Mercury Detoxification Research Articles

Detoxification of mercury in soil by selenite and related mechanisms

A better understanding of the benefits of selenium (Se) fertilization to alleviate the toxicity of mercury (Hg) on plants and of the underlying mechanisms involved in Hg stress is important for the remediation of soils contaminated by Hg. This study is aimed to explore the effects of the application of selenite to alleviate the toxicity of Hg in soils to plants and related mechanisms involved in this process. The chemical (Hg uptake of pak choi), biological (root and shoot length, root and shoot weight) and physiological effects (antioxidant enzyme activities, non-enzymatic antioxidant contents (proline) and lipid peroxidation products (malondialdehyde)) produced over plants by the application of different doses of Hg and Se to soil has been investigated through a pot experiment, which was conducted with exposure to different dosages of mercuric chloride (0, 1.0, 2.0, and 3.0 mg/kg soil) and sodium selenite (0, 0.5, 1.0, and 2.5 mg/kg soil). Results indicated that single high Hg treatment (3.0 mg/kg Hg) resulted in significantly increase in Hg uptake by plants (P < 0.01), thus the growth of pak choi was inhibited. However, the Se application at 1.0 and 2.5 mg/kg led to significantly alleviated Hg uptake by plants (P < 0.05). Meanwhile, the low Se (at 0.5 and 1.0 mg/kg) applied to soil induced significantly improvement the growth of pak choi (P < 0.05) by elevating the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione peroxidase (GSH-Px) enzymes and the content of chlorophyll (SPAD value) as well as suppressed the lipid peroxidation products contents (MDA) and proline. Results collectively indicated that applied Se played an important role in promoting the detoxification of Hg and growth of pak choi under oxidative stress. Notably, this role may only be significant when Se application at the appropriate concentration (≤ 1.0 mg/kg).

Mutual detoxification of mercury and selenium in unicellular Tetrahymena

Selenium (Se) is commonly recognized as a protective element with an antagonistic effect against mercury (Hg) toxicity. However, the mechanisms of this Hg–Se antagonism are complex and remain controversial. To gain insight into the Hg–Se antagonism, a type of unicellular eukaryotic protozoa (Tetrahymena malaccensis, T. malaccensis) was selected and individually or jointly exposed to two Hg and three Se species. We found that Se species showed different toxic effects on the proliferation of T. malaccensis with the toxicity following the order: selenite (Se(IV))>selenomethionine (SeMeth)>selenate (Se(VI)). The Hg–Se antagonism in Tetrahymena was observed because the joint toxicity significantly decreased under co-exposure to highly toxic dosages of Hg and Se versus individual toxicity. Unlike Se(IV) and Se(VI), non-toxic dosage of SeMeth significantly decreased the Hg toxicity, revealing the influence of the Se species and dosages on the Hg–Se antagonism. Unexpectedly, inorganic divalent Hg (Hg2+) and monomethylmercury (MeHg) also displayed detoxification towards extremely highly toxic dosages of Se, although their detoxifying efficiency was discrepant. These results suggested mutual Hg–Se detoxification in T. malaccensis, which was highly dependent on the dosages and species of both elements. As compared to other species, SeMeth and MeHg promoted the Hg–Se joint effects to a higher degree. Additionally, the Hg contents decreased for all the Hg–Se co-exposed groups, revealing a sequestering effect of Se towards Hg in T. malaccensis.

Solid-phase partitioning of mercury in artisanal gold mine tailings from selected key areas in Mindanao, Philippines, and its implications for mercury detoxification.

The solid-phase partitioning of mercury could provide necessary data in the identification of remediation techniques in contaminated artisanal gold mine tailings. This study was conducted to determine the total mercury content of mine wastes and identify its solid-phase partitioning through selective sequential extraction coupled with cold vapour atomic absorption spectroscopy. Samples from mine tailings and the carbon-in-pulp (CIP) process were obtained from selected key areas in Mindanao, Philippines. The results showed that mercury use is still prevalent among small-scale gold miners in the Philippines. Tailings after ball mill-gravity concentration (W-BM and Li-BM samples) from Mt Diwata and Libona contained high levels of mercury amounting to 25.024 and 6.5 mg kg-1, respectively. The most prevalent form of mercury in the mine tailings was elemental/amalgamated mercury, followed by water soluble, exchangeable, organic and strongly bound phases, respectively. In contrast, mercury content of carbon-in-pulp residues were significantly lower at only 0.3 and 0.06 mg kg-1 for P-CIP (Del Pilar) and W-CIP (Mt Diwata), respectively. The bulk of mercury in P-CIP samples was partitioned in residual fraction while in W-CIP samples, water soluble mercury predominated. Overall, this study has several important implications with regards to mercury detoxification of contaminated mine tailings from Mindanao, Philippines.

Low-Molecular-Weight Thiols and Thioredoxins Are Important Players in Hg(II) Resistance in Thermus thermophilus HB27.

Mercury (Hg), one of the most toxic and widely distributed heavy metals, has a high affinity for thiol groups. Thiol groups reduce and sequester Hg. Therefore, low-molecular-weight (LMW) and protein thiols may be important cell components used in Hg resistance. To date, the role of low-molecular-weight thiols in Hg detoxification remains understudied. The mercury resistance (mer) operon of Thermus thermophilus suggests an evolutionary link between Hg(II) resistance and low-molecular-weight thiol metabolism. The mer operon encodes an enzyme involved in methionine biosynthesis, Oah. Challenge with Hg(II) resulted in increased expression of genes involved in the biosynthesis of multiple low-molecular-weight thiols (cysteine, homocysteine, and bacillithiol), as well as the thioredoxin system. Phenotypic analysis of gene replacement mutants indicated that Oah contributes to Hg resistance under sulfur-limiting conditions, and strains lacking bacillithiol and/or thioredoxins are more sensitive to Hg(II) than the wild type. Growth in the presence of either a thiol-oxidizing agent or a thiol-alkylating agent increased sensitivity to Hg(II). Furthermore, exposure to 3 μM Hg(II) consumed all intracellular reduced bacillithiol and cysteine. Database searches indicate that oah2 is present in all Thermus sp. mer operons. The presence of a thiol-related gene was also detected in some alphaproteobacterial mer operons, in which a glutathione reductase gene was present, supporting the role of thiols in Hg(II) detoxification. These results have led to a working model in which LMW thiols act as Hg(II)-buffering agents while Hg is reduced by MerA.IMPORTANCE The survival of microorganisms in the presence of toxic metals is central to life's sustainability. The affinity of thiol groups for toxic heavy metals drives microbe-metal interactions and modulates metal toxicity. Mercury detoxification (mer) genes likely originated early in microbial evolution in geothermal environments. Little is known about how mer systems interact with cellular thiol systems. Thermus spp. possess a simple mer operon in which a low-molecular-weight thiol biosynthesis gene is present, along with merR and merA In this study, we present experimental evidence for the role of thiol systems in mercury resistance. Our data suggest that, in T. thermophilus, thiolated compounds may function side by side with mer genes to detoxify mercury. Thus, thiol systems function in consort with mer-mediated resistance to mercury, suggesting exciting new questions for future research.

Expression and Purification of Transmembrane Protein MerE from Mercury-Resistant Bacillus cereus.

Mercury-resistant (HgR) bacteria were isolated from heavy metal polluted wastewater and soil collected near to tanneries of district Kasur, Pakistan. Bacterial isolates AZ-1, AZ-2 and AZ-3 showed resistance up to 40 μg/ml against mercuric chloride (HgCl2). 16S rDNA ribotyping and phylogenetic analysis were performed for the characterization of selected isolates as Bacillus sp. AZ-1 (KT270477), Bacillus cereus AZ-2 (KT270478) and Bacillus cereus AZ-3 (KT270479). Phylogenetic relationship on the basis of merA nucleotide sequence confirmed 51- 100% homology with the corresponding region of the merA gene of already reported mercuryresistant Gram-positive bacteria. The merE gene involved in the transportation of elemental mercury (Hg0) via cell membrane was cloned for the first time into pHLV vector and transformed in overexpressed C43(DE3) E. coli cells. The recombinant plasmid (pHLMerE) was expressed and the native MerE protein was obtained after thrombin cleavage by size exclusion chromatography (SEC). The purification of fusion/recombinant and native protein MerE by Ni-NTA column, dialysis and fast protein liquid chromatography (FPLC/SEC) involved unfolding/refolding techniques. A small-scale reservoir of wastewater containing 30 μg/ml of HgCl2 was designed to check the detoxification ability of selected strains. It resulted in 83% detoxification of mercury by B. cereus AZ-2 and B. cereus AZ-3, and 76% detoxification by Bacillus sp. AZ-1 respectively (p < 0.05).

A Nanoselenium Sponge for Instantaneous Mercury Removal to Undetectable Levels

Selective removal of aqueous mercury to levels below 10 ng L−1 or part per trillion remains an elusive goal for public health and environmental agencies. Here, it is shown that a low‐cost nanocomposite sponge prepared by growing selenium (Se) nanomaterials on the surface and throughout the bulk of a polyurethane sponge exhibits a record breaking‐mercury ion (Hg2+) removal rate, regardless of the pH. The exposure of aqueous solutions containing 10 mg L−1–12 ng L−1 Hg2+ to the sponge for a few seconds results in clean water with undetectable mercury levels (detection limit: 0.2 ng L−1). Such performance is far below the acceptable limits in drinking water (2 µg L−1), industrial effluents (0.2 µg L−1), and the most stringent surface water quality standards (1.3 ng L−1). The sponge shows a unique preference for Hg, does not retain water nutrients, and can significantly reduce the concentration of other heavy metal pollutants. Furthermore, the sponge shows no cytotoxic effect on human cells while exhibiting strong antimicrobial properties. The high affinity of Hg for Se results in irreversible sequestration and detoxification of mercury by the sponge, confirming the suitability for landfill disposal.

The Foundation in Evidence of Medical and Dental Telephone Consultations.

Patients can only make well-informed decisions if the information they are given by health professionals is based on scientific evidence. In this study, we assessed the foundation in evidence of free, publicly available telephone consultations in Germany. From March 2013 to January 2014, four hidden clients seeking information asked standardized questions about three medical topics (screening for colorectal cancer, for glaucoma, and for trisomy 21) and three dental ones (the sealing of dental fissures, professional dental cleaning, and mercury detoxification). Depending on the topic, the questions addressed such issues as the risk of disease and the purpose, content, validity, benefits, and risks of potential diagnostic and therapeutic measures. All identifiable telephone consultation services that provided counselling on the above topics were included in the study (23 government-sponsored institutions, 31 institutions independently run by physicians, 521 institutions under religious auspices, 25 dental counselling services). Of the 599 telephone consultation services that were identified, 567 were contacted; 404 did not offer any relevant counselling. A total of 293 conversations were held with the remaining 163 consultation services. Six of these conversations fully met predefined criteria for evidence-based counselling. The percentage of appropriate answers to the key questions on each topic was 5% for colorectal cancer screening (7/140), 23.8% for glaucoma screening (25/105), 33.9% for trisomy 21 screening (121/357), 27.5% for the sealing of dental fissures (28/102), 16.2% for professional dental cleaning (19/117), and 12.9% for mercury detoxification (12/93). The percentage of appropriate answers also varied depending on the type of institution: 26.8% for government-sponsored institutions (67/250), 4.5% for institutions independently run by physicians (4/88), and 31.1% for institutions under religious auspices (82/264). The medical and dental counselling now offered over the telephone by the types of institutions included in this study does not satisfy the criteria for evidence-based health information.

Mercury species, selenium, metallothioneins and glutathione in two dolphins from the southeastern Brazilian coast: Mercury detoxification and physiological differences in diving capacity

In the present study, the concentration of trace elements, total mercury (Hg) and selenium (Se) and mercury forms (MeHg, Hginorg and HgSe) in the vulnerable coastal dolphins Pontoporia blainvillei and Sotalia guianensis were appraised and compared, using metallothioneins (MT) and glutathione (GSH) as biomarkers for trace element exposure. The trace element concentrations varied between muscle and liver tissues, with liver of all dolphin specimens showing higher Hg and Se concentrations than those found in muscle. Hg, MeHg and Hginorg molar concentrations showed a clear increase with Se molar concentrations in the liver of both dolphins, and Se concentrations were higher than those of Hg on a molar basis. Se plays a relevant role in the detoxification of MeHg in the hepatic tissue of both dolphins, forming Hg-Se amorphous crystals in liver. In contrast, MT were involved in the detoxification process of Hginorg in liver. GSH levels in P. blainvillei and S. guianensis muscle tissue suggest that these dolphins have different diving capacities. Muscle Hg concentrations were associated to this tripeptide, which protects dolphin cells against Hg stress.

Danio rerio ABC transporter genes abcb3 and abcb7 play a protecting role against metal contamination.

ATP-binding cassette (ABC) proteins are efflux transporters and some of them are involved in xenobiotic detoxification. The involvement of four zebrafish ABC transporters in cadmium, zinc and mercury detoxification was characterized in a metal hypersensitive mutant of Escherichia coli. The E. coli tolC mutant expressing ABCB3 or ABCB7 transporters exhibited higher survival ratios and lower metal accumulation under a metal exposure condition than the controls. For instance, in the presence of 8 and 10 μM of HgCl2 , the survival ratios of bacteria expressing ABCB3 were four and six-times higher than the control whereas the mercury concentrations were 2.5 and 2-times lower than in the control. This work provides new data on the function of zebrafish ABCB3 and ABCB7 transporters and highlights their significance in metal detoxification. Copyright © 2016 John Wiley & Sons, Ltd.

Genetic basis and importance of metal resistant genes in bacteria for bioremediation of contaminated environments with toxic metal pollutants.

Metal pollution is one of the most persistent and complex environmental issues, causing threat to the ecosystem and human health. On exposure to several toxic metals such as arsenic, cadmium, chromium, copper, lead, and mercury, several bacteria has evolved with many metal-resistant genes as a means of their adaptation. These genes can be further exploited for bioremediation of the metal-contaminated environments. Many operon-clustered metal-resistant genes such as cadB, chrA, copAB, pbrA, merA, and NiCoT have been reported in bacterial systems for cadmium, chromium, copper, lead, mercury, and nickel resistance and detoxification, respectively. The field of environmental bioremediation has been ameliorated by exploiting diverse bacterial detoxification genes. Genetic engineering integrated with bioremediation assists in manipulation of bacterial genome which can enhance toxic metal detoxification that is not usually performed by normal bacteria. These techniques include genetic engineering with single genes or operons, pathway construction, and alternations of the sequences of existing genes. However, numerous facets of bacterial novel metal-resistant genes are yet to be explored for application in microbial bioremediation practices. This review describes the role of bacteria and their adaptive mechanisms for toxic metal detoxification and restoration of contaminated sites.

Populasi Bakteri Pada Tanah Bekas Buangan Limbah Merkuri Tambang Emas di Kabupaten Bolaang Mongondow: Penelitian Pendahuluan

Mercury is one of the most toxic heavy metals found in nature. Athough adverse health effect of mercury have been known for a long time, exposure to mercury continues and is even increasing in some areas, for example, mercury is still used in gold mining in many parts of North Sulawesi Province. Most of the soil and aquatic bacteria that are continuously exposed to mercury usually develop a genetic adaptation to resist the toxicity of this compound. Bacteria have a specific operon called merOperon that functions to coordinate genes coding for proteins and enzymes involved in mercury disposal and detoxification. Therefore, this preliminary study aims to isolate and identify bacteria collected from gold mining area in the district of Bolaang Mongondow. Bacteria were isolated from soil samples collected from three locations of the gold mining waste disposal and the isolated bacteria were grown in agar media. Identification of the grown bacteria were then be performed using morphological, physiological and biochemical tests. The results showed that 36 bacteria were successfully isolated, of which, 11 isolates were gram positive bacteria and the remainders were gram negative. All isolates showed motility and all could be grouped into 4 species i.e. Bacillus sp., Escherichia coli, Enterobacter cloacea, and Enterobacter aerogenes.

Mercury in marine fish, mammals, seabirds, and human hair in the coastal zone of the southern Baltic.

Mercury (Hg), aside from having high toxicity, is characterized by its ability to biomagnify in the marine trophic chain. This is an important problem especially in estuaries, or in the coastal zone, particularly near the mouths of large rivers. This study was conducted in the years 2001–2011, in the coastal zone of the Baltic Sea near to the mouth of the River Vistula, which is the second biggest river discharging into the Baltic. Mercury concentration was measured in the tissues and organs of cod, flounder, herring, seals (living in the wild and in captivity), great black-backed gulls, and African penguins from Gdańsk Zoo, and also in human hair. Penguins and seals at the seal sanctuary in Hel were fed only herring. In marine birds and mammals and in the pelagic herring, the highest Hg concentration was observed in the kidney and in the liver, while in cod and flounder (located on a higher trophic level) the muscles were the most contaminated with mercury. In gray seals living in the seal sanctuary, Hg concentration in all analyzed tissues and organs except the kidneys was lower in comparison with seals living in the wild. The comparatively small share of fish in the diet of local Polish people and their preference towards the consumption of herring contributed to low concentration of Hg in their hair. The protective mechanisms related to detoxification and elimination of mercury were shown to be more effective in the seals than in the penguins, despite the former consuming around 10 times more food per day.

Identification of an archaeal mercury regulon by chromatin immunoprecipitation.

Mercury is a heavy metal and toxic to all forms of life. Metal exposure can invoke a response to improve survival. In archaea, several components of a mercury response system have been identified, but it is not known whether metal transport is a member of this system. To identify such missing components, a peptide-tagged MerR transcription factor was used to localize enriched chromosome regions by chromosome immunoprecipitation combined with DNA sequence analysis. Such regions could serve as secondary regulatory binding sites to control the expression of additional genes associated with mercury detoxification. Among the 31 highly enriched loci, a subset of five was pursued as potential candidates based on their current annotations. Quantitative reverse transcription-PCR analysis of these regions with and without mercury treatment in WT and mutant strains lacking merR indicated significant regulatory responses under these conditions. Of these, a Family 5 extracellular solute-binding protein and the MarR transcription factor shown previously to control responses to oxidation were most strongly affected. Inactivation of the solute-binding protein by gene disruption increased the resistance of mutant cells to mercury challenge. Inductively coupled plasma-MS analysis of the mutant cell line following metal challenge indicated there was less intracellular mercury compared with the isogenic WT strain. Together, these regulated genes comprise new members of the archaeal MerR regulon and reveal a cascade of transcriptional control not previously demonstrated in this model organism.

Biochemical and Structural Properties of a Thermostable Mercuric Ion Reductase from Metallosphaera sedula.

Mercuric ion reductase (MerA), a mercury detoxification enzyme, has been tuned by evolution to have high specificity for mercuric ions (Hg2+) and to catalyze their reduction to a more volatile, less toxic elemental form. Here, we present a biochemical and structural characterization of MerA from the thermophilic crenarchaeon Metallosphaera sedula. MerA from M. sedula is a thermostable enzyme, and remains active after extended incubation at 97°C. At 37°C, the NADPH oxidation-linked Hg2+ reduction specific activity was found to be 1.9 μmol/min⋅mg, increasing to 3.1 μmol/min⋅mg at 70°C. M. sedula MerA crystals were obtained and the structure was solved to 1.6 Å, representing the first solved crystal structure of a thermophilic MerA. Comparison of both the crystal structure and amino acid sequence of MerA from M. sedula to mesophillic counterparts provides new insights into the structural determinants that underpin the thermal stability of the enzyme.

Microbes track pollution through time

Mercury Microbiology Genes present in buried lake sediments can help reconstruct how microbial ecosystems responded to past exposure to mercury. When atmospheric mercury transported around the globe is eventually deposited in terrestrial or aquatic environments, microorganisms control its cycling and bioaccumulation. Poulain et al. isolated ancient microbial DNA from sediments in remote subarctic lakes in the Hudson Bay Lowlands of Canada. The distribution of genes coding for mercuric reductase (merA), the primary enzyme involved in mercury detoxification, correlates with historical mercury deposition, including a rapid population shift at the onset of the Industrial Revolution. ISME J. 10.1038/ismej.2015.86 (2015).

OVERPRODUCTION OF MERCURIC REDUCTASE FROM MERCURY-RESISTANT BACTERIA KLEBSIELLA PNEUMONIAE ISOLATE A1.1.1

Mercury is a highly toxic compound in human. It can, however, be detoxified by mercuric reductase (MerA) protein derived from mercury resistant bacteria. This study aims to obtaine MerA protein by transforming merA gene into Escherichia coli BL21. Nucleotide sequence of merA gene of mercury resistant bacteria Klebsiella pneumoniae isolates A111, optimized by using gene program designers (www.dna20/com) then commercially synthesized and cloned in pET32b expression plasmid vector. Plasmid was transformed into Escherichia coli BL21 to produce MerA protein recombinant, induced with isopropyl-β-D-thiogalactopyranoside (IPTG). MerA proteins were analyzed by 10% sodium dodecyl sulphate poly acrylamide gel electrophoresis (SDS PAGE). The result showed that MerA protein with 60kDa was detected on SDS PAGE. The obtained MerA protein can be used in further research for the enzymatic detoxification of inorganic mercury. Key words : mercuric reductase, mer A gene, MerA protein, Escherichia coli BL21

Mercury Complexes with Tripodal Pseudopeptides Derived from D ‐Penicillamine Favour a HgS 3 Coordination

The pseudopeptides L4 and L5 are built on a nitrilotriacetic scaffold and functionalized with three D-penicillamine (D-Pen) units. D-Pen is an interesting building block and a bulkier analogue of cysteine with the β-methylene hydrogen atoms replaced by larger methyl groups. The two sulfur ligands L4 and L5 are studied here with the toxic HgII ion. The formation of the two mononuclear complexes HgL4 and HgL5 is demonstrated by 1H NMR spectroscopy and mass spectrometry and the tristhiolato coordination is unambiguously evidenced by the UV absorption signatures. An important feature is the stabilization of the HgS3 coordination mode at the expense of the generally preferred HgS2 linear coordination. It appears that the hindered D-Pen pseudopeptide L5 is capable of stabilizing the unusual HgS3 coordination over a large pH range in water, pH > 5.5. This water-soluble compound can therefore be considered as a promising chelating agent for mercury detoxification.

Detoxification of Mercury by Bacteria Using Crude Glycerol from Biodiesel as a Carbon Source

Bacteria that harbor the mer operon in their genome are able to enzymatically reduce mercury (II) to the volatile form of mercury Hg (0). Detoxification of contaminated waste by using these bacteria may be an alternative to conventional methods for mercury removal. Residual glycerol from the biodiesel industry can be used as a carbon source to accelerate the process. This work shows for the first time the feasibility of using residual glycerol as a carbon source for Hg removal by bacteria prospected from contaminated environments. Eight bacterial isolates were able to remove mercury and degrade glycerol in mineral medium and residual glycerol. Mercury removal was monitored by atomic absorption spectroscopy and glycerol degradation by high performance liquid chromatography. The best results of mercury removal and glycerol degradation were obtained using isolates of Serratia marcescens M25C (85 and 100 %), Klebsiella pneumoniae PLB (90 and 100 %), Klebsiella oxytoca (90 and 100 %), and Arthrobacter sp. U3 (80 and 75 %), with addition of 0.5 g L−1 yeast extract. The Arthrobacter sp. U3 isolate is common in soils and has proven to be a promising candidate for environment applications due to its low pathogenicity and higher Hg removal and glycerol degradation rates.

Metallothionein, essential elements and lipid peroxidation in mercury-exposed suckling rats pretreated with selenium.

Detoxification of mercury (Hg) with selenium (Se) in the early postnatal period with regard to the expression of metallothionein protein (MT), essential element status, and lipid peroxidation level in tissues has not been studied. Seven-day-old Wistar pups were orally pretreated with Se [6 μmol Na2SeO3/kg body weight (b.w.)] for 3 days and then cotreated with Hg (6 μmol HgCl2/kg b.w.) for the following 4 days. This group (Se + Hg) was compared to the groups treated with Hg, Se, or vehicle (control). Compared to the Hg-group, Se + Hg-group exhibited lower renal MT expression, reduced accumulation of Hg, Cu and Zn, and reduced excretion of Se, Hg and Zn in urine. In the liver, MT was stimulated by Se treatment in both, Se and Se + Hg-group. Hepatic and brain levels of the endogenous essential elements Cu, Fe, Mg, and Zn remained unchanged in all of the studied groups. Brain Hg levels and oxidation of lipids measured as thiobarbituric acid reactive substances were diminished in Se + Hg-group of pups compared to the Hg-group. This study suggests that Se pretreatment can help reduce Hg in the tissues of suckling rats, simultaneously preventing impairment of essential element levels in the kidneys and their excessive excretion via urine. Also, Se was shown to prevent oxidative damage of lipids in the brain, which is particularly susceptible to Hg during the early postnatal period.

The Inhibition Analysis of Two Heavy Metal ATPase Genes (NtHMA3a and NtHMA3b) in Nicotiana tabacum

ABSTRACTPrevious studies suggested that plants detoxified mercury and cadmium through similar mechanisms. A heavy metal ATPase (adenosine triphosphatase) gene, HMA3, plays a key role in the plant's cadmium detoxification. To prove whether HMA3 also participates in mercury detoxification in plants, an experiment was designed to inhibit the expressions of HMA3 genes (NtHMA3a and NtHMA3b) in tobacco plants. Results showed that plants’ tolerance to mercury ions had not changed after the expressions of NtHMA3a and NtHMA3b were inhibited. When mercury content was measured from the whole seedlings, no differences had been observed among wild-type, NtHMA3a-NtHMA3b-RNAi, and the empty-vector transgenic plants. HMA3 was not the key gene responsible for plants’ mercury ion uptake from soil. Although the mercury content in the root was higher than that in the shoot for each seedling, in each treatment, neither in shoots nor in roots were statistical differences in mercury content found among NtHMA3a-NtHMA3b-RNAi, empty-vector transgenic, and wild-type plants. After the expressions of NtHMA3a and NtHMA3b were inhibited, the movement of mercury ions from root to shoot had not been affected. HMA3 was not the key gene responsible for mercury ion transportation from root to shoot. When mercury content was measured from the whole seedling, no significant difference had been found among wild-type, NtHMA3a-NtHMA3b-RNAi, and the empty-vector transgenic plants. For mercury ion translocation across tonoplast, the main pathway might not be HMA3, but ABC (ATP-binding cassette) transporters.