Molecular characterization of an anion pump. The arsA gene product is an arsenite(antimonate)-stimulated ATPase

Identification of bacterial dissimilatory antimonate reductase AnrA: genes and proteins involved in antimonate respiration and resistance in Geobacter sp. strain SVR

Geobacter sp. strain SVR uses antimonate [Sb(V)] as a terminal electron acceptor for anaerobic respiration. Here, we visualized a possible key enzyme, periplasmic Sb(V) reductase (Anr), via active staining and non-denaturing gel electrophoresis. Liquid chromatography-tandem mass spectrometry analysis revealed that a novel dimethyl sulfoxide (DMSO) reductase family protein, WP_173201954.1, is involved in Anr. This protein was closely related with AnrA, a protein suggested to be the catalytic subunit of a respiratory Sb(V) reductase in Desulfuribacillus stibiiarsenatis. The anr genes of strain SVR (anrXSRBAD) formed an operon-like structure, and their transcription was upregulated under Sb(V)-respiring conditions. The expression of anrA gene was induced by more than 1 µM of antimonite [Sb(III)]; however, arsenite [As(III)] did not induce the expression of anrA gene. Tandem mass tag-based proteomic analysis revealed that, in addition to Anr proteins, proteins in the following categories were upregulated under Sb(V)-respiring conditions: (i) Sb(III) efflux systems such as Ant and Ars; (ii) antioxidizing proteins such as ferritin, rubredoxin, and thioredoxin; (iii) protein quality control systems such as HspA, HslO, and DnaK; and (iv) DNA repair proteins such as UspA and UvrB. These results suggest that strain SVR copes with antimony stress by modulating pleiotropic processes to resist and actively metabolize antimony. To the best of our knowledge, this is the first report to demonstrate the involvement of AnrA in Sb(V) respiration at the protein level. Furthermore, this is the first example to show high expression of the Ant system proteins in the Sb(V)-respiring bacterium.IMPORTANCEAntimony (Sb) exists mainly as antimonite [Sb(III)] or antimonate [Sb(V)] in the environment, and Sb(III) is more toxic than Sb(V). Recently, microbial involvement in Sb redox reactions has received attention. Although more than 90 Sb(III)-oxidizing bacteria have been reported, information on Sb(V)-reducing bacteria is limited. Especially, the enzyme involved in dissimilatory Sb(V) reduction, or Sb(V) respiration, is unclear, despite this pathway being very important for the circulation of Sb in nature. In this study, we demonstrated that the Sb(V) reductase (Anr) of an Sb(V)-respiring bacterium (Geobacter sp. SVR) is a novel member of the dimethyl sulfoxide (DMSO) reductase family. In addition, we found that strain SVR copes with Sb stress by modulating pleiotropic processes, including the Ant and Ars systems, and upregulating the antioxidant and quality control protein levels. Considering the abundance and diversity of putative anr genes in the environment, Anr may play a significant role in global Sb cycling in both marine and terrestrial environments.

Single Strain-Triggered Biogeochemical Cycle of Arsenic

The microbial metabolism of arsenic plays a prominent role in governing the biogeochemical cycle of arsenic. Although diverse microbes are known to be involved in the redox transformation of inorganic arsenic, the underlying mechanisms about the arsenic redox cycle mediated by a single microbial strain remain unclear yet. Herein, we discover that Shewanella putrefaciens CN32, a well-known arsenate-respiring and dissimilatory metal-reducing bacterium, could mediate the reversible arsenic redox transformation under aerobic conditions. Genetic analysis shows that S. putrefaciens CN32 contains both ars and arr operon but lacks an As(III) oxidase encoding gene. Arsenic(V) reduction tests demonstrate that the ars operon is advantageous but not essential for As(V) respiration in S. putrefaciens CN32. The Arr complex encoded by the arr operon not only plays a crucial role in arsenate respiration under anaerobic conditions but also participates in the sequential process of As(V) reduction and As(III) oxidation under aerobic conditions. The Arr enzyme also contributes to the microbial As(III) resistance. The expression and catalysis directionality of Arr in S. putrefaciens CN32 are regulated by the carbon source types. Our results highlight the complexity of arsenic redox biotransformation in environments and provide new insights into the important contribution of Arr to the As biogeochemical cycle in nature.

Recent advances in exploring the heavy metal(loid) resistant microbiome

Heavy metal(loid)s exert selective pressure on microbial communities and evolution of metal resistance determinants. Despite increasing knowledge concerning the impact of metal pollution on microbial community and ecological function, it is still a challenge to identify a consistent pattern of microbial community composition along gradients of elevated metal(loid)s in natural environments. Further, our current knowledge of the microbial metal resistome at the community level has been lagging behind compared to the state-of-the-art genetic profiling of bacterial metal resistance mechanisms in a pure culture system. This review provides an overview of the core metal resistant microbiome, development of metal resistance strategies, and potential factors driving the diversity and distribution of metal resistance determinants in natural environments. The impacts of biotic factors regulating the bacterial metal resistome are highlighted. We finally discuss the advances in multiple technologies, research challenges, and future directions to better understand the interface of the environmental microbiome with the metal resistome. This review aims to highlight the diversity and wide distribution of heavy metal(loid)s and their corresponding resistance determinants, helping to better understand the resistance strategy at the community level.

Transcriptomic Analysis of Two Thioalkalivibrio Species Under Arsenite Stress Revealed a Potential Candidate Gene for an Alternative Arsenite Oxidation Pathway

The genus Thioalkalivibrio includes haloalkaliphilic chemolithoautotrophic sulfur-oxidizing bacteria isolated from various soda lakes worldwide. Some of these lakes possess in addition to their extreme haloalkaline environment also other harsh conditions, to which Thioalkalivibrio needs to adapt. An example is arsenic in soda lakes in eastern California, which is found there in concentrations up to 3000 μM. Arsenic is a widespread element that can be an environmental issue, as it is highly toxic to most organisms. However, resistance mechanisms in the form of detoxification are widespread and some prokaryotes can even use arsenic as an energy source. We first screened the genomes of 76 Thioalkalivibrio strains for the presence of known arsenic oxidoreductases and found 15 putative ArxA (arsenite oxidase) and two putative ArrA (arsenate reductase). Subsequently, we studied the resistance to arsenite in detail in Thioalkalivibrio jannaschii ALM2^T, and Thioalkalivibrio thiocyanoxidans ARh2^T by comparative genomics and by growing them at different arsenite concentrations followed by arsenic species and transcriptomic analysis. Tv. jannaschii ALM2^T, which has been isolated from Mono Lake, an arsenic-rich soda lake, could resist up to 5 mM arsenite, whereas Tv. thiocyanoxidans ARh2^T, which was isolated from a Kenyan soda lake, could only grow up to 0.1 mM arsenite. Interestingly, both species oxidized arsenite to arsenate under aerobic conditions, although Tv. thiocyanoxidans ARh2^T does not contain any known arsenite oxidases, and in Tv. jannaschii ALM2^T, only arxB2 was clearly upregulated. However, we found the expression of a SoeABC-like gene, which we assume might have been involved in arsenite oxidation. Other arsenite stress responses for both strains were the upregulation of the vitamin B₁₂ synthesis pathway, which can be linked to antioxidant activity, and the up- and downregulation of different DsrE/F-like genes whose roles are still unclear. Moreover, Tv. jannaschii ALM2^T induced the ars gene operon and the Pst system, and Tv. thiocanoxidans ARh2^T upregulated the sox and apr genes as well as different heat shock proteins. Our findings for Thioalkalivibrio confirm previously observed adaptations to arsenic, but also provide new insights into the arsenic stress response and the connection between the arsenic and the sulfur cycle.

Use of heavy metals resistant bacteria-a strategy for arsenic bioremediation

A large number of industries release their untreated wastes in the environment causing an increase in the concentration of toxic pollutants including heavy metal ions in ground and drinking water which is above the WHO limit. The presence of toxic pollutants in the industrial wastes pollutes our environment. Arsenic (As) is a ubiquitous toxic metalloid. Its amount varies in different parts on the earth, and its concentration is increasing in our environment day by day both by natural and anthropogenic activities. It is found in two forms; one is arsenate (As⁵⁺) and other is arsenite (As³⁺) and the latter is more toxic due to high mobility across the cell membrane. The long-term use of arsenic-containing water causes arsenicosis. High arsenic consumption, revealed by skin harms, color change, and spots on hands and feet, may cause skin cancer and affect lungs and kidneys. Hypertension, a state of high blood pressure, and lack of insulin which causes diabetes and many other disorders which relate to reproduction are the consequences of arsenic contamination. Several methods have been employed to decontaminate arsenic pollution, but the bioremediation by using biomass of bacteria, algae, fungi, and yeasts is the most compromising approach and has gained much attention from researchers in the last few decades. The microbial detoxification of arsenic can be achieved by reduction, oxidation, and methylation. High bioremediation potential and feasibility of the process make bacteria an impending foundation for green chemistry to exterminate arsenic in the environment.

The organoarsenical biocycle and the primordial antibiotic methylarsenite

Arsenic is the most pervasive environmental toxic substance. As a consequence of its ubiquity, nearly every organism has genes for resistance to inorganic arsenic. In bacteria these genes are found largely in bacterial arsenic resistance (ars) operons. Recently a parallel pathway for synthesis and degradation of methylated arsenicals has been identified. The arsM gene product encodes the ArsM (AS3MT in animals) As(iii) S-adenosylmethionine methyltransferase that methylates inorganic trivalent arsenite in three sequential steps to methylarsenite MAs(iii), dimethylarsenite (DMAs(iii) and trimethylarsenite (TMAs(iii)). MAs(iii) is considerably more toxic than As(iii), and we have proposed that MAs(iii) was a primordial antibiotic. Under aerobic conditions these products are oxidized to nontoxic pentavalent arsenicals, so that methylation became a detoxifying pathway after the atmosphere became oxidizing. Other microbes have acquired the ability to regenerate MAs(v) by reduction, transforming it again into toxic MAs(iii). Under this environmental pressure, MAs(iii) resistances evolved, including the arsI, arsH and arsP genes. ArsI is a C-As bond lyase that demethylates MAs(iii) back to less toxic As(iii). ArsH re-oxidizes MAs(iii) to MAs(v). ArsP actively extrudes MAs(iii) from cells. These proteins confer resistance to this primitive antibiotic. This oscillation between MAs(iii) synthesis and detoxification is an essential component of the arsenic biogeocycle.

Synergistic interaction of glyceraldehydes-3-phosphate dehydrogenase and ArsJ, a novel organoarsenical efflux permease, confers arsenate resistance

Microbial biotransformations are major contributors to the arsenic biogeocycle. In parallel with transformations of inorganic arsenic, organoarsenicals pathways have recently been recognized as important components of global cycling of arsenic. The well-characterized pathway of resistance to arsenate is reduction coupled to arsenite efflux. Here, we describe a new pathway of arsenate resistance involving biosynthesis and extrusion of an unusual pentavalent organoarsenical. A number of arsenic resistance (ars) operons have two genes of unknown function that are linked in these operons. One, gapdh, encodes the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase. The other, arsJ, encodes a major facilitator superfamily (MFS) protein. The two genes were cloned from the chromosome of Pseudomonas aeruginosa. When expressed together, but not alone, in Escherichia coli, gapdh and arsJ specifically conferred resistance to arsenate and decreased accumulation of As(V). Everted membrane vesicles from cells expressing arsJ accumulated As(V) in the presence of purified GAPDH, D-glceraldehylde 3-phosphate (G3P) and NAD(+) . GAPDH forms the unstable organoarsenical 1-arseno-3-phosphoglycerate (1As3PGA). We propose that ArsJ is an efflux permease that extrudes 1As3PGA from cells, where it rapidly dissociates into As(V) and 3-phosphoglycerate (3PGA), creating a novel pathway of arsenate resistance.

Mutations in the ArsA ATPase that restore interaction with the ArsD metallochaperone

The ArsA ATPase is the catalytic subunit of the ArsAB As(III) efflux pump. It receives trivalent As(III) from the intracellular metallochaperone ArsD. The interaction of ArsA and ArsD allows for resistance to As(III) at environmental concentrations. A quadruple mutant in the arsD gene encoding a K2A/K37A/K62A/K104A ArsD is unable to interact with ArsA. An error-prone mutagenesis approach was used to generate random mutations in the arsA gene that restored interaction with the quadruple arsD mutant in yeast two-hybrid assays. A number of arsA genes with multiple mutations were isolated. These were analyzed in more detail by separation into single arsA mutants. Three such mutants encoding Q56R, F120I and D137V ArsA were able to restore interaction with the quadruple ArsD mutant in yeast two-hybrid assays. Each of the three single ArsA mutants also interacted with wild type ArsD. Only the Q56R ArsA derivative exhibited significant metalloid-stimulated ATPase activity in vitro. Purified Q56R ArsA was stimulated by wild type ArsD and to a lesser degree by the quadruple ArsD derivative. The F120I and D137V ArsAs did not show metalloid-stimulated ATPase activity. Structural models generated by in silico docking suggest that an electrostatic interface favors reversible interaction between ArsA and ArsD. We predict that mutations in ArsA propagate changes in hydrogen bonding and salt bridges to the ArsA-ArsD interface that affect their interactions.

Pathways of arsenic uptake and efflux

Arsenic is the most prevalent environmental toxic substance and ranks first on the U.S. Environmental Protection Agency's Superfund List. Arsenic is a carcinogen and a causative agent of numerous human diseases. Paradoxically arsenic is used as a chemotherapeutic agent for treatment of acute promyelocytic leukemia. Inorganic arsenic has two biological important oxidation states: As(V) (arsenate) and As(III) (arsenite). Arsenic uptake is adventitious because the arsenate and arsenite are chemically similar to required nutrients. Arsenate resembles phosphate and is a competitive inhibitor of many phosphate-utilizing enzymes. Arsenate is taken up by phosphate transport systems. In contrast, at physiological pH, the form of arsenite is As(OH)(3), which resembles organic molecules such as glycerol. Consequently, arsenite is taken into cells by aquaglyceroporin channels. Arsenic efflux systems are found in nearly every organism and evolved to rid cells of this toxic metalloid. These efflux systems include members of the multidrug resistance protein family and the bacterial exchangers Acr3 and ArsB. ArsB can also be a subunit of the ArsAB As(III)-translocating ATPase, an ATP-driven efflux pump. The ArsD metallochaperone binds cytosolic As(III) and transfers it to the ArsA subunit of the efflux pump. Knowledge of the pathways and transporters for arsenic uptake and efflux is essential for understanding its toxicity and carcinogenicity and for rational design of cancer chemotherapeutic drugs.

Microbial interactions in the arsenic cycle: adoptive strategies and applications in environmental management

Arsenic (As) is a nonessential element that is often present in plants and in other organisms. However, it is one of the most hazardous of toxic elements globally. In many parts of the world, arsenic contamination in groundwater is a serious and continuing threat to human health. Microbes play an important role in regulating the environmental fate of arsenic. Different microbial processes influence the biogeochemical cycling of arsenic in ways that affect the accumulation of different arsenic species in various ecosystem compartments. For example, in soil, there are bacteria that methylate arsenite to trimethylarsine gas, thereby releasing arsenic to the atmosphere.In marine ecosystems, microbes exist that can convert inorganic arsenicals to organic arsenicals (e.g., di- and tri-methylated arsenic derivatives, arsenocholine,arsenobetaine, arsenosugars, arsenolipids). The organo arsenicals are further metabolized to complete the arsenic cycle.Microbes have developed various strategies that enable them to tolerate arsenic and to survive in arsenic-rich environments. Such strategies include As exclusion from cells by establishing permeability barrier, intra- and extracellular sequestration,active efflux pumps, enzymatic reduction, and reduction in the sensitivity of cellular targets. These strategies are used either singly or in combination. In bacteria,the genes for arsenic resistance/detoxification are encoded by the arsenic resistance operons (ars operon).In this review, we have addressed and emphasized the impact of different microbial processes (e.g., arsenite oxidation, cytoplasmic arsenate reduction, respiratory arsenate reduction, arsenite methylation) on the arsenic cycle. Microbes are the only life forms reported to exist in heavy arsenic-contaminated environments. Therefore,an understanding of the strategies adopted by microbes to cope with arsenic stress is important in managing such arsenic-contaminated sites. Further future insights into the different microbial genes/proteins that are involved in arsenic resistance may also be useful for developing arsenic resistant crop plants.

Analysis of the capability of ultra-highly diluted glucose to increase glucose uptake in arsenite-stressed bacteria Escherichia coli

OBJECTIVE

Whether ultra-highly diluted homeopathic remedies can affect living systems is questionable. Therefore, this study sees value in the analysis of whether homeopathically diluted glucose 30C has any effect on Escherichia coli exposed to arsenite stress.

METHODS

E. coli were cultured to their log phase in standard Luria-Bertani medium and then treated with either 1 mmol/L or 2 mmol/L sodium arsenite, with or without supplementation of either 1% or 3% glucose, an ultra-highly diluted and agitated ethanolic solution (70%) of glucose (diluted 10(60) times), glucose 30C or 70% ethanol (placebo) in the medium. Glucose uptake, specific activities of hexokinase and glucokinase, membrane potential, intracellular adenosine triphosphate (ATP) and expression of glucose permease in E. coli were analyzed at two different time intervals. Arsenic content in E. coli (intracellular) and in the spent medium (extracellular) was also determined.

RESULTS

In arsenite-exposed E. coli, the glucose uptake increased along with decreases in the specific activities of hexokinase and glucokinase, intracellular ATP and membrane potential and an increase in the gene expression level of glucose permease. Glucose uptake increased further by addition of 1%, 3% or ultra-highly diluted glucose in the medium, but not by the placebo.

CONCLUSION

The results demonstrated the efficacy of the ultra-highly diluted and agitated glucose in mimicking the action of actual glucose supplementation and its ability to modulate expressions of hexokinase and glucokinase enzymes and glucose permease genes, thereby validating the efficacy of ultra-high dilutions used in homeopathy.

Role of arsenic and its resistance in nature

Contamination of the environment with heavy metals has increased drastically over the last few decades. The heavy metals that are toxic include mercury, cadmium, arsenic, and selenium. Of these heavy metals, arsenic is one of the most important global environmental pollutants and is a persistent bioaccumulative carcinogen. It is a toxic metalloid that exists in two major inorganic forms: arsenate and arsenite. Arsenite disrupts enzymatic functions in cells, while arsenate behaves as a phosphate analog and interferes with phosphate uptake and utilization. Despite its toxicity, arsenic may be actively sequestered in plant and animal tissues. Various microbes interact with this metal and have shown resistance to arsenic exposure, and they appear to possess the ars operon for arsenic resistance consisting of three to five genes, i.e., arsRBC or arsRDABC, organized into a single transcriptional unit; some microbes even use it for respiration. Microbial interactions with metals may have several implications for the environment. Microbes may play a role in cycling of toxic heavy metals and in remediation of metal-contaminated sites. There is a correlation between tolerance to heavy metals and antibiotic resistance, a global problem currently threatening the treatment of infections in plants, animals, and humans. The purpose of this review is to highlight the nature and role of toxic arsenic in bacterial systems and to discuss the various genes responsible for this heavy-metal resistance in nature and the mechanisms to detoxify this element.

Isolation and characterization of arsenic resistant Geobacillus kaustophilus strain from geothermal soils

A thermophilic, arsenate resistant bacterial strain was isolated from a geothermal field located in the area surrounding Monterotondo (Tuscany, Italy). Based on 16S rRNA gene analysis and recN comparisons the strain was identified as Geobacillus kaustophilus. Cells of the strain, designated A1, were rod-shaped, 2-3 μm long and reacted negatively to Gram staining, despite its taxonomic classification as a Gram positive microorganism. Strain A1 is a thermophilic spore-forming bacterium, and grows optimally at pH 6.5 and 55 °C. An arsenate MIC of 80 mM was determined for strain A1, and the close relative G. kaustophilus DSM 7263(T) showed similar levels of arsenate resistance. These observations were consistent with the presence of arsenic detoxification genes in the genome of G. kaustophilus HTA426. Furthermore, strain A1 growth was not inhibited by 5 mM antimonite and 15 mM arsenite, the highest tested concentrations. This is the first description of arsenic resistance in a Geobacillus strain and supports the hypothesis that members of the genus may have a role in the biogeochemical cycling of arsenic.

The yeast permease Acr3p is a dual arsenite and antimonite plasma membrane transporter

The Acr3p permease from the yeast Saccharomyces cerevisiae is a prototype member of the arsenical resistance-3 (Acr3) family of transporters, which are found in all domains of life. Remarkably little is known about substrate specificity, localization and regulation of Acr3 proteins. Here, we show that the yeast Acr3p mediates not only high-level resistance to arsenite but also moderate tolerance to antimonite. The acr3 deletion mutant shows increased sensitivity to antimonite. In addition, overexpression of the ACR3 gene complements antimonite sensitivity of cells lacking the vacuolar ABC transporter Ycf1p. Moreover, both antimonite and arsenite induce transcription of the ACR3 gene resulting in the accumulation of Acr3 transporter at the plasma membrane. However, antimonite is much weaker inducer of the ACR3 gene transcription comparing to arsenite. Interestingly, the presence of metalloids does not influence either stability of Acr3 protein or its intracellular localization suggesting that Acr3p is mainly regulated at the transcriptional level. Finally, transport experiments confirmed that Acr3p indeed mediates efflux of antimonite and thus possesses a dual arsenite and antimonite specificity.

Biochemical characterization of a novel ArsA ATPase complex from Alkaliphilus metalliredigens QYMF

The two putative ars operons in Alkaliphilus metalliredigens QYMF are distinctive in that the arsA gene is split in halves, amarsA1 and amarsA2, and, acr3 but not an arsB gene coexists with arsA. Heterologous expression of one of the A. metalliredigensars operons (ars1) conferred arsenite but not antimonite resistance to DeltaarsEscherichia coli. Only the co-expressed AmArsA1 and AmArsA2 displayed arsenite or antimonite stimulated ATPase activity. The results show that AmArsA1-AmArsA2 interaction is needed to form the functional ArsA ATPase. This novel AmArsA1-AmArsA2 complex may provide insight in how it participates with Acr3 in arsenite detoxification.

Role of signature lysines in the deviant walker a motifs of the ArsA ATPase

The ArsA ATPase belongs to the P-loop GTPase subgroup within the GTPase superfamily of proteins. Members of this subgroup have a deviant Walker A motif which contains a signature lysine that is predicted to make intermonomer contact with the bound nucleotides and to play a role in ATP hydrolysis. ArsA has two signature lysines located at positions 16 and 335. The role of Lys16 in the A1 half and Lys335 in the A2 half was investigated by altering the lysines individually to alanine, arginine, leucine, methionine, glutamate, and glutamine by site-directed mutagenesis. While Lys16 mutants show similar resistance phenotypes as the wild type, the Lys335 mutants are sensitive to higher concentrations of arsenite. K16Q ArsA shows 70% of wild-type ATPase activity while K335Q ArsA is inactive. ArsA is activated by binding of Sb(III), and both wild-type and mutant ArsAs bind Sb(III) with a 1:1 stoichiometry. Although each ArsA binds nucleotide, the binding affinity decreases in the order wild type > K16Q > K335Q. The results of limited trypsin digestion analysis indicate that both wild type and K16Q adopt a similar conformation during activated catalysis, whereas K335Q adopts a conformation that is resistant to trypsin cleavage. These biochemical data along with structural modeling suggest that, although Lys16 is not critical for ATPase activity, Lys335 is involved in intersubunit interaction and activation of ATPase activity in both halves of the protein. Taken together, the results indicate that Lys16 and Lys335, located in the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that the nucleotide binding domains in these two halves are functionally nonequivalent.

Construction and purification of His-tagged staphylococcal ArsB protein, an integral membrane protein that is involved in arsenical salt resistance

Bacterial resistance to arsenical salts encoded on plasmid pI258 occurs by active extrusion of toxic oxyanions from cells of Staphylococcus aureus. The operon encodes for three gene products: ArsR, ArsB and ArsC. The gene product of arsB is an integral membrane protein and it is sufficient to provide resistance to arsenite and antimonite. A poly His-ArsB fusion protein was generated to purify the staphylococcal ArsB protein. Cells containing the His-tagged arsB gene were resistant to arsenite and antimonite. The levels of resistance to these toxic oxyanions by the His-tagged construct were greater than the levels obtained with the wild type gene. These data would indicate that the His-tagged protein is functionally active. A new 36 kDa protein band was visualized on 10% SDS-polyacrylamide gel electrophoresis (PAGE), which was confirmed as the His-ArsB protein by immunodetection with polyclonal Hisantibodies. The His-ArsB fusion protein was purified by the use of metal-chelate affinity chromatography with a Ni(+2)-nitrilotriacetic acid column and size-exclusion chromatography suggests that the protein was a homodimer.

ARS5 is a component of the 26S proteasome complex, and negatively regulates thiol biosynthesis and arsenic tolerance in Arabidopsis

A forward-genetic screen in Arabidopsis led to the isolation of several arsenic tolerance mutants. ars5 was the strongest arsenate- and arsenite-resistant mutant identified in this genetic screen. Here, we report the characterization and cloning of the ars5 mutant gene. ars5 is shown to exhibit an increased accumulation of arsenic and thiol compounds during arsenic stress. Rough mapping together with microarray-based expression mapping identified the ars5 mutation in the alpha subunit F (PAF1) of the 26S proteasome complex. Characterization of an independent paf1 T-DNA insertion allele and complementation by PAF1 confirmed that paf1 mutation is responsible for the enhanced thiol accumulation and arsenic tolerance phenotypes. Arsenic tolerance was not observed in a knock-out mutant of the highly homologous PAF2 gene. However, genetic complementation of ars5 by the overexpression of PAF2 suggests that the PAF2 protein is functionally equivalent to PAF1 when expressed at high levels. No detectible difference was observed in total ubiquitinylated protein profiles between ars5 and wild-type (WT) Arabidopsis, suggesting that the arsenic tolerance observed in ars5 is not derived from a general impairment in proteasome-mediated protein degradation. Quantitative RT-PCR showed that arsenic induces the enhanced transcriptional activation of several key genes that function in glutathione and phytochelatin biosynthesis in the WT, and this arsenic induction of gene expression is more dramatic in ars5. The enhanced transcriptional response to arsenic and the increased accumulation of thiol compounds in ars5, compared with WT, suggest the presence of a positive regulation pathway for thiol biosynthesis that is enhanced in the ars5 background.

Role of conserved aspartates in the ArsA ATPase

The ArsA ATPase is the catalytic subunit of the arsenite-translocating ArsAB pump that is responsible for resistance to arsenicals and antimonials in Escherichia coli. ATPase activity is activated by either arsenite or antimonite. ArsA is composed of two homologous halves A1 and A2, each containing a nucleotide binding domain, and a single metalloid binding or activation domain is located at the interface of the two halves of the protein. The metalloid binding domain is connected to the two nucleotide binding domains through two DTAPTGH sequences, one in A1 and the other in A2. The DTAPTGH sequences are proposed to be involved in information communication between the metal and catalytic sites. The roles of Asp142 in A1 D 142TAPTGH sequence, and Asp447 in A2 D 447TAPTGH sequence was investigated after altering the aspartates individually to alanine, asparagine, and glutamate by site-directed mutagenesis. Asp142 mutants were sensitive to As(III) to varying degrees, whereas the Asp447 mutants showed the same resistance phenotype as the wild type. Each altered protein exhibited varying levels of both basal and metalloid-stimulated activity, indicating that neither Asp142 nor Asp447 is essential for catalysis. Biochemical characterization of the altered proteins imply that Asp142 is involved in Mg (2+) binding and also plays a role in signal transduction between the catalytic and activation domains. In contrast, Asp447 is not nearly as critical for Mg (2+) binding as Asp142 but appears to be in communication between the metal and catalytic sites. Taken together, the results indicate that Asp142 and Asp447, located on the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that these two halves are functionally nonequivalent.

ASNA1, an ATPase targeting tail-anchored proteins, regulates melanoma cell growth and sensitivity to cisplatin and arsenite

PURPOSE

ASNA1 is homologous to E. coli ArsA, a well characterized ATPase involved in efflux of arsenite and antimonite. Cells resistant to arsenite and antimonite are cross-resistant to the chemotherapeutic drug cisplatin. ASNA1 is also an essential ATPase for the insertion of tail-anchored proteins into ER membranes and a novel regulator of insulin secretion. The aim of this study was to determine if altered ASNA1 levels influenced growth and sensitivity to arsenite and cisplatin in human melanoma cells.

METHODS

Cultured melanoma T289 cells were transfected with plasmids containing sense or antisense ASNA1. Cells were exposed to cisplatin, arsenite and zinc. Cell growth and chemosensitivity were evaluated by the MTT assay and apoptosis by a TUNEL assay.

RESULTS

ASNA1 expression was necessary for growth. T289 clones with decreased ASNA1 expression exhibited 51 +/- 5% longer doubling times than wildtype T289 (P = 0.0091). After exposure to cisplatin, ASNA1 downregulated cells displayed a significant increase in apoptosis. The cisplatin IC(50) in ASNA1 underexpressing cells was 41.7 +/- 1.8% compared to wildtype (P = 0.00097) and the arsenite IC(50) was 59.9 +/- 3.2% of wildtype IC(50) (P = 0.0067).

CONCLUSIONS

Reduced ASNA1 expression is associated with significant inhibition of cell growth, increased apoptosis and increased sensitivity to cisplatin and arsenite.

Responses to arsenate stress by Comamonas sp. strain CNB-1 at genetic and proteomic levels

Comamonas sp. strain CNB-1, a chloronitrobenzene-degrading bacterium, was demonstrated to possess higher arsenate tolerance as compared with the mutant strain CNB-2. pCNB1, a plasmid harboured by CNB-1 but not CNB-2, contained the genetic cluster ars(RPBC)Com, which putatively encodes arsenate-resistance regulator, family II arsenate reductase, arsenite efflux pump and family I arsenate reductase, respectively, in Comamonas strain CNB-1. The arsC-negative Escherichia coli could gain arsenate resistance by transformation with arsPCom or arsCCom, indicating that these two genes might express functional forms of arsenate reductases. Intriguingly, when CNB-1 cells were exposed to arsenate, the transcription of arsPCom and arsCCom was measurable by RT-PCR, but only ArsPCom was detectable at protein level. To explore the proteins responding to arsenate stress, CNB-1 cells were cultured with and without arsenate and differential proteomics was carried out by two-dimensional PAGE (2-DE) and MALDI-TOF MS. A total of 31 differential 2-DE spots were defined upon image analysis and 23 proteins were identified to be responsive specifically to arsenate. Of these spots, 18 were unique proteins. These proteins were identified to be phosphate transporters, heat-shock proteins involved in protein refolding, and enzymes participating in carbon and energy metabolism.

Characterization of the metalloactivation domain of an arsenite/antimonite resistance pump

The ArsAB extrusion pump encoded by the ars operon of Escherichia coli plasmid R773 confers resistance to the toxic trivalent metalloids arsenite [As(III)] and antimonite [Sb(III)]. The ArsA ATPase, the catalytic subunit of the pump, has two homologous halves, A1 and A2. At the interface of these two halves are two nucleotide-binding domains and a metalloid-binding domain. Cys-113 and Cys-422 have been shown to form a high-affinity metalloid binding site. The crystal structure of ArsA shows two other bound metalloid atoms, one liganded to Cys-172 and His-453, and the other liganded to His-148 and Ser-420. The contribution of those putative metalloid sites was examined. There was little effect of mutagenesis of residues His-148 and Ser-420 on metalloid binding. However, a C172A ArsA mutant and C172A/H453A double mutant exhibited significantly decreased affinity for Sb(III). These results suggest first that there is only a single high-affinity metalloid binding site in ArsA, and second that Cys-172 controls the affinity of this site for metalloid and hence the efficiency of metalloactivation of the ArsAB efflux pump.

Cys-113 and Cys-422 form a high affinity metalloid binding site in the ArsA ATPase

The arsRDABC operon of Escherichia coli plasmid R773 encodes the ArsAB extrusion pump for the trivalent metalloids As(III) and Sb(III). ArsA, the catalytic subunit has two homologous halves, A1 and A2. Each half has a consensus signal transduction domain that physically connects the nucleotide-binding domain to the metalloid-binding domain. The relation between metalloid binding by ArsA and transport through ArsB is unclear. In this study, direct metalloid binding to ArsA was examined. The results show that ArsA binds a single Sb(III) with high affinity only in the presence of Mg(2+)-nucleotide. Mutation of the codons for Cys-113 and Cys-422 eliminated Sb(III) binding to purified ArsA. C113A/C422A ArsA has basal ATPase activity similar to that of the wild type but lacks metalloid-stimulated activity. Accumulation of metalloid was assayed in intact cells, where reduced uptake results from active extrusion by the ArsAB pump. Cells expressing the arsA(C113A/C422A)B genes had an intermediate level of metalloid resistance and accumulation between those expressing only arsB alone and those expressing wild type arsAB genes. The results indicate that, whereas metalloid stimulation of ArsA activity enhances the ability of the pump to reduce the intracellular concentration of metalloid, high affinity binding of metalloid by ArsA is not obligatory for transport or resistance. Yet, in mixed populations of cells bearing either arsAB or arsA(C113A/C422A)B growing in subtoxic concentrations of arsenite, cells bearing wild type arsAB replaced cells with mutant arsA(C113A/C422A)B in less than 1 week, showing that the metalloid binding site confers an evolutionary advantage.

Nonequivalence of the nucleotide binding domains of the ArsA ATPase

The arsRDABC operon of Escherichia coli plasmid R773 encodes the ArsAB pump that catalyzes extrusion of the metalloids As(III) and Sb(III), conferring metalloid resistance. The catalytic subunit, ArsA, is an ATPase with two homologous halves, A1 and A2, connected by a short linker. Each half contains a nucleotide binding domain. The overall rate of ATP hydrolysis is slow in the absence of metalloid and is accelerated by metalloid binding. The results of photolabeling of ArsA with the ATP analogue 8-azidoadenosine 5'-[alpha-(32)P]-triphosphate at 4 degrees C indicate that metalloid stimulation correlates with a >10-fold increase in affinity for nucleotide. To investigate the relative contributions of the two nucleotide binding domains to catalysis, a thrombin site was introduced in the linker. This allowed discrimination between incorporation of labeled nucleotides into the two halves of ArsA. The results indicate that both the A1 and A2 nucleotide binding domains bind and hydrolyze trinucleotide, even in the absence of metalloid. Sb(III) increases the affinity of the A1 nucleotide binding domain to a greater extent than the A2 nucleotide binding domain. The ATP analogue labeled with (32)P at the gamma position was used to measure hydrolysis of trinucleotide at 37 degrees C. Under these catalytic conditions, both nucleotide binding domains hydrolyze ATP, but hydrolysis in A1 is stimulated to a greater degree by Sb(III) than A2. These results suggest that the two homologous halves of the ArsA may be functionally nonequivalent.

Pink-eyed dilution protein modulates arsenic sensitivity and intracellular glutathione metabolism

Mutations in the mouse p (pink-eyed dilution) and human P genes lead to melanosomal defects and ocular developmental abnormalities. Despite the critical role played by the p gene product in controlling tyrosinase processing and melanosome biogenesis, its precise biological function is still not defined. We have expressed p heterologously in the yeast Saccharomyces cerevisiae to study its function in greater detail. Immunofluorescence studies revealed that p reaches the yeast vacuolar membrane via the prevacuolar compartment. Yeast cells expressing p exhibited increased sensitivity to a number of toxic compounds, including arsenicals. Similarly, cultured murine melanocytes expressing a functional p gene were also found to be more sensitive to arsenical compounds compared with p-null cell lines. Intracellular glutathione, known to play a role in detoxification of arsenicals, was diminished by 50% in p-expressing yeast. By using the glutathione-conjugating dye monochlorobimane, in combination with acivicin, an inhibitor of vacuolar gamma-glutamyl cysteine transpeptidase, involved in the breakdown of glutathione, we found that p facilitates the vacuolar accumulation of glutathione. Our data demonstrate that the pink-eyed dilution protein increases cellular sensitivity to arsenicals and other metalloids and can modulate intracellular glutathione metabolism.

Conformational changes in four regions of the Escherichia coli ArsA ATPase link ATP hydrolysis to ion translocation

Structures of ArsA with ATP, AMP-PNP, or ADP.AlF(3) bound at the A2 nucleotide binding site were determined. Binding of different nucleotides modifies the coordination sphere of Mg(2+). In particular, the changes elicited by ADP.AlF(3) provide insights into the mechanism of ATP hydrolysis. In-line attack by water onto the gamma-phosphate of ATP would be followed first by formation of a trigonal intermediate and then by breaking of the scissile bond between the beta- and gamma-phosphates. Motions of amino acid side chains at the A2 nucleotide binding site during ATP binding and hydrolysis propagate at a distance, producing conformational changes in four different regions of the protein corresponding to helices H4-H5, helices H9-H10, helices H13-H15, and to the S1-H2-S2 region. These elements are extensions of, respectively, the Switch I and Switch II regions, the A-loop (a small loop near the nucleotide adenine moiety), and the P-loop. Based on the observed conformational changes, it is proposed that ArsA functions as a reciprocating engine that hydrolyzes 2 mol of ATP per each cycle of ion translocation across the membrane.

Confounding variables in the environmental toxicology of arsenic

Arsenic is one of the most important global environmental toxicants. For example, in regions of West Bengal and Inner Mongolia, more than 100000 persons are chronically exposed to well water often strongly contaminated with As. Unfortunately, a toxicologically safe risk assessment and standard setting, especially for long-term and low-dose exposures to arsenic, is not possible. One reason is that the key mechanism of arsenic's tumorigenicity still is not elucidated. Experimental data indicate that either DNA repair inhibition or DNA methylation status alteration may be causal explanations. Moreover, when comparing epidemiological data, it cannot be ruled out that the susceptibility to arsenic's carcinogenicity may be different between Mexican and Taiwanese people. Some other studies indicate that some Andean populations do not develop skin cancer after long-term exposure to As. It is not known yet how this resistance could be mediated. Finally, the situation is even more complicated when taking into consideration that there are several compounds suspected to modulate the chronic environmental toxicity of arsenic, variables that may either enhance or suppress the in vivo genotoxicity and carcinogenicity of the metalloid. Among them are nutritional factors like selenium and zinc as well as drinking water co-contaminants like antimony. Further, yet unidentified factors influencing the body burden and/or the excretion of arsenic are possibly prevailing: preliminary data from own human biomonitoring studies showed a peaking of As in urine samples of non-exposed people which was not caused by elevated exposure to As through seafood consumption. The relevance of these putative confounding variables cannot be finally evaluated yet. Further experimental as well as epidemiological studies are needed to answer these questions. This would help to conduct a toxicologically improved risk assessment, especially for low-dose and long-term exposures to arsenic.

Trimeric ring-like structure of ArsA ATPase

ArsA protein is the soluble subunit of the Ars anion pump in the Escherichia coli membrane which extrudes arsenite or antimonite from the cytoplasm. The molecular weight of the subunit is 63 kDa. In the cell it hydrolyzes ATP, and the energy released is used by the membrane-bound subunit ArsB to transport the substrates across the membrane. We have obtained two-dimensional crystals of ArsA in the presence of arsenite on negatively-charged lipid monolayer composed of DMPS and DOPC. These crystals have been studied using electron microscopy of negatively-stained specimens followed by image processing. The projection map obtained at 2.4 nm resolution reveals a ring-like structure with threefold symmetry. Many molecular assemblies with the same ring-shape and dimensions were also seen dispersed on electron microscopy grids, prepared directly from purified ArsA protein solution. Size-exclusion chromatography of the protein sample with arsenite present revealed that the majority of the protein particles in solution have a molecular weight of about 180 kDa. Based on these experiments, we conclude that in solution the ArsA ATPase with substrate bound is mainly in a trimeric form.

The linker peptide of the ArsA ATPase

Plasmid R773 encodes an As(III)/Sb(III)-translocating ATPase that confers resistance to those metalloids in Escherichia coli. The catalytic subunit of the pump, the ArsA ATPase, consists of homologous N- and C-terminal nucleotide-binding domains connected by a 25-residue linker. The role of this linker sequence was examined by deletion of five, 10, 15 or 23 residues or insertion of five glycine residues. Cells expressing arsA with the 5-residue insertion had wild-type arsenite resistance. Resistance of cells expressing modified arsA genes with deletions was dependent on the linker length. Cells with five or 10 deleted residues exhibited slightly reduced resistance. Deletion of 15 or 23 residues resulted in further decreases in resistance. Each altered ArsA was purified. The enzyme with the 5-residue insertion had the same affinity for ATP and Sb(III) as the wild-type enzyme. Enzymes with 5-, 10-, 15- or 23-residue deletions exhibited decreased affinity for both Sb(III) and ATP. The enzyme with a 23-residue deletion exhibited only basal ATPase activity and was unable to be allosterically activated by Sb(III). These results suggest that the linker has evolved to a length optimal for bringing the two halves of the protein into proper contact with each other, facilitating catalysis.

Mechanism of the ArsA ATPase

The ArsAB ATPase confers metalloid resistance in Escherichia coli by pumping toxic anions out of the cells. This transport ATPase shares structural and perhaps mechanism features with ABC transporters. The ArsAB pump is composed of a membrane subunit that has two groups of six transmembrane segments, and the catalytic subunit, the ArsA ATPase. As is the case with many ABC transporters, ArsA has an internal repeat, each with an ATP binding domain, and is allosterically activated by substrates of the pump. The mechanism of allosteric activation of the ArsA ATPase has been elucidated at the molecular level. Binding of the activator produces a conformational change that forms a tight interface of the nucleotide binding domains. In the rate-limiting step in the overall reaction, the enzyme undergoes a slow conformational change. The allosteric activator accelerates catalysis by increasing the velocity of this rate-limiting step. We postulate that similar conformational changes may be rate-limiting in the mechanism of ABC transporters.

The anion-stimulated ATPase ArsA shows unisite and multisite catalytic activity

ArsA, an anion-stimulated ATPase, consists of two nucleotide binding domains, A1 in the N terminus and A2 in the C terminus of the protein, connected by a linker. The A1 domain contains a high affinity ATP binding site, whereas the A2 domain has low affinity and it requires the allosteric ligand antimonite for binding ATP. ArsA is known to form a UV-activated adduct with [alpha-(32)P]ATP in the linker region. This study shows that on addition of antimonite, much more adduct is formed. Characterization of the nature of the adduct suggests that it is between ArsA and ADP, instead of ATP, indicating that the adduct formation reflects hydrolysis of ATP. The present study also demonstrates that the A1 domain is capable of carrying out unisite catalysis in the absence of antimonite. On addition of antimonite, multisite catalysis involving both A1 and A2 sites occurs, resulting in a 40-fold increase in ATPase activity. Studies with mutant proteins suggest that the A2 site may be second in the sequence of events, so that its role in catalysis is dependent on a functional A1 site. It is also proposed that ArsA goes through an ATP-bound and an ADP-bound conformation, and the linker region, where ADP binds under both unisite and multisite catalytic conditions, may play an important role in the energy transduction process.

The ATPase mechanism of ArsA, the catalytic subunit of the arsenite pump

The ArsA ATPase is the catalytic subunit of a novel arsenite pump, with two nucleotide-binding consensus sequences in the N- and C-terminal halves of the protein. The single tryptophan-containing Trp159 ArsA was used to elucidate the elementary steps of the ATPase mechanism by fluorescence stopped-flow experiments. The binding and hydrolysis of MgATP is a multistep process with a minimal kinetic mechanism (Mechanism 1). A notable feature of the reaction is that MgATP binding induces a slow transient increase in fluorescence of ArsA, which is independent of the ATP concentration, indicative of the build-up of a pre-steady state intermediate. This finding, coupled with a phosphate burst, implies that the steady-state intermediate builds up subsequent to product release. We propose that the rate-limiting step is an isomerization between different conformational forms of ArsA. kcat is faster than the phosphate burst, indicating that both nucleotide binding sites of ArsA are catalytic. Consistent with this interpretation, approximately 2 mol of phosphate are released per mole of ArsA during the phosphate burst.

A chromosomal ars operon homologue of Pseudomonas aeruginosa confers increased resistance to arsenic and antimony in Escherichia coli

Operons encoding homologous arsenic-resistance determinants (ars) have been discovered in bacterial plasmids from Gram-positive and Gram-negative organisms, as well as in the Escherichia coli chromosome. However, evidence for this arsenic-resistance determinant in the medically and environmentally important bacterial species Pseudomonas aeruginosa is conflicting. Here the identification of a P. aeruginosa chromosomal ars operon homologue via cloning and complementation of an E. coli ars mutant is reported. The P. aeruginosa chromosomal ars operon contains three potential ORFs encoding proteins with significant sequence similarity to those encoded by the arsR, arsB and arsC genes of the plasmid-based and E. coli chromosomal ars operons. The cloned P. aeruginosa chromosomal ars operon confers augmented resistance to arsenic and antimony oxyanions in an E. coli arsB mutant and in wild-type P. aeruginosa. Expression of the operon was induced by arsenite at the mRNA level. DNA sequences homologous with this operon were detected in some, but not all, species of the genus Pseudomonas, suggesting that its conservation follows their taxonomic-based evolution.

Steric limitations in the interaction of the ATP binding domains of the ArsA ATPase

ArsA, the catalytic subunit of an anion-translocating ATPase, has two consensus nucleotide binding sites, one N-terminal and one C-terminal. A mutation producing a G15C substitution in the N-terminal domain resulted in substantial reductions in arsenite resistance, transport, and ATPase activity. A second site revertant (A344V) adjacent to the C-terminal nucleotide binding site was previously shown to restore arsenite resistance, suggesting the interaction of the nucleotide binding sites in ArsA (Li, J., Liu, S., and Rosen, B. P. (1996) J. Biol. Chem. 271, 25247-25252). In this study, it is shown that alteration of Ala-344 to bulkier residues, including Cys, Thr, Pro, Asp, Leu, Phe, Tyr, or Arg, also suppressed the G15C substitution. However, A344G or A344S substitutions only marginally suppressed the primary mutation. Alteration of Gly-15 to Ala, Cys, Asp, Tyr, or Arg each resulted in decreased arsenite resistance. The larger the residue volume of the substitution, the lower the resistance, with a G15R substitution producing the least resistance. Resistance in a strain expressing an arsA gene encoding the G15R substitution could be rescued by A344S, A344T, A344D, A344R, or A344V second site suppressors. The larger the residue is then the greater the suppression is. The in vitro ArsA ATPase activities from purified wild type, G15A, G15C, and G15R exhibits an inverse relationship between activity and residue volume. Purified G15A and G15C exhibited both an increase in the Km for ATP and a decrease in Vmax. The results are consistent with a physical interaction of the two nucleotide binding domains and indicate that the geometry at the interface between the N- and C-terminal nucleotide binding sites places spatial constraints on allowable residues in that interface.

Mechanisms of absorption of As2O5 from rat small intestine: the effect of different parameters

The main purpose of this research was to study the effects of water movement on arsenic absorption. In order to appreciate and measure the interaction between these two variables we investigated the perfusion of isotonic and hypotonic buffers (containing 3.2 microgram/ml As2 O5 into rat small intestines. The As (V) depended on osmolarity sensitive. This implied the presence of a saturable uptake kinetic mechanism and suggested the participation of some kind of carrier-mediated transport system. An increase in arsenate absorption and its accumulation in organs was detected in the presence of an Na+ gradient. The same technique, combining isotonic buffers with different pH values and buffers containing valinomycin, was used in order to study the effects of intraluminal pH on membrane potential. A significant decrease in arsenic levels and As(V) absorption in organs was detected as a result of an increase in the pH. This influence of pH on the As(V) absorption mechanism indicated dependence on a proton (H+) gradient. An inside negative potassium potential induced by valinomycin increased significantly the uptake of arsenate, suggesting an electrogenic transport of arsenate. In conclusion, the As(V) might be absorbed from the small intestine through a carrier-mediated mechanism, which would depend on the Na+(-) and H+(-)gradients and the membrane potential differential from both sides of the intestinal epithelial cells.

Arsenic and antimony: comparative approach on mechanistic toxicology

A chemico-toxicological similarity between arsenic and antimony exists and their toxicology is often seen. Indeed, both elements possess several common properties, e.g. they are clastogenic but not mutagenic in the trivalent state and they have a carcinogenic potential: trivalent arsenicals are known to be human carcinogens and antimony(III) oxide (by inhalation) has been shown to cause lung cancer in female rats. For years, arsenic has been known to be environmentally toxic. Elevated human exposure to this element, mostly caused by the intake of contaminated tap water, is associated with increased incidences of cancer at various sites. It is still not clear how arsenic compounds exert their genotoxic effect. It may be connected with an inhibition of DNA repair or the induction of oxidative stress. Little work has been done on the toxicology of antimony as it is less widely present in the environment. There is evidence that in mammals antimony, unlike arsenic, is not detoxified via methylation but it still remains unclear what mechanism is responsible for antimony's genotoxicity. In general, there is little information known about this element to accurately determine its impact on human health. Thus, the aim of this paper is to review current knowledge for future risk assessment and further scientific work.

The Saccharomyces cerevisiae ACR3 gene encodes a putative membrane protein involved in arsenite transport

The cluster of three genes, ACR1, ACR2, and ACR3, previously was shown to confer arsenical resistance in Saccharomyces cerevisiae. The overexpression of ACR3 induced high level arsenite resistance. The presence of ACR3 together with ACR2 on a multicopy plasmid was conducive to increased arsenate resistance. The function of ACR3 gene has now been investigated. Amino acid sequence analysis of Acr3p showed that this hypothetical protein has hydrophobic character with 10 putative transmembrane spans and is probably located in yeast plasma membrane. We constructed the acr3 null mutation. The resulting disruptants were 5-fold more sensitive to arsenate and arsenite than wild-type cells. The acr3 disruptants showed wild-type sensitivity to antimony, tellurite, cadmium, and phenylarsine oxide. The mechanism of arsenical resistance was assayed by transport experiments using radioactive arsenite. We did not observe any significant differences in the accumulation of 76AsO33- in wild-type cells, acr1 and acr3 disruptants. However, the high dosage of ACR3 gene resulted in loss of arsenite uptake. These results suggest that arsenite resistance in yeast is mediated by an arsenite transporter (Acr3p).

Tryptophan fluorescence reports nucleotide-induced conformational changes in a domain of the ArsA ATPase

The ars operon of plasmid R773 encodes an ATP-dependent extrusion pump for arsenite and antimonite in Escherichia coli. The ArsA ATPase is the catalytic subunit of the pump protein, with two nucleotide binding consensus sequences, one in the NH2-terminal half and one in the COOH-terminal half of the protein. A 12-residue consensus sequence (DTAPTGHTIRLL) has been identified in ArsA homologs from eubacteria, archebacteria, fungi, plants, and animals. ArsA enzymes were constructed containing single tryptophan residues at either end of this conserved sequence. The emission spectrum of the fluorescence of the tryptophan on the COOH-terminal end (Trp-159) indicated a relatively hydrophilic environment for this residue. An increase in intrinsic tryptophan fluorescence and a blue shift of the maximum emission wavelength were observed upon addition of MgATP, indicating movement of Trp-159 into a relatively less polar environment. No fluorescence response was observed with MgADP, with nonhydrolyzable ATP analogs, or with MgATP by catalytically inactive enyzmes. This suggests that the location Trp-159 is shifted only during hydrolysis of ATP. In contrast, the emission spectrum of Trp-141, located on the NH2-terminal side of the consensus sequence, indicated a relatively nonpolar environment. The maximum emission wavelength red shifted upon addition of MgADP. MgATP slowly produced a response that correlated with product formation, suggesting that the environment of Trp-141 is sensitive only to MgADP binding. Thus, during ATP hydrolysis the COOH-terminal end of the conserved domain moves into a less polar environment, whereas the NH2-terminal end moves into a more hydrophilic environment as product is formed. A hypothesis is presented in which the conserved domain of ArsA and homologs is an energy transduction domain involved in transmission of the energy of ATP hydrolysis to biological functions such as transport.

Interaction of ATP binding sites in the ArsA ATPase, the catalytic subunit of the Ars pump

The ArsA ATPase is the catalytic subunit of the Ars pump that catalyzes arsenical extrusion in Escherichia coli, thus providing resistance. The active form of ArsA is a homodimer with four nucleotide binding sites, two from each monomer. The codons for Gly-15 in the N-terminal consensus nucleotide binding sequence and Gly-334 in the C-terminal sequence were individually mutated to cysteine codons. Cells expressing an arsAG334C mutation retained arsenite resistance, while an arsAG15C mutation resulted in substantial reductions in arsenite resistance, transport, and ATPase activity. Selection for suppression of the G15C mutation that restored arsenite resistance yielded an A344V substitution. Ala-344 is located adjacent to the C-terminal nucleotide binding sequence. The second site mutation did not suppress the loss of resistance resulting from G18D, G20S, or T22I substitutions in the N-terminal nucleotide binding site. Cells expressing the G15C/A344V double mutant regained arsenite extrusion. These results suggest a spatial proximity of Gly-15 and Ala-344 and support a model for interaction of the nucleotide binding sites in ArsA.

Spatial proximity of Cys113, Cys172, and Cys422 in the metalloactivation domain of the ArsA ATPase

ArsA ATPase activity is allosterically activated by salts of the semimetal arsenic or antimony. Activation is associated with the presence of three cysteine residues in ArsA: Cys113, Cys172, and Cys422. To determine the distance between cysteine residues, wild type ArsA and ArsA proteins with cysteine to serine substitutions were treated with the bifunctional alkylating agent dibromobimane, which reacts with thiol pairs within 3-6 A of each other to form a fluorescent adduct. ArsA proteins in which single cysteine residues were altered by site-directed mutagenesis still formed fluorescent adducts. Proteins in which two of the three cysteine residues were substituted did not form fluorescent adducts. These results demonstrate that Cys113, Cys172, and Cys422 are in close proximity of each other. We propose a model in which As(III) or Sb(III) interacts with these three cysteines in a trigonal pyramidal geometry, forming a novel soft metal-thiol cage.

Expression of the Escherichia coli chromosomal ars operon

A chromosomally located operon (ars) of Escherichia coli has been previously shown to be functional in arsenic detoxification. DNA sequencing revealed three open reading frames homologous to the arsR, arsB, and arsC open reading frames of plasmid-based arsenic resistance operons isolated from both E. coli and staphylococcal species. To examine the outline of transcriptional regulation of the chromosomal ars operon, several transcriptional fusions, using the luciferase-encoding luxAB genes of Vibrio harveyi, were constructed. Measurement of the expression of these gene fusions demonstrated that the operon was rapidly induced by sodium arsenite and negatively regulated by the trans-acting arsR gene product. Northern blotting and primer extension analyses revealed that the chromosomal ars operon is most likely transcribed as a single mRNA of approximately 2100 nucleotides in length and processed into two smaller mRNA products in a manner similar to that found in the E. coli R773 plasmid-borne ars operon. However, transcription was found to initiate at a position that is relatively further upstream of the initiation codon of the arsR coding sequence than that determined for the E. coli R773 plasmid's ars operon.

The arsenical resistance operon of IncN plasmid R46

The arsenical resistance operon of the IncN plasmid R46 consists of 4696 bp and starts with predicted transcriptional control and initiation signals, followed by five genes, arsD, arsA, and arsC. The corresponding Escherichia coli chromosomal ars operon and two staphylococcal ars operons lack arsA and arsD genes. The R46 system contains only the second known versions of arsA and arsD, after those of plasmid R773. Western blot analysis identified the R46 proteins using antibodies against R773 ArsA, ArsD and ArsR.

The proteins encoded by the rbs operon of Escherichia coli: I. Overproduction, purification, characterization, and functional analysis of RbsA

The nucleotide-binding component of the high-affinity ribose transport system of Escherichia coli, RbsA, was overproduced from a T7-7 expression vector, and the protein was purified. Biochemical analyses of the purified protein indicated that the ATP analogues, 5'-FSBA and 8-azido ATP, covalently labeled the protein, a reaction that was inhibited by ATP, but not by GTP or CTP. The pure protein exhibited low-level ATPase activity with a K(m) of about 140 microM. Analyses of bacterial strains carrying chromosomal deletions of rbsA and other rbs genes suggested that RbsA is important for the chemotaxis function, a surprising result that was not anticipated from previous studies. However, an inconsistency between the several results from deletion strains raises questions regarding the interpretations of the in vivo data.

Cloning and expression of the unique Ca2+-ATPase from Flavobacterium odoratum

The 60-kDa Ca2+-ATPase from Flavobacterium odoratum is kinetically and mechanistically similar to other P-type ATPases, suggesting its use as a model system for structure-function studies of ion transport. A portion of the gene was amplified by polymerase chain reaction of genomic DNA with degenerate oligonucleotide primers, one based on the N-terminal amino acid sequence of the purified protein and the other based on a consensus sequence for the phosphorylation site of P-type ATPases. This gene fragment was used to screen a lambda library of F. odoratum 29979 DNA. Clone "C" is 3.3 kilobases in length and contains one complete and part of a second open reading frame, the first of which encodes a 58-kDa protein containing the exact N-terminal amino acid sequence of the purified protein. We have named this gene cda, for calcium-dependent ATPase. Escherichia coli, transformed with clone C, demonstrates high levels of calcium-dependent and vanadate-sensitive ATP hydrolysis activity, forms a 60-kDa phosphointermediate, and cross-reacts with antibodies to the purified Ca2+-ATPase. The gene has almost no sequence homology to even the highly conserved regions characteristic of P-type ATPases but does possess significant homology to a protein with alkaline phosphatase activity (PhoD) from Zymomonas mobilis. The putative phosphorylation site is a Walker A (P-loop) ATP binding sequence and is modified relative to P-type ATPases, suggesting that the F. odoratum Ca2+-ATPase may represent an ancestral link between the F- and the P-type ATPases or perhaps a new class of ATPases.

Role of cysteinyl residues in metalloactivation of the oxyanion-translocating ArsA ATPase

The ArsA protein, the catalytic subunit of the oxyanion-translocating ATPase responsible for resistance to arsenicals and antimonials in Escherichia coli, is activated by arsenite or antimonite. Activation is associated with dimerization of the ArsA protein. Enzymatic activity was rapidly but reversibly inhibited by the sulfhydryl reagent methyl methanethiosulfonate, suggesting that at least one cysteinyl residue is required for catalytic activity. Each of the four cysteinyl residues in the ArsA protein, Cys26, Cys113, Cys172, and Cys422, were individually changed to seryl residues. The C26S protein had normal properties. Cells expressing the other three mutations lost resistance to arsenite and antimonite. The C113S, C172S, and C422S enzymes each had relatively normal Km values for ATP but reduced affinity for antimonite and arsenite. The Vmax of the activated enzymes ranged from very low for the C113S and C422S enzymes to near normal for the C172S enzyme. These results suggest a mechanism of activation by formation of a tricoordinate complex between Sb(III) or As(III) and the cysteine thiolates 113, 172, and 422.