosphorylation, as the deletion of CKA1 significantly reduces Ser251 phosphorylation in vivo. Furthermore, purified recombinant Cka1p phosphorylates a Ycf1p-derived peptide containing Ser251. We also demonstrate that Ycf1p function is induced in response to high salt stress. Induction of the Ycf1p function strongly correlates with reduced phosphorylation of Ser251. Importantly, Cka1p activity in vivo is similarly reduced in response to salt stress, consistent with our finding that Cka1p directly phosphorylates Ser251 of Ycf1p. We provide genetic and biochemical evidence that strongly suggests that the induction of Ycf1p function is the result of decreased phosphorylation of Ser251. In conclusion, our work demonstrates a novel biochemical role for Cka1p regulation of Ycf1p function in the cellular response of yeast to salt stress.

The relevance of the individual genetic background for the toxicokinetics of two significant neurodevelopmental toxicants: mercury and lead

The heavy metals mercury and lead are well-known and significant developmental neurotoxicants. This review summarizes the genetic factors that modify their toxicokinetics. Understanding toxicokinetics (uptake, biotransformation, distribution, and elimination processes) is a key precondition to understanding the individual health risks associated with exposure. We selected candidate susceptibility genes when evidence was available for (1) genes/proteins playing a significant role in mercury and lead toxicokinetics, (2) gene expression/protein activity being induced by these metals, and (3) mercury and lead toxicokinetics being affected by gene knockout/knockdown or (4) by functional gene polymorphisms. The genetic background is far better known for mercury than for lead toxicokinetics. Involved are genes encoding L-type amino acid transporters, organic anion transporters, glutathione (GSH)-related enzymes, metallothioneins, and transporters of the ABC family. Certain gene variants can influence mercury toxicokinetics, potentially explaining part of the variable susceptibility to mercury toxicity. Delta-aminolevulinic acid dehydratase (ALAD), vitamin D receptor (VDR) and hemochromatosis (HFE) gene variants are the only well-established susceptibility markers of lead toxicity in humans. Many gaps remain in our knowledge about the functional genomics of this issue. This calls for studies to detect functional gene polymorphisms related to mercury- and lead-associated disease phenotypes, to demonstrate the impact of functional polymorphisms and gene knockout/knockdown in relation to toxicity, to confirm the in vivo relevance of genetic variation, and to examine gene-gene interactions on the respective toxicokinetics. Another crucial aspect is knowledge on the maternal-fetal genetic background, which modulates fetal exposure to these neurotoxicants. To completely define the genetically susceptible risk groups, research is also needed on the genes/proteins involved in the toxicodynamics, i.e., in the mechanisms causing adverse effects in the brain. Studies relating the toxicogenetics to neurodevelopmental disorders are lacking (mercury) or very scarce (lead). Thus, the extent of variability in susceptibility to heavy metal-associated neurological outcomes is poorly characterized.

ABC transporters in Saccharomyces cerevisiae and their interactors: new technology advances the biology of the ABCC (MRP) subfamily

Members of the ATP-binding cassette (ABC) transporter superfamily exist in bacteria, fungi, plants, and animals and play key roles in the efflux of xenobiotic compounds, physiological substrates, and toxic intracellular metabolites. Based on sequence relatedness, mammalian ABC proteins have been divided into seven subfamilies, ABC subfamily A (ABCA) to ABCG. This review focuses on recent advances in our understanding of ABC transporters in the model organism Saccharomyces cerevisiae. We propose a revised unified nomenclature for the six yeast ABC subfamilies to reflect the current mammalian designations ABCA to ABCG. In addition, we specifically review the well-studied yeast ABCC subfamily (formerly designated the MRP/CFTR subfamily), which includes six members (Ycf1p, Bpt1p, Ybt1p/Bat1p, Nft1p, Vmr1p, and Yor1p). We focus on Ycf1p, the best-characterized yeast ABCC transporter. Ycf1p is located in the vacuolar membrane in yeast and functions in a manner analogous to that of the human multidrug resistance-related protein (MRP1, also called ABCC1), mediating the transport of glutathione-conjugated toxic compounds. We review what is known about Ycf1p substrates, trafficking, processing, posttranslational modifications, regulation, and interactors. Finally, we discuss a powerful new yeast two-hybrid technology called integrated membrane yeast two-hybrid (iMYTH) technology, which was designed to identify interactors of membrane proteins. iMYTH technology has successfully identified novel interactors of Ycf1p and promises to be an invaluable tool in future efforts to comprehensively define the yeast ABC interactome.

Compartment-specific synthesis of phosphatidylethanolamine is required for normal heavy metal resistance

The enzyme Psd2 catalyzes endosomal synthesis of the phospholipid PE. While this pool of PE represents a minority of total cellular PE, function of Psd2 is required for normal activity of the vacuolar ABC transporter Ycf1. Psd2 controls vacuolar PE levels by acting at the level of the endosome.

Control of lipid composition of membranes is crucial to ensure normal cellular functions. Saccharomyces cerevisiae has two different phosphatidylserine decarboxylase enzymes (Psd1 and Psd2) that catalyze formation of phosphatidylethanolamine. The mitochondrial Psd1 provides roughly 70% of the phosphatidylethanolamine (PE) biosynthesis in the cell with Psd2 carrying out the remainder. Here, we demonstrate that loss of Psd2 causes cells to acquire sensitivity to cadmium even though Psd1 remains intact. This cadmium sensitivity results from loss of normal activity of a vacuolar ATP-binding cassette transporter protein called Ycf1. Measurement of phospholipid levels indicates that loss of Psd2 causes a specific reduction in vacuolar membrane PE levels, whereas total PE levels are not significantly affected. The presence of a phosphatidylinositol transfer protein called Pdr17 is required for Psd2 function and normal cadmium tolerance. We demonstrate that Pdr17 and Psd2 form a complex in vivo that seems essential for maintenance of vacuolar PE levels. Finally, we refine the localization of Psd2 to the endosome arguing that this enzyme controls vacuolar membrane phospholipid content by regulating phospholipids in compartments that will eventually give rise to the vacuole. Disturbance of this regulation of intracellular phospholipid balance leads to selective loss of membrane protein function in the vacuole.

Phytochelatins: peptides involved in heavy metal detoxification

Phytochelatins (PCs) are enzymatically synthesized peptides known to involve in heavy metal detoxification and accumulation, which have been measured in plants grown at high heavy metal concentrations, but few studies have examined the response of plants even at lower environmentally relevant metal concentrations. Recently, genes encoding the enzyme PC synthase have been identified in plants and other species enabling molecular biological studies to untangle the mechanisms underlying PC synthesis and its regulation. The present paper embodies review on recent advances in structure of PCs, their biosynthetic regulation, roles in heavy metal detoxification and/or accumulation, and PC synthase gene expression for better understanding of mechanism involved and to improve phytoremediation efficiency of plants for wider application.

Negative regulation of the yeast ABC transporter Ycf1p by phosphorylation within its N-terminal extension

The yeast vacuolar membrane protein Ycf1p and its mammalian counterpart, MRP1, belong to the ABCC subfamily of ATP-binding cassette (ABC) transporters that rid cells of toxic endogenous and xenobiotic compounds. Like most members of the ABCC subfamily, Ycf1p contains an N-terminal extension in addition to its ABC "core" domain and transports substrates in the form of glutathione conjugates. Ycf1p is subject to complex regulation to ensure its optimal function. Previous studies showed that Ycf1p activity is stimulated by a guanine nucleotide exchange factor, Tus1p, and is positively regulated by phosphorylation in its ABC core domain at residues Ser-908 and Thr-911. Here we provide evidence that phosphorylation of Ser-251 in the Ycf1p N-terminal extension negatively regulates activity. Mutant Ycf1p-S251A exhibits increased resistance to cadmium in vivo and increased Ycf1p-dependent transport of [(3)H]estradiol-beta-17-glucuronide in vitro as compared with wild-type Ycf1p. Activity is restored to the wild-type level for Ycf1-S251E. To identify kinase(s) that negatively regulate Ycf1p function, we conducted an integrated membrane yeast two-hybrid (iMYTH) screen and identified two kinase genes, CKA1 and HAL5, deletion of which increases Ycf1p function. Genetic evidence suggests that Cka1p may regulate Ycf1p function through phosphorylation of Ser-251 either directly or indirectly. Overall, this study provides compelling evidence that negative, as well as positive, regulation of Ycf1p is mediated by phosphorylation.

Identification of the cadmium-inducible Hansenula polymorpha SEO1 gene promoter by transcriptome analysis and its application to whole-cell heavy-metal detection systems

The genomewide gene expression profiling of the methylotrophic yeast Hansenula polymorpha exposed to cadmium (Cd) allowed us to identify novel genes responsive to Cd treatment. To select genes whose promoters can be useful for construction of a cellular Cd biosensor, we further analyzed a set of H. polymorpha genes that exhibited >6-fold induction upon treatment with 300 muM Cd for 2 h. The putative promoters, about 1,000-bp upstream fragments, of these genes were fused with the yeast-enhanced green fluorescence protein (GFP) gene. The resultant reporter cassettes were introduced into H. polymorpha to evaluate promoter strength and specificity. The promoter derived from the H. polymorpha SEO1 gene (HpSEO1) was shown to drive most strongly the expression of GFP upon Cd treatment among the tested promoters. The Cd-inducible activity was retained in the 500-bp deletion fragment of the HpSEO1 promoter but was abolished in the further truncated 250-bp fragment. The 500-bp HpSEO1 promoter directed specific expression of GFP upon exposure to Cd in a dose-dependent manner, with Cd detection ranging from 1 to 900 muM. Comparative analysis of the Saccharomyces cerevisiae SEO1 (ScSEO1) promoter revealed that the ScSEO1 promoter has a broader specificity for heavy metals and is responsive to arsenic and mercury in addition to Cd. Our data demonstrate the potential use of the HpSEO1 promoter as a bioelement in whole-cell biosensors to monitor heavy metal contamination, particularly Cd.

Phytochelatins are synthesized by two vacuolar serine carboxypeptidases inSaccharomyces cerevisiae

Phytochelatins (PCs) are cysteine-rich peptides that chelate heavy metal ions, thereby mediating heavy metal tolerance in plants, fission yeast, and Caenorhabditis elegans. They are synthesized from glutathione by PC synthase, a specific dipeptidyltransferase. While Saccharomyces cerevisiae synthesizes PCs upon exposure to heavy metal ions, the S. cerevisiae genome does not encode a PC synthase homologue. How PCs are synthesized in yeast is unclear. This study shows that the vacuolar serine carboxypeptidases CPY and CPC are responsible for PC synthesis in yeast. The finding of a PCS-like activity of these enzymes in vivo discloses another route for PC biosynthesis in eukaryotes.

Effect of mercury and organomercurials on cellular glucose utilization: a study using resting mercury-resistant yeast cells

AIMS

Mercury compounds are highly toxic to all types of living cells. Isolated yeast strains of Rhodotorula rubra showed high and low resistance pattern towards mercury and organomercurial compounds. To investigate the basis of differential sensitivity of these two types of strains, glucose utilization was measured in the presence of mercury compounds.

METHODS AND RESULTS

Glucose utilization process remained unaffected in resting cells of highly Hg(2+)-resistant strain in the presence of HgCl(2) but not in the presence of phenylmercuric acetate and thimerosal. However, HgCl(2) significantly affected glucose utilization in the case of low-resistant cells. The Hg-retaining ability of the cell wall of highly Hg(2+)-resistant yeast strain was greater than that of the weakly Hg(2+)-resistant strain. The spheroplast-bound Hg(2+) was also significantly less in the highly Hg(2+)-resistant strain than in the weakly Hg(2+)-resistant strain.

CONCLUSIONS

Glucose uptake machinery was not affected in the presence of toxic metal ions in the case of high-resistant strains. But in the case of low Hg(2+)-resistant strain, glucose transport system may be affected either by inactivation of sensor proteins containing -SH group associated with glucose uptake.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cell wall of mercury-resistant yeast cells may play an important role in heavy metal bioremediation process.

Transport of antimony salts byArabidopsis thalianaprotoplasts over-expressing the human multidrug resistance-associated protein 1 (MRP1/ABCC1)

ABC transporters from the multidrug resistance-associated protein (MRP) subfamily are glutathione S-conjugate pumps exhibiting a broad substrate specificity illustrated by numerous xenobiotics, such as anticancer drugs, herbicides, pesticides and heavy metals. The engineering of MRP transporters into plants might be interesting either to reduce the quantity of xenobiotics taken up by the plant in the context of "safe-food" strategies or, conversely, in the development of phytoremediation strategies in which xenobiotics are sequestered in the vacuolar compartment. In this report, we obtained Arabidopsis transgenic plants overexpressing human MRP1. In these plants, expression of MRP1 did not increase plant resistance to antimony salts (Sb(III)), a classical glutathione-conjugate substrate of MRP1. However, the transporter was fully translated in roots and shoots, and targeted to the plasma membrane. In order to investigate the functionality of MRP1 in Arabidopsis, mesophyll cell protoplasts (MCPs) were isolated from transgenic plants and transport activities were measured by using calcein or Sb(III) as substrates. Expression of MRP1 at the plasma membrane was correlated with an increase in the MCPs resistance to Sb(III) and a limitation of the metalloid content in the protoplasts due to an improvement in Sb(III) efflux. Moreover, Sb(III) transport was sensitive to classical inhibitors of the human MRP1, such as MK571 or glibenclamide. These results demonstrate that a human ABC transporter can be functionally introduced in Arabidopsis, which might be useful, with the help of stronger promoters, to reduce the accumulation of xenobiotics in plants, such as heavy metals from multi-contaminated soils.

An ATP-binding cassette transporter related to yeast vacuolar ScYCF1 is important for Cd sequestration in Chlamydomonas reinhardtii

We generated a Cd-sensitive insertional mutant, Cds18, in Chlamydomonas reinhardtii and elucidated the deletion of a 10 kb fragment containing the promoter and a portion of the coding region for CrMRP2 gene that silenced the transcription of CrMRP2 in mutant Cds18. The association between CrMRP2 and Cd sensitivity was confirmed by complementing mutant Cds18 with a cloned genomic DNA fragment containing the promoter and complete coding sequence for CrMRP2. The genomic region and the full-length cDNA for CrMRP2 were cloned and sequenced. Computer searches detected the significant resemblance of CrMRP2 with HsMRP1, AtMRP3 and ScYCF1, in Homo sapiens, Arabidopsis thaliana and Saccharomyces cerevisiae, respectively. All are members of the multidrug resistance-associated protein (MRP)/cystic fibrosis transmembrane conductance regulator (CFTR) subfamily of ATP-binding cassette (ABC) transporters. When the cDNA of CrMRP2 was cloned into the yeast expression vector pEGKT and transformed into the yeast mutant strain DTY168 lacking ScYCF1, it restored the function of ScYCF1, a yeast vacuolar glutathione (GSH)-conjugate ABC transporter. A putative vacuolar-targeting motif (T/I/K)LP(L/K/I) was detected in the N-terminal part of CrMRP2. In wild-type C. reinhardtii, CrMRP2 transcription was significantly up-regulated upon Cd treatment. Comparing with mutant Cds18, the wild-type algal cells accumulated and sequestered more Cd in the stable high molecular weight (HMW) phytochelatin (PC)-Cd complex; the labile low molecular weight (LMW) PC-Cd complex was detected in mutant Cds18 at an earlier stage of Cd treatment. This study demonstrated the expression of CrMRP2 in C. reinhardtii and implicated its function in the formation/accumulation of stable HMW PC-Cd complex.

Gene transcription profiling in pollutant exposed mussels (Mytilus spp.) using a new low-density oligonucleotide microarray

In this study we describe the design and implementation of a novel low-density oligonucleotide microarray (the "Mytox-chip"). It consists of 24 mussel genes involving both normalizing elements and stress response related genes, each represented on the array with one or two different 50 mer oligonucleotide-probe reporters spotted in replicated samples on glass-activated slides. Target genes were selected on the basis of their potential involvement in mechanisms of pollutant and xenobiotic response. They are implicated in both basic and stress related cellular processes such as shock response, biotransformation and excretion, cell-cycle regulation, immune defense, drug metabolism, etc. The microarray was tested on mussels exposed to sublethal concentrations of mercury or a crude North Sea oil mixture. RNA samples were extracted from digestive glands of control and treated mussels for the synthesis of fluorescence labeled cDNAs to be used in dual color hybridizations. Transcription rates of two metallothionein iso-genes (mt10 and mt20), a p53-like gene and actin were quantitatively estimated also by real-time PCR to confirm microarray data. Significant alterations in the gene transcription patterns were seen in response to both treatments.

Metal(loid)s and radionuclides cytotoxicity in Saccharomyces cerevisiae. Role of YCF1, glutathione and effect of buthionine sulfoximine

The presence of heavy metal(loid)s in soils and waters is an important issue with regards to human health. Taking into account speciation problems, in the first part of this report, we investigated under identical growth conditions, yeast tolerance to a set of 15 cytotoxic metal(loid)s and radionuclides. The yeast cadmium factor 1 (YCF1) is an ATP-Binding Cassette transporter mediating the glutathione detoxification of heavy metals. In the second part, metal(loid)s that could be handled by YCF1 and a possible re-localisation of the transporter after heavy metal exposure were evaluated. YCF1 and a C-terminal GFP fusion, YCF1-GFP, were overexpressed in wild-type and Deltaycf1 strains. Both forms were functional, conferring a tolerance to Cd, Sb, As, Pb, Hg but not to Ni, Zn, Cu, Ag, Se, Te, Cr, Sr, Tc, U. Confocal experiments demonstrated that during exposure to cytotoxic metals, the localisation of YCF1-GFP was restricted to the yeast vacuolar membrane. In the last part, the role of glutathione in this resistance mechanism to metal(loid)s was studied. In the presence of heavy metals, application of buthionine sulfoximine (BSO), a well-known inhibitor of gamma-glutamylcysteine synthetase, led to a decrease in the cytosolic pool of GSH and to a limitation of yeast growth. Surprisingly, BSO was able to phenocopy the deletion of gamma-glutamylcysteine synthetase after exposure to Cd but not to Sb or As. In the genetic context of gsh1 and gsh2 yeast mutants, the critical role of GSH for Cd, As, Sb and Hg tolerance was compared to that of wild-type and Deltaycf1.

Regulation of the cadmium stress response through SCF-like ubiquitin ligases: comparison between Saccharomyces cerevisiae, Schizosaccharomyces pombe and mammalian cells

Saccharomyces cerevisiae has developed several mechanisms to cope with exposure to cadmium. In particular, the sulfur compound glutathione plays a pivotal role in cadmium detoxification, and exposure to cadmium leads to a wide reorganization of S. cerevisiae transcriptome and proteome, resulting in a significant increase in glutathione synthesis. Met4, the transcriptional activator of the sulfur metabolism enzymes, is a critical actor in this reorganization. Recent work has uncovered a part of the mechanism of cadmium-induced Met4 regulation, and showed that it occurs trough the SCF ubiquitin ligase complex SCF(Met30). We discuss this regulation in S. cerevisiae and compare it with the regulation of two other transcriptional activators involved in cadmium detoxification: the Schizosaccharomyces pombe Zip1, regulated by SCF(Pof1), and the mammalian Nrf2, regulated by the SCF-like ubiquitin ligase Cul3:Rbx1:Keap1.

Extracellular and cellular mechanisms sustaining metal tolerance in ectomycorrhizal fungi

This review focuses on recent evidence that identifies potential extracellular and cellular mechanisms that may be involved in the tolerance of ectomycorrhizal fungi to excess metals in their environment. It appears likely that mechanisms described in the nonmycorrhizal fungal species are used in the ectomycorrhizal fungi as well. These include mechanisms that reduce uptake of metals into the cytosol by extracellular chelation through extruded ligands and binding onto cell-wall components. Intracellular chelation of metals in the cytosol by a range of ligands (glutathione, metallothioneins), or increased efflux from the cytosol out of the cell or into sequestering compartments are also key mechanisms conferring tolerance. Free-radical scavenging capacities through the activity of superoxide dismutase or production of glutathione add another line of defence against the toxic effect of metals.

The biotransformation of mercury in pH-stat cultures of microfungi

The biotransformation of mercury was monitored in Hymenoscyphus ericae (Read) Korf & Kernan, Neocosmospora vasinfecta E. F. Sm., and Verticillium terrestre (Link) Lindau following the exposure of these fungi to environmentally relevant doses of HgII(HgCl2) in aerated pH-controlled cultures. Mercury applied at 120 and 300 μg·L–1quickly associated with cells of N. vasinfecta. Within 3 h, approximately 80% of the remaining Hg in both treatments had been converted to β-HgS, and at 8 h, 15% and 53% of the original doses had been converted into volatile Hg0. Hymenoscyphus ericae was less efficient at converting HgIIto β-HgS, but it volatilized larger amounts of Hg0. A mercury-tolerant isolate of V. terrestre withstood levels of Hg in the parts per million. When exposed to 270, 1000, or 2000 μg·L–1HgCl2, 93% ± 3% of the amounts remaining in cultures of V. terrestre after 2 h was β-HgS. At 2 mg·L–1, 72% of the Hg was reduced to Hg0in 2 h. Volatilization accounts for the tolerance of V. terrestre to high doses of Hg, whereas the conversion of Hg to β-HgS was the major detoxifying mechanism at lower doses. The formation of β-HgS occurs preferentially with volatilization operating as the default pathway, when the former biotransformation mechanism is saturated.

Yeast ABC transporters-- a tale of sex, stress, drugs and aging

Yeast ATP-binding cassette (ABC) proteins are implicated in many biological phenomena, often acting at crossroads of vital cellular processes. Their functions encompass peptide pheromone secretion, regulation of mitochondrial function, vacuolar detoxification, as well as pleiotropic drug resistance and stress adaptation. Because yeast harbors several homologues of mammalian ABC proteins with medical importance, understanding their molecular mechanisms, substrate interaction and three-dimensional structure of yeast ABC proteins might help identifying new approaches aimed at combating drug resistance or other ABC-mediated diseases. This review provides a comprehensive discussion on the functions of the ABC protein family in the yeast Saccharomyces cerevisiae.

Micromolar HgCl2 concentrations transitorily duplicate the ATP level in Saccharomyces cerevisiae cells

Low concentrations of HgCl2 elicited, in Saccharomyces cerevisiae, a transitory increase in the ATP level followed by a decrease of its concentration, until almost disappearance. At 1 microM HgCl2, the increase in ATP lasted for about 30 min, while at 10 microM the increase was only observed in the first 5 min of treatment. The initial burst of ATP was accompanied by a decrease in the level of hexose phosphates, whereas during the decrease of ATP an increase in the inosine and hexose phosphates levels took place. The treatment with HgCl2 inhibited the plasma membrane proton ATPase but not the activities of hexokinase or 6-phosphofructokinase.

Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation

A relatively small group of hyperaccumulator plants is capable of sequestering heavy metals in their shoot tissues at high concentrations. In recent years, major scientific progress has been made in understanding the physiological mechanisms of metal uptake and transport in these plants. However, relatively little is known about the molecular bases of hyperaccumulation. In this paper, current progresses on understanding cellular/molecular mechanisms of metal tolerance/hyperaccumulation by plants are reviewed. The major processes involved in hyperaccumulation of trace metals from the soil to the shoots by hyperaccumulators include: (a) bioactivation of metals in the rhizosphere through root-microbe interaction; (b) enhanced uptake by metal transporters in the plasma membranes; (c) detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; (d) sequestration of metals into the vacuole by tonoplast-located transporters. The growing application of molecular-genetic technologies led to the well understanding of mechanisms of heavy metal tolerance/accumulation in plants, and subsequently many transgenic plants with increased resistance and uptake of heavy metals were developed for the purpose of phytoremediation. Once the rate-limiting steps for uptake, translocation, and detoxification of metals in hyperaccumulating plants are identified, more informed construction of transgenic plants would result in improved applicability of the phytoremediation technology.