Characterization of phytochelatin synthase-like protein encoded by alr0975 from a prokaryote, Nostoc sp. PCC 7120

Phytochelatins (PCs) are well known as the heavy metal-detoxifying peptides in higher plants, eukaryotic algae, fungi, and nematode. In contrast, neither PCs nor PC synthase genes have ever been identified in any prokaryotes. The genome sequences for the cyanobacterium Nostoc sp. PCC 7120 were recently completed and allowed us to identify a gene encoding a PC synthase-like protein, termed alr0975. The predicted product of alr0975 contains the conserved N-terminal domain but not the variable C-terminal domain found in eukaryotic PC synthases. The recombinant alr0975 protein strongly catalyzed the first step of PC synthesis, in which glutathione (GSH) is converted to gamma-glutamylcysteine (gamma-EC), although the protein only weakly catalyzed the second step of PC synthesis, namely the transfer of gamma-EC moiety to an acceptor GSH molecule to form PC(2). These results suggest alr0975 protein may be a more primitive form of the PC synthases found in eukaryotes.     

Mutagenic definition of a papain-like catalytic triad, sufficiency of the N-terminal domain for single-site core catalytic enzyme acylation, and C-terminal domain for augmentative metal activation of a eukaryotic phytochelatin synthase

Phytochelatin (PC) synthases are gamma-glutamylcysteine (gamma-Glu-Cys) dipeptidyl transpeptidases that catalyze the synthesis of heavy metal-binding PCs, (gamma-Glu-Cys)nGly polymers, from glutathione (GSH) and/or shorter chain PCs. Here it is shown through investigations of the enzyme from Arabidopsis (Arabidopsis thaliana; AtPCS1) that, although the N-terminal half of the protein, alone, is sufficient for core catalysis through the formation of a single-site enzyme acyl intermediate, it is not sufficient for acylation at a second site and augmentative stimulation by free Cd2+. A purified N-terminally hexahistidinyl-tagged AtPCS1 truncate containing only the first 221 N-terminal amino acid residues of the enzyme (HIS-AtPCS1_221tr) is competent in the synthesis of PCs from GSH in media containing Cd2+ or the synthesis of S-methyl-PCs from S-methylglutathione in media devoid of heavy metal ions. However, whereas its full-length hexahistidinyl-tagged equivalent, HIS-AtPCS1, undergoes gamma-Glu-Cys acylation at two sites during the Cd2+-dependent synthesis of PCs from GSH and is stimulated by free Cd2+ when synthesizing S-methyl-PCs from S-methylglutathione, HIS-AtPCS1_221tr undergoes gamma-Glu-Cys acylation at only one site when GSH is the substrate and is not directly stimulated, but instead inhibited, by free Cd2+ when S-methylglutathione is the substrate. Through the application of sequence search algorithms capable of detecting distant homologies, work we reported briefly before but not in its entirety, it has been determined that the N-terminal half of AtPCS1 and its equivalents from other sources have the hallmarks of a papain-like, Clan CA Cys protease. Whereas the fold assignment deduced from these analyses, which substantiates and is substantiated by the recent determination of the crystal structure of a distant prokaryotic PC synthase homolog from the cyanobacterium Nostoc, is capable of explaining the strict requirement for a conserved Cys residue, Cys-56 in the case of AtPCS1, for formation of the biosynthetically competent gamma-Glu-Cys enzyme acyl intermediate, the primary data from experiments directed at determining whether the other two residues, His-162 and Asp-180 of the putative papain-like catalytic triad of AtPCS1, are essential for catalysis have yet to be presented. This shortfall in our basic understanding of AtPCS1 is addressed here by the results of systematic site-directed mutagenesis studies that demonstrate that not only Cys-56 but also His-162 and Asp-180 are indeed required for net PC synthesis. It is therefore established experimentally that AtPCS1 and, by implication, other eukaryotic PC synthases are papain Cys protease superfamily members but ones, unlike their prokaryotic counterparts, which, in addition to having a papain-like N-terminal catalytic domain that undergoes primary gamma-Glu-Cys acylation, contain an auxiliary metal-sensing C-terminal domain that undergoes secondary gamma-Glu-Cys acylation.     

A reassessment of substrate specificity and activation of phytochelatin synthases from model plants by physiologically relevant metals

Phytochelatin synthases (PCS) catalyze phytochelatin (PC) synthesis from glutathione (GSH) in the presence of certain metals. The resulting PC-metal complexes are transported into the vacuole, avoiding toxic effects on metabolism. Legumes have the unique capacity to partially or completely replace GSH by homoglutathione (hGSH) and PCs by homophytochelatins (hPCs). However, the synthesis of hPCs has received little attention. A search for PCS genes in the model legume Lotus (Lotus japonicus) resulted in the isolation of a cDNA clone encoding a protein (LjPCS1) highly homologous to a previously reported homophytochelatin synthase (hPCS) of Glycine max (GmhPCS1). Recombinant LjPCS1 and Arabidopsis (Arabidopsis thaliana) PCS1 (AtPCS1) were affinity purified and their polyhistidine-tags removed. AtPCS1 catalyzed hPC synthesis from hGSH alone at even higher rates than did LjPCS1, indicating that GmhPCS1 is not a genuine hPCS and that a low ratio of hPC to PC synthesis is an inherent feature of PCS1 enzymes. For both enzymes, hGSH is a good acceptor, but a poor donor, of gamma-glutamylcysteine units. Purified AtPCS1 and LjPCS1 were activated (in decreasing order) by Cd2+, Zn2+, Cu2+, and Fe3+, but not by Co2+ or Ni2+, in the presence of 5 mm GSH and 50 microm metal ions. Activation of both enzymes by Fe3+ was proven by the complete inhibition of PC synthesis by the iron-specific chelator desferrioxamine. Plants of Arabidopsis and Lotus accumulated (h)PCs only in response to a large excess of Cu2+ and Zn2+, but to a much lower extent than did with Cd2+, indicating that (h)PC synthesis does not significantly contribute in vivo to copper, zinc, and iron detoxification.     

Heavy metals toxicity in plants: An overview on the role of glutathione and phytochelatins in heavy metal stress tolerance of plants

Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage. In addition, plants accumulate metal ions that disturb cellular ionic homeostasis. To minimize the detrimental effects of heavy metal exposure and their accumulation, plants have evolved detoxification mechanisms. Such mechanisms are mainly based on chelation and subcellular compartmentalization. Chelation of heavy metals is a ubiquitous detoxification strategy described in wide variety of plants. A principal class of heavy metal chelator known in plants is phytochelatins (PCs), a family of Cys-rich peptides. PCs are synthesized non-translationally from reduced glutathione (GSH) in a transpeptidation reaction catalyzed by the enzyme phytochelatin synthase (PCS). Therefore, availability of glutathione is very essential for PCs synthesis in plants at least during their exposure to heavy metals. Here, I reviewed on effect of heavy metals exposure to plants and role of GSH and PCs in heavy metal stress tolerance. Further, genetic manipulations of GSH and PCs levels that help plants to ameliorate toxic effects of heavy metals have been presented.     

Homo-Phytochelatins Are Synthesized in Response to Cadmium in Azuki Beans

In a recent report, it was claimed that azuki beans (Vigna angularis) do not synthesize phytochelatins (PCs) upon exposure to cadmium, although glutathione (GSH), the substrate for PC synthesis, is present in this plant. This legume species thus would be the first exception in the plant kingdom that would fail to complex heavy metals by PCs. Here, we report that not GSH, but only homoglutathione can be detected in this plant and that homo-phytochelatins are formed when azuki beans are challenged with heavy metals such as cadmium. We also show that the 5,5'-dithiobis(2-nitrobenzoic acid)-oxidized GSH reductase recycling assay, used for GSH quantification in the recent study of heavy metal tolerance in azuki beans, reacts both with GSH and homoglutathione and therefore cannot be used when biological samples should be analyzed exclusively for GSH.     

Enhanced arsenic accumulation by engineered yeast cells expressing Arabidopsis thaliana phytochelatin synthase

Phytochelatins (PCs) are naturally occurring peptides with high-binding capabilities for a wide range of heavy metals including arsenic (As). PCs are enzymatically synthesized by phytochelatin synthases and contain a (gamma-Glu-Cys)(n) moiety terminated by a Gly residue that makes them relatively proteolysis resistant. In this study, PCs were introduced by expressing Arabidopsis thaliana Phytochelatin Synthase (AtPCS) in the yeast Saccharomyces cerevisiae for enhanced As accumulation and removal. PCs production in yeast resulted in six times higher As accumulation as compared to the control strain under a wide range of As concentrations. For the high-arsenic concentration, PCs production led to a substantial decrease in levels of PC precursors such as glutathione (GSH) and gamma-glutamyl cysteine (gamma-EC). The levels of As(III) accumulation were found to be similar between AtPCS-expressing wild type strain and AtPCS-expressing acr3Delta strain lacking the arsenic efflux system, suggesting that the arsenic uptake may become limiting. This is further supported by the roughly 1:3 stoichiometric ratio between arsenic and PC2 (n = 2) level (comparing with a theoretical value of 1:2), indicating an excess availability of PCs inside the cells. However, at lower As(III) concentration, PC production became limiting and an additive effect on arsenic accumulation was observed for strain lacking the efflux system. More importantly, even resting cells expressing AtPCS pre-cultured in Zn(2+) enriched media showed PCs production and two times higher arsenic removal than the control strain. These results open up the possibility of using cells expressing AtPCS as an inexpensive sorbent for the removal of toxic arsenic.     

Phytochelatins are synthesized by two vacuolar serine carboxypeptidases in Saccharomyces 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.     

Comparative analysis of the two-step reaction catalyzed by prokaryotic and eukaryotic phytochelatin synthase by an ion-pair liquid chromatography assay

Genes encoding phytochelatin (PC) synthase have been found in higher plants, fission yeast and worm. Recently, kinetic and mutagenic analyses of recombinant PC synthase have been revealing the molecular mechanisms underlying PC synthesis, however, a conclusive model has not been established. To clarify the mechanism of PC synthase found in eukaryotes, we have compared the two-step reactions catalyzed by the prokaryotic Nostoc PC synthase (NsPCS) and the eukaryotic Arabidopsis PC synthase (AtPCS1). Comparative analysis shows that in the first step of PC synthesis corresponding to the cleavage of -glutamylcysteine (-EC) from glutathione (GSH), free GSH or PCs acts as a donor molecule to supply a -EC unit for elongation of the PC chain, and heavy metal ions are required to carry out the cleavage. Furthermore, functional analyses of various mutants of NsPCS and AtPCS1, selected by comparing the sequences of NsPCS and AtPCS1, indicate that the N-terminal region (residues 1–221) in AtPCS1 is the catalytic domain, and in this region, the Cys⁵⁶ residue is associated with the PC synthesis reaction. These results enable us to propose an advanced model of PC synthesis, describing substrate specificity, heavy metal requirement, and the active site in the enzyme.     

A pseudo-phytochelatin synthase in the ciliated protozoan Tetrahymena thermophila

Phytochelatins (PCs) and metallothioneins (MTs) are the two major heavy metal chelating peptides in eukaryotes. We report here on the identification of a biosynthetically inactive pseudo-phytochelatin synthase enzyme (TtψPCS) in the ciliate Tetrahymena thermophila, the first of this kind (pseudo-PCS) to be described in eukaryotes. TtψPCS which resembles a true PCS at the N-terminal region, while it is most divergent in its Cys-poor C-terminal region, was found to be up-regulated under cadmium stress conditions. However, only glutathione (GSH) hydrolysis products, but not PCs, could be detected in extracts from Cd-treated cells. The latter feature is reminiscent of pseudo-PCS enzymes recently identified in cyanobacteria, which are also biosynthetically inactive, but capable to hydrolyze GSH.     

植物螯合肽及其在抗重金属胁迫中的作用

植物螯合肽(PCs)广泛存在于植物体中,与植物抗重金属胁迫关系密切。植物螯合肽及其复合物是一类富含半胱氨酸的低分子量化合物。现有研究证明,PCS由谷胱甘肽(GSH)为底物的酶促反应合成,其合成受相关基因的调控,从模式植物拟南芥的突变体中已分离到与PCS合成有关的几个基因。植物螯合肽首先与重金属离子结合形成低分子量(LMW)复合物,以此形态经由细胞质进入液泡后,再与一个分子的植物螯合肽结合,形成对植物组织毒性较小的高分子量(HMW)复合物,从而达到缓解重金属对植物的危害作用。就植物螯合肽及其复合物的结构、生物合成、基因调控及重金属解毒机理等进行了综述,并对今后的研究方向提出了一些看法。     

Enhancing the tolerance of zebrafish (Danio rerio) to heavy metal toxicity by the expression of plant phytochelatin synthase

Phytochelatin synthase (PC synthase) catalyzes a biosynthesis of phytochelatins (PCs), which are small molecules and glutathione (GSH)-derived metal-binding peptides that are essential for the detoxification of heavy metal ions in plants, fungi and worms. In order to enhance tolerance to heavy metal cytotoxicity, mRNA coding for PC synthase from Arabidopsis thaliana (AtPCS1) was introduced into the early embryos of zebrafish. As a result, the heterogeneous expression of PC synthase and the synthesis of PCs from GSH in embryos could be detected. The developing embryos expressing PC synthase (PC-embryos) became more tolerant to Cd toxicity (500 microM exposure). PC-embryos had significantly longer apparent lethal times for 50% of the population (LT50) of 8.17+/-1.08 days, although control embryos had apparent LT50 of 5.43+/-0.66 days. These data suggest that PC synthase can function in developmental zebrafish, and that PCs are highly effective in detoxifying Cd toxicity even in the whole body of a vertebrate species.     

植物螯合肽及其在重金属耐性中的作用

综述植物螯合肽的生物合成及其在重金属耐性中的作用．有毒重金属在土壤中的积累不仅影响作物的生长和产量形成,而且严重威胁农产品的安全性．植物对重金属的耐性和积累在种间和基因型之间存在着很大的差异,在重金属胁迫条件下植物螯合肽(PC)的合成是植物对胁迫的一种适应性反应,耐性基因型合成较多的PC谷胱苷肽是合成PC的前体,PC可与重金属螯合,并进一步转运至液泡贮存,使细胞质的重金属浓度降低,从而达到解毒效果．重金属诱导植物合成PC的遗传机理和生化途径有赖于分子生物学的深入研究,cD-敏感型拟南芥突变体Cad1-1(缺失GSH)和Cad2-1(缺失PC合成诱导酶)的分离及相关研究,佐证了PC在Cd-解毒中起关键作用．对PC在重金属污染土壤或水体的植物修复和农作物安全生产中的意义进行了讨论．     

PRODUCTION OF PHYTOCHELATINS AND GLUTATHIONE BY MARINE PHYTOPLANKTON IN RESPONSE TO METAL STRESS1

Phytoplankton deal with metal toxicity using a variety of biochemical strategies. One of the strategies involves glutathione (GSH) and phytochelatins (PCs), which are metal‐binding thiol peptides produced by eukaryotes and these compounds have been related to several intracellular functions, including metal detoxification, homeostasis, metal resistance and protection against oxidative stress. This paper assesses our state of knowledge on the production of PCs and GSH by marine phytoplankton in laboratory and field conditions and the possible applications of PCs for environmental purposes. Good relationships have been observed between metal exposure and PC production in phytoplankton in the laboratory with Cd, Pb, and Zn showing the greatest efficacy, thereby indicating that PCs have a potential for application as a biomarker. Fewer studies on PC distributions in particulate material have been undertaken in the field. These studies show that free Cu has a strong relationship with the levels of PC in the particulate material. The reason for this could be because Cu is a common contaminant in coastal waters. However it could also be due to the lack of measurements of other metals and their speciation. GSH shows a more complex relationship to metal levels both in the laboratory and in the field. This is most likely due to its multifunctionality. However, there is evidence that phytoplankton act as an important source of dissolved GSH in marine waters, which may form part of the strong organic ligands that control metal speciation, and hence metal toxicity.     

Synthesis of Phytochelatins and Homo-Phytochelatins in Pisum sativum L

In the roots of pea plants (Pisum sativum L.) cultivated with 20 [mu]M CdCl2 for 3 d, synthesis of phytochelatins [PCs or ([gamma]EC)nG, where [gamma]EC is [gamma]glutamylcysteine and G is glycine] and homophytochelatins [h-PCs, ([gamma]EC)n[beta]-alanine] is accompanied by a drastic decrease in glutathione (GSH) content, but an increase in homoglutathione (h-GSH) content. In contrast, the in vitro activity of GSH synthetase increases 5-fold, whereas h-GSH synthetase activity increases regardless of Cd exposure. The consititutive enzyme PC synthase, which catalyzes the transfer of the [gamma]-EC moiety of GSH to an acceptor GSH molecule thus producing ([gamma]EC)2G, is activated by heavy metals, with Cd and Cu being strong activators and Zn being a very poor activator. Using h-GSH or hm-GSH for substrate, the synthesis rate of([gamma]EC)2[beta]-alanine and [gamma]EC)2-serine is only 2.4 and 0.3%, respectively, of the sythesis rate of ([gamma]EC)2G with GSH as substrate. However, in the presence of a constant GSH level, increasing the concentration of h-GSH or hm-GSH results in increased synthesis of ([gamma]EC)2[beta]-alanine or ([gamma]EC)2-serine, respecively; simultaneously, the synthesis of ([gamma]EC)2G is inhibited. [gamma]EC is not a substrate of PC synthase. These results are best explained by assuming that PC synthase has a [gamma]EC donor binding site, which is very specific for GSH, and a [gamma]EC acceptor binding site, which is less specific and accepts several tripeptides, namely GSH, h-GSH, and hm-GSH.     

Characterization of phytochelatin synthase from tomato

The enzyme that synthesizes Cd-binding phytochelatins (PCs), PC synthase, has been studied in tomato ( Lycopersicon esculentum ) cell cultures and plants. This enzyme transfers γ-GluCys from GSH or PC to either GSH or an existing polymer of (γ-GluCys)_nGly. PC synthase from tomato requires GSH or PCs as substrates but cannot utilise γ-GluCys or GSSG. PC synthase is activated both in vivo and in vitro by a variety of heavy metal ions, including Cd²⁺, Ag⁺, Cu²⁺, Au⁺, Zn²⁺, Fe²⁺, Hg²⁺ and Pb²⁺. In crude protein extracts from tomato cells the enzyme has an apparent K_m of 7.7 m M for GSH in the presence of 0.5 m M Cd²⁺, and exhibits maximum activity at pH 8.0 and 35°C. PC synthase is present in tomato cells grown in the absence of Cd. The level of enzyme activity is regulated during the cell culture cycle, with the highest activity occurring 3 days after subculture. Cadmium-resistant tomato cells growing in medium containing 6 m M CdCl₂ have a 65% increase in PC synthase activity compared to unselected cells. PC synthase is also present in roots and stems of tomato plants, but not in leaves or fruits. The distribution of the enzyme in tomato plants and regulation of PC synthase activity in tomato cells indicate that PC synthase, and PCs, may have additional functions in plant metabolism that are not directly related to the formation of Cd-PC complexes in response to cadmium.     

Phytochelatin synthase, a dipeptidyltransferase that undergoes multisite acylation with gamma-glutamylcysteine during catalysis: stoichiometric and site-directed mutagenic analysis of arabidopsis thaliana PCS1-catalyzed phytochelatin synthesis

Phytochelatin (PC) synthase has been assumed to be a gamma-glutamylcysteine dipeptidyl transpeptidase (EC 2.3.2.15) and, more recently, as exemplified by analyses of the immunopurified recombinant enzyme from Arabidopsis thaliana (AtPCS1-FLAG), has been shown to catalyze a PC synthetic reaction with kinetics that approximates a bisubstrate-substituted enzyme mechanism in which millimolar concentrations of free GSH and micromolar concentrations of heavy metal.GSH thiolates (e.g. cadmium.GS(2)) or millimolar concentrations of S-alkylglutathiones serve as cosubstrates. Here, we show, by direct analyses of the stoichiometry of AtPCS1-FLAG-catalyzed PC synthesis, the kinetics and stoichiometry of acylation of the enzyme and release of free glycine from gamma-Glu-Cys donors, and the effects of the Cys-to-Ser or -Ala and Ser-to-Ala substitution of conserved residues in the catalytic N-terminal half of the enzyme, that PC synthase is indeed a dipeptidyltransferase that undergoes gamma-Glu-Cys acylation at two sites during catalysis, one of which, in accord with a cysteine protease model, likely corresponds to or is at least tightly coupled with Cys(56). The identity of the second site of enzyme modification remains to be determined, but it is distinguishable from the first Cys(56)-dependent site, which is amenable to gamma-Glu-Cys acylation by free GSH, because its acylation not only depends on the provision of Cd(2+) or GSH with a blocked, S-alkylated thiol group, but is also necessary for net PC synthesis. We conclude that des-Gly-PCs are not generated as an immediate by-product, but rather that the enzyme catalyzes a dipeptidyl transfer reaction in which some of the energy liberated upon cleavage of the Cys-Gly bonds of the gamma-Glu-Cys donors in the first phase of the catalytic cycle is conserved through the formation of a two site-substituted gamma-Glu-Cys acyl-enzyme intermediate whose hydrolysis provides the energy required for the formation of the new peptide bond required for the extension of PC chain length by one gamma-Glu-Cys repeat per catalytic cycle.     

Comparative genomics and evolution of eukaryotic phospholipid biosynthesis

Phospholipid biosynthetic enzymes produce diverse molecular structures and are often present in multiple forms encoded by different genes. This work utilizes comparative genomics and phylogenetics for exploring the distribution, structure and evolution of phospholipid biosynthetic genes and pathways in 26 eukaryotic genomes. Although the basic structure of the pathways was formed early in eukaryotic evolution, the emerging picture indicates that individual enzyme families followed unique evolutionary courses. For example, choline and ethanolamine kinases and cytidylyltransferases emerged in ancestral eukaryotes, whereas, multiple forms of the corresponding phosphatidyltransferases evolved mainly in a lineage specific manner. Furthermore, several unicellular eukaryotes maintain bacterial-type enzymes and reactions for the synthesis of phosphatidylglycerol and cardiolipin. Also, base-exchange phosphatidylserine synthases are widespread and ancestral enzymes. The multiplicity of phospholipid biosynthetic enzymes has been largely generated by gene expansion in a lineage specific manner. Thus, these observations suggest that phospholipid biosynthesis has been an actively evolving system. Finally, comparative genomic analysis indicates the existence of novel phosphatidyltransferases and provides a candidate for the uncharacterized eukaryotic phosphatidylglycerol phosphate phosphatase.     

Cadmium hyperaccumulation leads to an increase of glutathione rather than phytochelatins in the cadmium hyperaccumulator Sedum alfredii

Sedum alfredii has been reported to be a cadmium (Cd) hyperaccumulator. Phytochelatins (PCs) and other thiol (SH)-containing compounds have been proposed to play an important role in the detoxification and tolerance of some heavy metals, but it is not clear whether PCs are responsible for Cd hyperaccumulation and tolerance in S. alfredii. In this study, two geographically isolated populations of S. alfredii were studied: one population grew on an old Pb/Zn mine site, while the other on a non-mine site. The mine population of this species exhibited a stronger heavy metal tolerance than in the other population. Root-to-shoot transport of Cd was higher in population located at the mine site than at the non-mine site. Considerable amounts of Cd were accumulated in leaves and stems of mine plants, while most Cd was distributed in roots of non-mine plants. Non-protein SH in plant tissues of two populations were further investigated by a HPLC pre-column derivatization system. Upon exposure to Cd, no PCs were detected in all tissues of mine population, while an appreciable amount of glutathione (GSH) was observed in the descending order of stem>root>leaf. The concentrations of GSH consistently increased with the increase of exogenous Cd concentrations and time. On the contrary, Cd exposure strongly induced the production of PCs (mainly PC(2) and PC(3)) and GSH in plant tissues of non-mine population, and the concentrations of GSH showed an initial drop over the duration of 7-d exposure. The present results provided strong evidence that PCs are not involved in Cd transport, hyperaccumulation and tolerance in mine population of S. alfredii.     

A new pathway for heavy metal detoxification in animals. Phytochelatin synthase is required for cadmium tolerance in Caenorhabditis elegans

Increasing emissions of heavy metals such as cadmium, mercury, and arsenic into the environment pose an acute problem for all organisms. Considerations of the biochemical basis of heavy metal detoxification in animals have focused exclusively on two classes of peptides, the thiol tripeptide, glutathione (GSH, gamma-Glu-Cys-Gly), and a diverse family of cysteine-rich low molecular weight proteins, the metallothioneins. Plants and some fungi, however, not only deploy GSH and metallothioneins for metal detoxification but also synthesize another class of heavy metal binding peptides termed phytochelatins (PCs) from GSH. Here we show that PC-mediated heavy metal detoxification is not restricted to plants and some fungi but extends to animals by demonstrating that the ce-pcs-1 gene of the nematode worm Caenorhabditis elegans encodes a functional PC synthase whose activity is critical for heavy metal tolerance in the intact organism.