Molecular and biochemical characterization of the selenocysteine Se-methyltransferase gene and Se-methylselenocysteine synthesis in broccoli

Evaluation of germplasm effect on Fe, Zn and Se content in wheat seedlings

Micronutrients are essential for human health and crucial for plant survival. The capacity of food crops in acquiring mineral nutrients affects plant growth and potentially the yield and nutrient content in edible tissues/organs. In this study, we selected 20 wheat (Triticum aestivum L.) accessions and evaluated genotypic variations of the young seedlings in response to iron (Fe), zinc (Zn), and selenium (Se) treatments. Wheat accessions exhibited different growth responses to these minerals and possessed various abilities to accumulate them. Wheat seedlings in general were less tolerable to excess of Fe and benefits from increased levels of Zn supply. They were sensitive to selenite and profited from selenate treatment at low dosages. Limited mineral interactions were observed between Fe or Zn with other nutrients. In contrast, selenate supply enhanced Fe, Zn, sulfur (S), molybdenum (Mo), magnesium (Mg), calcium (Ca) and manganese (Mn) content in wheat seedlings, supporting its beneficial role in promoting plant growth; Selenite supplement reduced Zn, S, Mo, Mg, Ca and Mn levels in the plants, consisting with its detrimental role in inhibiting seedling growth. Based on nutrient accumulation, plant growth, and mineral interaction, a number of accessions such as EMB 38 and BRS 264 appeared to be good lines for breeding wheat cultivars with better plant health and potential to accumulate essential micronutrients in edible grains.

Assessment of the anticancer compounds Se-methylselenocysteine and glucosinolates in Se-biofortified broccoli (Brassica oleracea L. var. italica) sprouts and florets

Broccoli (Brassica oleracea L. var. italica) is a rich source of chemopreventive compounds. Here, we evaluated and compared the effect of selenium (Se) treatment on the accumulation of anticancer compounds Se-methylselenocysteine (SeMSCys) and glucosinolates in broccoli sprouts and florets. Total Se and SeMSCys content in sprouts increased concomitantly with increasing Se doses. Selenate was superior to selenite in inducing total Se accumulation, but selenite is equally effective as selenate in promoting SeMSCys synthesis in sprouts. Increasing sulfur doses reduced total Se and SeMSCys content in sprouts treated with selenate, but not in those with selenite. Examination of five broccoli cultivars reveals that sprouts generally have better fractional ability than florets to convert inorganic Se into SeMSCys. Distinctive glucosinolate profiles between sprouts and florets were observed, and sprouts contained approximately 6-fold more glucoraphanin than florets. In contrast to florets, glucosinolate content was not affected by Se treatment in sprouts. Thus, Se-enriched broccoli sprouts are excellent for simultaneous accumulation of chemopreventive compounds SeMSCys and glucoraphanin.

Changes in SeMSC, glucosinolates and sulforaphane levels, and in proteome profile in broccoli (Brassica oleracea var. Italica) fertilized with sodium selenate

The aim of this work was to analyze the effect of sodium selenate fortification on the content of selenomethyl selenocysteine (SeMSC), total glucosinolates and sulforaphane, as well as the changes in protein profile of the inflorescences of broccoli (Brassica oleracea var. Italica). Two experimental groups were considered: plants treated with 100 μmol/L sodium selenate (final concentration in the pot) and control plants treated with water. Fortification began 2 weeks after transplantation and was repeated once a week during 10 weeks. Broccoli florets were harvested when they reached appropriate size. SeMSC content in broccoli florets increased significantly with sodium selenate fortification; but total glucosinolates and sulforaphane content as well as myrosinase activity were not affected. The protein profile of broccoli florets changed due to fortification with sodium selenate. Some proteins involved in general stress-responses were up-regulated, whereas down-regulated proteins were identified as proteins involved in protection against pathogens. This is the first attempt to evaluate the physiological effect of fortification with sodium selenate on broccoli at protein level. The results of this work will contribute to better understanding the metabolic processes related with selenium uptake and accumulation in broccoli.

Evaluation of genotypic variation of broccoli (Brassica oleracea var. italic) in response to selenium treatment

Broccoli (Brassica oleracea var. italic) fortified with selenium (Se) has been promoted as a functional food. Here, we evaluated 38 broccoli accessions for their capacity to accumulate Se and for their responses to selenate treatment in terms of nutritional qualities and sulfur gene expresion. We found that the total Se content varied with over 2-fold difference among the leaf tissues of broccoli accessions when the plants were treated with 20 μM Na(2)SeO(4). Approximately half of total Se accumulated in leaves was Se-methylselenocysteine and selenomethionine. Transcriptional regulation of adenosine 5'-phosphosulfate sulfurylase and selenocysteine Se-methyltransferase gene expression might contribute to the different levels of Se accumulation in broccoli. Total glucosinolate contents were not affected by the concentration of selenate application for the majority of broccoli accessions. Essential micronutrients (i.e., Fe, Zn, Cu, and Mn) remained unchanged among half of the germplasm. Moreover, the total antioxidant capacity was greatly stimulated by selenate in over half of the accessions. The diverse genotypic variation in Se, glucosinolate, and antioxidant contents among accessions provides the opportunity to breed broccoli cultivars that simultaneously accumulate Se and other health benefit compounds.

Selenium accumulation in lettuce germplasm

Selenium (Se) is an essential micronutrient for animals and humans. Increasing Se content in food crops offers an effective approach to reduce the widespread selenium deficiency problem in many parts of the world. In this study, we evaluated 30 diverse accessions of lettuce (Lactuca sativa L.) for their capacity to accumulate Se and their responses to different forms of Se in terms of plant growth, nutritional characteristics, and gene expression. Lettuce accessions responded differently to selenate and selenite treatment, and selenate is superior to selenite in inducing total Se accumulation. At least over twofold change in total Se levels between cultivars with high and low Se content was found. Synergistic relationship between Se and sulfur accumulation was observed in nearly all accessions at the selenate dosage applied. The change in shoot biomass varied between lettuce accessions and the forms of Se used. The growth-stimulated effect by selenate and the growth-inhibited effect by selenite were found to be correlated with the alteration of antioxidant enzyme activities. The different ability of lettuce accessions to accumulate Se following selenate treatment appeared to be associated with an altered expression of genes involved in Se/S uptake and assimilation. Our results provide important information for the effects of different forms of Se on plant growth and metabolism. They will also be of help in selecting and developing better cultivars for Se biofortification in lettuce.

Generation of Se-fortified broccoli as functional food: impact of Se fertilization on S metabolism

Selenium (Se)-fortified broccoli (Brassica oleracea var. italica) has been proposed as a functional food for cancer prevention, based on its high glucosinolate (GSL) content and capacity for Se accumulation. However, as selenate and sulphate share the initial assimilation route, Se fertilization could interfere with sulphur metabolism and plant growth. Consequently, GSL accumulation could be compromised. To evaluate these potentially adverse effects of Se fertilization, we performed a comprehensive study on sand-grown young broccoli plants (weekly selenate applications of 0.8 µmol plant(-1) via the root) and field-grown adult broccoli plants during head formation (single foliar selenate application: 25.3 or 253 µmol plant(-1) ). The results show that under these conditions, Se application does not affect plant growth, contents of cysteine, glutathione, total GSL, glucoraphanin (major aliphatic GSL) or the expression of BoMYB28 (encoding a functionally confirmed master regulator for aliphatic GSL biosynthesis). Conversely, due to the changed expression of sulphate transporters (BoSULTR1;1, 1;2, 2;1, and 2;2), sulphate and total S contents increased in the shoot of young plants while decreasing in the root. We conclude that broccoli can be fertilized with Se without reduction in GSL content, even with Se accumulation exceeding the level recommended for human consumption.

Involvement of a broccoli COQ5 methyltransferase in the production of volatile selenium compounds

Selenium (Se) is an essential micronutrient for animals and humans but becomes toxic at high dosage. Biologically based Se volatilization, which converts Se into volatile compounds, provides an important means for cleanup of Se-polluted environments. To identify novel genes whose products are involved in Se volatilization from plants, a broccoli (Brassica oleracea var italica) cDNA encoding COQ5 methyltransferase (BoCOQ5-2) in the ubiquinone biosynthetic pathway was isolated. Its function was authenticated by complementing a yeast coq5 mutant and by detecting increased cellular ubiquinone levels in the BoCOQ5-2-transformed bacteria. BoCOQ5-2 was found to promote Se volatilization in both bacteria and transgenic Arabidopsis (Arabidopsis thaliana) plants. Bacteria expressing BoCOQ5-2 produced an over 160-fold increase in volatile Se compounds when they were exposed to selenate. Consequently, the BoCOQ5-2-transformed bacteria had dramatically enhanced tolerance to selenate and a reduced level of Se accumulation. Transgenic Arabidopsis expressing BoCOQ5-2 volatilized three times more Se than the vector-only control plants when treated with selenite and exhibited an increased tolerance to Se. In addition, the BoCOQ5-2 transgenic plants suppressed the generation of reactive oxygen species induced by selenite. BoCOQ5-2 represents, to our knowledge, the first plant enzyme that is not known to be directly involved in sulfur/Se metabolism yet was found to mediate Se volatilization. This discovery opens up new prospects regarding our understanding of the complete metabolism of Se and may lead to ways to modify Se-accumulator plants with increased efficiency for phytoremediation of Se-contaminated environments.

Characterization of selenocysteine methyltransferases from Astragalus species with contrasting selenium accumulation capacity

A group of selenium (Se)-hyperaccumulating species belonging to the genus Astragalus are known for their capacity to accumulate up to 0.6% of their foliar dry weight as Se, with most of this Se being in the form of Se-methylselenocysteine (MeSeCys). Here, we report the isolation and molecular characterization of the gene that encodes a putative selenocysteine methyltransferase (SMT) enzyme from the non-accumulator Astragalus drummondii and biochemically compare it with an authentic SMT enzyme from the Se-hyperaccumulator Astragalus bisulcatus, a related species that lives within the same native habitat. The non-accumulator enzyme (AdSMT) shows a high degree of homology with the accumulator enzyme (AbSMT) but lacks the selenocysteine methyltransferase activity in vitro, explaining why little or no detectable levels of MeSeCys accumulation are observed in the non-accumulator plant. The insertion of mutations on the coding region of the non-accumulator AdSMT enzyme to better resemble enzymes that originate from Se accumulator species results in increased selenocysteine methyltransferase activity, but these mutations were not sufficient to fully gain the activity observed in the AbSMT accumulator enzyme. We demonstrate that SMT is localized predominantly within the chloroplast in Astragalus, the principal site of Se assimilation in plants. By using a site-directed mutagenesis approach, we show that an Ala to Thr amino acid mutation at the predicted active site of AbSMT results in a new enzymatic capacity to methylate homocysteine. The mutated AbSMT enzyme exhibited a sixfold higher capacity to methylate selenocysteine, thereby establishing the evolutionary relationship of SMT and homocysteine methyltransferase enzymes in plants.

Alpha-keto acid metabolites of naturally occurring organoselenium compounds as inhibitors of histone deacetylase in human prostate cancer cells

Histone deacetylase (HDAC) inhibitors are gaining interest as cancer therapeutic agents. We tested the hypothesis that natural organoselenium compounds might be metabolized to HDAC inhibitors in human prostate cancer cells. Se-Methyl-L-selenocysteine (MSC) and selenomethionine are amino acid components of selenium-enriched yeast. In a cell-free system, glutamine transaminase K (GTK) and L-amino acid oxidase convert MSC to the corresponding alpha-keto acid, beta-methylselenopyruvate (MSP), and L-amino acid oxidase converts selenomethionine to its corresponding alpha-keto acid, alpha-keto-gamma-methylselenobutyrate (KMSB). Although methionine (sulfur analogue of selenomethionine) is an excellent substrate for GTK, selenomethionine is poorly metabolized. Structurally, MSP and KMSB resemble the known HDAC inhibitor butyrate. We examined androgen-responsive LNCaP cells and androgen-independent LNCaP C4-2, PC-3, and DU145 cells and found that these human prostate cancer cells exhibit endogenous GTK activities. In the corresponding cytosolic extracts, the metabolism of MSC was accompanied by the concomitant formation of MSP. In MSP-treated and KMSB-treated prostate cancer cell lines, acetylated histone 3 levels increased within 5 hours, and returned to essentially baseline levels by 24 hours, suggesting a rapid, transient induction of histone acetylation. In an in vitro HDAC activity assay, the selenoamino acids, MSC and selenomethionine, had no effect at concentrations up to 2.5 mmol/L, whereas MSP and KMSB both inhibited HDAC activity. We conclude that, in addition to targeting redox-sensitive signaling proteins and transcription factors, alpha-keto acid metabolites of MSC and selenomethionine can alter HDAC activity and histone acetylation status. These findings provide a potential new paradigm by which naturally occurring organoselenium might prevent the progression of human prostate cancer.

Accumulation of an organic anticancer selenium compound in a transgenic Solanaceous species shows wider applicability of the selenocysteine methyltransferase transgene from selenium hyperaccumulators

Tolerance to high selenium (Se) soils in Se-hyperaccumulating plant species is correlated with the ability to biosynthesise methylselenocysteine (MeSeCys), due to the activity of selenocysteine methyltransferase (SMT). In mammals, inclusion of MeSeCys in the diet reduces the incidence of certain cancers, so increasing the range of crop plants that can produce this compound is an attractive biotechnology target. However, in the non-Se accumulator Arabidopsis, overexpression of SMT does not result in biosynthesis of MeSeCys from selenate because the rate at which selenate is reduced to selenite by ATP sulfurylase (ATPS) is low. This limitation is less problematic in other species of the Brassicaceae that can produce MeSeCys naturally. We investigated the potential for biosynthesis of MeSeCys in other plant families using Nicotiana tabacum L., a member of the Solanaceae. When plants were watered with 200 microM selenate, overexpression of a SMT transgene caused a 2- to 4-fold increase in Se accumulation (resulting in increased numbers of leaf lesions and areas of necrosis), production of MeSeCys (up to 20% of total Se) and generation of volatile dimethyl diselenide derived directly from MeSeCys. Despite the greatly increased accumulation of total Se, this did not result in increased Se toxicity effects on growth. Overexpression of ATPS did not increase Se accumulation from selenate. Accordingly, lines overexpressing both ATPS and SMT did not show a further increase in total Se accumulation or in leaf toxicity symptoms relative to overexpression of SMT alone, but directed a greater proportion of Se into MeSeCys. This work demonstrates that the production of the cancer-preventing compound MeSeCys in plants outside the Brassicaceae is possible. We conclude that while the SMT gene from Se hyperaccumulators can probably be utilised universally to increase the metabolism of Se into MeSeCys, the effects of enhancing ATPS activity will vary depending on the species involved.

Physiological functions of beneficial elements

Aluminum (Al), cobalt (Co), sodium (Na), selenium (Se), and silicon (Si) are considered beneficial elements for plants: they are not required by all plants but can promote plant growth and may be essential for particular taxa. These beneficial elements have been reported to enhance resistance to biotic stresses such as pathogens and herbivory, and to abiotic stresses such as drought, salinity, and nutrient toxicity or deficiency. The beneficial effects of low doses of Al, Co, Na and Se have received little attention compared to toxic effects that typically occur at higher concentrations. Better understanding of the effects of beneficial elements is important to improve crop productivity and enhance plant nutritional value for a growing world population.

Seed storage protein deficiency improves sulfur amino acid content in common bean (Phaseolus vulgaris L.): redirection of sulfur from gamma-glutamyl-S-methyl-cysteine

The contents of sulfur amino acids in seeds of common bean ( Phaseolus vulgaris L.) are suboptimal for nutrition. They accumulate large amounts of a gamma-glutamyl dipeptide of S-methyl-cysteine, a nonprotein amino acid that cannot substitute for methionine or cysteine in the diet. Protein accumulation and amino acid composition were characterized in three genetically related lines integrating a progressive deficiency in major seed storage proteins, phaseolin, phytohemagglutinin, and arcelin. Nitrogen, carbon, and sulfur contents were comparable among the three lines. The contents of S-methyl-cysteine and gamma-glutamyl-S-methyl-cysteine were progressively reduced in the mutants. Sulfur was shifted predominantly to the protein cysteine pool, while total methionine was only slightly elevated. Methionine and cystine contents (mg per g protein) were increased by up to ca. 40%, to levels slightly above FAO guidelines on amino acid requirements for human nutrition. These findings may be useful to improve the nutritional quality of common bean.

Reduced activity of Arabidopsis thaliana HMT2, a methionine biosynthetic enzyme, increases seed methionine content

In the S-methylmethionine cycle of plants, homocysteine methyltransferase (HMT) catalyzes the formation of two molecules of methionine from homocysteine and S-methylmethionine, and methionine methyltransferase (MMT) catalyzes the formation of methionine from S-methylmethionine using S-adenosylmethionine as a methyl group donor. Somewhat surprisingly, two independently isolated knockdown mutations of HMT2 (At3g63250), one of three Arabidopsis thaliana genes encoding homocysteine methyltransferase, increased free methionine abundance in seeds. Crosses and flower stalk grafting experiments demonstrate that the maternal genotype at the top of the flower stalk determines the seed S-methylmethionine and methionine phenotype of hmt2 mutants. Uptake, transport and inter-conversion of [(13)C]S-methylmethionine and [(13)C]methionine in hmt2, mmt and wild-type plants show that S-methylmethionine is a non-essential intermediate in the movement of methionine from vegetative tissue to the seeds. Together, these results support a model whereby elevated S-methylmethionine in hmt2 vegetative tissue is transported to seeds and either directly or indirectly results in the biosynthesis of additional methionine. Manipulation of the S-methylmethionine cycle may provide a new approach for improving the nutritional value of major grain crops such as rice, as methionine is a limiting essential amino acid for mammalian diets.

Selenium transformation studies during Broccoli (Brassica oleracea) growing process by liquid chromatography–inductively coupled plasma mass spectrometry (LC–ICP-MS)

Selenium uptake and transformation was studied in Se-enriched Broccoli (Brassica olearacea). Plants were grown in hydroponic culture and exposed during 40 days to Na2SeO3 (1 mg L(-1)). After growing, the plants were harvested and their different parts (roots, stems and fruit) were analyzed by ICP-MS or LC-ICP-MS. Se-species were identified and quantified after enzymatic extraction by using both an anion exchange (PRP-X100), and a size exclusion/ion exchange (Shodex Asahipak) chromatographic columns. Selenium translocation and transformation Se species in plants was studied through the Se-speciation in root, stem and fruit. After 40 days of exposure, selenomethionine was the major species found in roots, however, Se-methylselenocysteine was the main species found in the fruit, suggesting Broccoli as a source of this important selenoamino acid in human diet. However, the degree of meal processing influences the stability of Se-aminoacids. Speciation studies in boiled Broccoli and in the extraction water were also carried out. This experiment revealed a noticeable degradation of Se-methylselenocysteine in the boiled Broccoli fruit. Proteins soluble in Tris-HCl were analyzed by two-dimensional chromatography coupled to ICP-MS. The results obtained contribute not only to a deeper understanding of Se accumulation mechanisms by plants but also to further functional food complements preparation and the effect of food processing on species stability.

Biochemical and molecular characterization of the homocysteine S-methyltransferase from broccoli (Brassica oleracea var. italica)

Plants are known for their unique ability to synthesize methionine from S-methylmethionine (SMM) and homocysteine using the enzyme SMM: homocysteine S-methyltransferase (HMT) in the SMM cycle. Two cDNAs exhibiting HMT activity were cloned from broccoli and functionally expressed in E. coli. One cDNA, that encodes an enzyme with high substrate specificity for homocysteine, was designated as BoHMT1. The other cDNA was the BoSMT gene that we previously characterized and encodes a selenocysteine methyltransferase (Lyi, S.M., Heller, L.I., Rutzke, M., Welch, R.M., Kochian, L.V., Li, L., 2005. Molecular and biochemical characterization of the selenocysteine Se-methyltransferase gene and Se-methylselenocysteine synthesis in broccoli. Plant Physiol. 138, 409-420). Both exist as single gene sequences in the broccoli genome. While BoSMT expression was extremely low or undetectable in broccoli plants unless the plants were exposed to selenium, the BoHMT1 mRNA accumulated in most tissues of the plant except older leaves. In contrast to BoSMT whose expression was dramatically upregulated by treating plants with selenate, the transcript levels of BoHMT1 were not markedly affected in plants exposed to selenium. BoHMT1 expression responded significantly to changes in plant sulfur status. However, its expression was not dramatically affected in plants treated with methionine, SMM, homocysteine, or the heavy metal, cadmium. The differences in the substrate specificity and gene expression in response to changes in plant sulfur and selenium status between BoHMT1 and BoSMT suggest that the enzymes encoded by these two genes play distinct roles in sulfur and selenium metabolism in broccoli.

Differential effects of selenium compounds on glucose synthesis in rabbit kidney-cortex tubules and hepatocytes. In vitro and in vivo studies

Although selenium is taken with diet mainly as selenoamino acids, its hypoglycaemic action on hepatic gluconeogenesis has been studied with the use of inorganic selenium derivatives. The aim of the present investigation was to compare relative efficacies of inorganic and organic selenium compounds in reducing glucose synthesis in hepatocytes and renal tubules, significantly contributing to the glucose homeostasis. In contrast to hepatocytes, both selenite and methylselenocysteine inhibited renal gluconeogenesis by about 40-45% in control rabbits. Selenate did not affect this process, whereas selenomethionine inhibited gluconeogenesis by about 20% in both hepatocytes and renal tubules. In contrast to methylselenocysteine, selenite decreased intracellular ATP content, glutathione reduced/glutathione oxidized (GSH/GSSG) ratio and pyruvate carboxylase, PEPCK and FBPase activities, while methylselenocysteine diminished PEPCK activity due to elevation of intracellular 2-oxoglutarate and GSSG, inhibitors of this enzyme. Experiments in vivo indicate that in 3 of 9 alloxan-diabetic rabbits treated for 14 days with methylselenocysteine (0.182mg/kg body weight) blood glucose level was normalized, whereas in all diabetic rabbits plasma creatinine and urea levels decreased from 2.52+/-0.18 and 87.4+/-9.7 down to 1.63+/-0.11 and 39.0+/-2.8, respectively. In view of these data selenium supplementation might be beneficial for protection against diabetes-induced nephrotoxicity despite selenium accumulation in kidneys and liver.

Chemical and biological characterisation of nutraceutical compounds of broccoli

People's diet offers a greater and more diverse group of plant bioactives than do drugs, and they often do not realise that many drugs are derived from the compounds originally discovered in plant foods. Numerous epidemiological studies indicate that Brassica vegetables in general, and broccoli in particular, protect humans against cancer since they are rich sources of glucosinolates as well as possessing a high content of flavonoids, vitamins and mineral nutrients. One unusual phytotherapeutic role of broccoli is for skin diseases--the juice of the leaves is used to treat warts. However, the main use of broccoli stems from its health-promoting properties. Some criteria have been proposed to evaluate the possibilities of developing new "functional foods" to reduce the risk of specific cancers; largely in broccoli, which is associated with cancer protection. Processing conditions, transport, domestic cooking, etc., affect the health-promoting properties of broccoli and these have been widely studied. This review makes an in-depth study of the chemical and biological characterization of the phytochemicals of broccoli and the effects on the bioactive composition of broccoli.

Methylation of sulfhydryl groups: a new function for a family of small molecule plant O-methyltransferases

In plants, type I and II S-adenosyl-l-methionine-dependent O-methyltransferases (OMTs) catalyze most hydroxyl group methylations of small molecules. A homology-based RT-PCR strategy using Catharanthus roseus (Madagascar periwinkle) RNA previously identified six new type I plant OMT family members. We now describe the molecular and biochemical characterization of a seventh protein. It shares 56-58% identity with caffeic acid OMTs (COMTs), but it failed to methylate COMT substrates, and had no activity with flavonoids. However, the in vitro incubations revealed unusually high background levels without added substrates. A search for the responsible component revealed that the enzyme methylated dithiothreitol (DTT), the reducing agent added for enzyme stabilization. Unexpectedly, product analysis revealed that the methylation occurred on a sulfhydryl moiety, not on a hydroxyl group. Analysis of 34 compounds indicated a broad substrate range, with a preference for small hydrophobic molecules. Benzene thiol (Km 220 microm) and furfuryl thiol (Km 60 microm) were the best substrates (6-7-fold better than DTT). Small isosteric hydrophobic substrates with hydroxyl groups, like phenol and guaiacol, were also methylated, but the activities were at least 5-fold lower than with thiols. The enzyme was named C. roseus S-methyltransferase 1 (CrSMT1). Models based on the COMT crystal structure suggest that S-methylation is mechanistically identical to O-methylation. CrSMT1 so far is the only recognized example of an S-methyltransferase in this protein family. Its properties indicate that a few changes in key residues are sufficient to convert an OMT into a S-methyltransferase (SMT). Future functional investigations of plant methyltransferases should consider the possibility that the enzymes may direct methylation at sulfhydryl groups.

Arsenic detoxification and evolution of trimethylarsine gas by a microbial arsenite S-adenosylmethionine methyltransferase

In this article, a mechanism of arsenite [As(III)]resistance through methylation and subsequent volatization is described. Heterologous expression of arsM from Rhodopseudomonas palustris was shown to confer As(III) resistance to an arsenic-sensitive strain of Escherichia coli. ArsM catalyzes the formation of a number of methylated intermediates from As(III), with trimethylarsine as the end product. The net result is loss of arsenic, from both the medium and the cells. Because ArsM homologues are widespread in nature, this microbial-mediated transformation is proposed to have an important impact on the global arsenic cycle.