Arsenic Methyltransferase is Involved in Arsenosugar Biosynthesis by Providing DMA

Selective Methylation by an ArsM S-Adenosylmethionine Methyltransferase from Burkholderia gladioli GSRB05 Enhances Antibiotic Production

Arsenic methylation contributes to the formation and diversity of environmental organoarsenicals, an important process in the arsenic biogeochemical cycle. The arsM gene encoding an arsenite (As(III)) S-adenosylmethionine (SAM) methyltransferase is widely distributed in members of every kingdom. A number of ArsM enzymes have been shown to have different patterns of methylation. When incubated with inorganic As(III), Burkholderia gladioli GSRB05 has been shown to synthesize the organoarsenical antibiotic arsinothricin (AST) but does not produce either methylarsenate (MAs(V)) or dimethylarsenate (DMAs(V)). Here, we show that cells of B. gladioli GSRB05 synthesize DMAs(V) when cultured with either MAs(III) or MAs(V). Heterologous expression of the BgarsM gene in Escherichia coli conferred resistance to MAs(III) but not As(III). The cells methylate MAs(III) and the AST precursor, reduced trivalent hydroxyarsinothricin (R-AST-OH) but do not methylate inorganic As(III). Similar results were obtained with purified BgArsM. Compared with ArsM orthologs, BgArsM has an additional 37 amino acid residues in a linker region between domains. Deletion of the additional 37 residues restored As(III) methylation activity. Cells of E. coli co-expressing the BgarsL gene encoding the noncanonical radical SAM enzyme that catalyzes the synthesis of R-AST-OH together with the BgarsM gene produce much more of the antibiotic AST compared with E. coli cells co-expressing BgarsL together with the CrarsM gene from Chlamydomonas reinhardtii, which lacks the sequence for additional 37 residues. We propose that the presence of the insertion reduces the fitness of B. gladioli because it cannot detoxify inorganic arsenic but concomitantly confers an evolutionary advantage by increasing the ability to produce AST.

The Need to Unravel Arsenolipid Transformations in Humans

The main source of arsenic exposure to humans worldwide is the diet, in particular, drinking water, rice, and seafood. Although arsenic is often considered toxic, it can exist in food as more than 300 chemical species with different toxicities. This diversity makes it difficult for food safety and health authorities to regulate arsenic levels in food, which are currently based on a few arsenic species. Of particular interest are arsenolipids, a type of arsenic species widely found in seafood. Emerging evidence indicates that there are risks associated with human exposure to arsenolipids (e.g., accumulation in breast milk, ability to cross the blood-brain barrier and accumulate in the brain, and potential development of neurodegenerative disorders). Still, more research is needed to fully understand the impact of arsenolipid exposure, which requires establishing interdisciplinary collaborations.

Impact of rhizosphere microorganisms on arsenic (As) transformation and accumulation in a traditional Chinese medical plant

Panax notoginseng is an important traditional medicinal plant, but the commercial value is threatened by root-rot disease caused by rhizosphere microbes and a potential health risk caused by plant arsenic (As) accumulation. Whether rhizospheric microbes isolated from P. notoginseng rhizosphere soil could impact As uptake and transport into P. notoginseng is not yet known. Among the three root-rot disease-causing pathogens Fusarium flocciferum (PG 1), Fusarium oxysporum (PG 2), and Fusarium solani (PG 3) and one root-rot disease biocontrol fungus Trichoderma koningiopsis (FC 1) and five biocontrol-exerting bacterial species Bacillus siamensis (BC 1), Delftia acidovorans (BC 2), Brevibacillus formosus (BC 3), Mortierella alpine (BC 4), and Bacillus subtilis (BC 5), one As-resistant pathogen and four biocontrol microorganisms with As-resistant ability were identified. The As-transforming ability of the identified fungi and bacteria was ranked in the order of FC 1 > PG 1 and BC 2 > BC 3 > BC 1, respectively. Then, the As-resistant biocontrol and pathogenic microbes were initiated to colonize the rhizosphere of 1-year-old P. notoginseng seedlings growing in artificially As(V)-contaminated soil to evaluate the impact of microbe inoculation on P. notoginseng As uptake and transport capacity. Concentration of As in P. notoginseng tissues decreased in the order of the sequence stem > root > leaf. Compared to treatment without colonization by microorganism, inoculation with microorganisms increased As root uptake efficiency and root As concentration, especially under treatment of inoculation by BC 2 and PG 1 + BC 2. As transport efficiency from root to stem decreased by inoculation with microorganism, especially under treatment with inoculation of BC 2 and PG 1 + BC 2. However, the impact of microorganism colonization on As stem to leaf transport efficiency was not obvious. In summary, inoculation with rhizosphere microbes may increase As accumulation in P. notoginseng root, especially when using bacteria with high As transformation ability. Therefore, it is necessary to evaluate the As transformation capacity before applying biological control microorganism to the rhizosphere of P. notoginseng.

Arsenic: Various species with different effects on cytochrome P450 regulation in humans

Arsenic is well-recognized as one of the most hazardous elements which is characterized by its omnipresence throughout the environment in various chemical forms. From the simple inorganic arsenite (iAsIII) and arsenate (iAsV) molecules, a multitude of more complex organic species are biologically produced through a process of metabolic transformation with biomethylation being the core of this process. Because of their differential toxicity, speciation of arsenic-based compounds is necessary for assessing health risks posed by exposure to individual species or co-exposure to several species. In this regard, exposure assessment is another pivotal factor that includes identification of the potential sources as well as routes of exposure. Identification of arsenic impact on different physiological organ systems, through understanding its behavior in the human body that leads to homeostatic derangements, is the key for developing strategies to mitigate its toxicity. Metabolic machinery is one of the sophisticated body systems targeted by arsenic. The prominent role of cytochrome P450 enzymes (CYPs) in the metabolism of both endobiotics and xenobiotics necessitates paying a great deal of attention to the possible effects of arsenic compounds on this superfamily of enzymes. Here we highlight the toxicologically relevant arsenic species with a detailed description of the different environmental sources as well as the possible routes of human exposure to these species. We also summarize the reported findings of experimental investigations evaluating the influence of various arsenicals on different members of CYP superfamily using human-based models.

Factors and pathways regulating the release and transformation of arsenic mediated by reduction processes of dissimilated iron and sulfate

The driving process and explanatory factors regulating the transformation and migration of arsenic (As) mediated by dissimilatory iron reducing bacteria (DFeRB) and sulfate reducing bacteria (SRB) remain poorly understood. The novelty of this study is to explore the driving process and key environmental factors governing As mobilization mediated by DFeRB and SRB based on continuous As speciation and environmental parameter monitoring in a sediment-water system. The results illustrate the reduction process mediated by DFeRB and SRB significantly promotes the reduction of As(V) and the endogenous release of As. However, in the DFeRB and SRB mediated reductions, the main driving process and key explanatory factors that dominate the As mobility are significantly different. DFeRB has significant effects on the reductive dissolution and re-distribution of Fe(III) oxyhydroxides and As-containing Fe(III) minerals and on adsorption-desorption, which in turn influenced the transformation of iron species and the release and ecotoxicity of As. Meanwhile, the environmental factors that affect As mobility depend on Fe²⁺ and Fe³⁺ in DFeRB-induced reduction, presenting two main pathways: the process of As mobilization mediated by DFeRB, and the process influenced by the inorganic phosphorus involved in the competitive adsorption and anion exchange. Significantly different from DFeRB, the effects of SRB on As behavior mainly occur by influencing the adsorbed As, pyrite, and As sulfides in the sediments and through the formation of sulfides during the sulfate reduction. The main pathways of As mobilization reflect the direct effects of SRB, S^2-, and Fe²⁺. In addition, the role of NH₄⁺-N in the driving process of As mobility is more pronounced in SRB-induced reduction. NO₃^--N is an essential factor affecting As mobility, but the effects of NO₃^--N on As lead to non-significant pathways. This work provides insights into the environmental effects of DFeRB and SRB on the biogeochemical cycle of As.

Characterization and Mechanistic Study of the Radical SAM Enzyme ArsS Involved in Arsenosugar Biosynthesis

Arsenosugars are a group of arsenic-containing ribosides that are found predominantly in marine algae but also in terrestrial organisms. It has been proposed that arsenosugar biosynthesis involves a key intermediate 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), but how DDMAA is produced remains elusive. Now, we report characterization of ArsS as a DDMAA synthase, which catalyzes a radical S-adenosylmethionine (SAM)-mediated alkylation (adenosylation) of dimethylarsenite (DMAs^III ) to produce DDMAA. This radical-mediated reaction is redox neutral, and multiple turnover can be achieved without external reductant. Phylogenomic and biochemical analyses revealed that DDMAA synthases are widespread in distinct bacterial phyla with similar catalytic efficiencies; these enzymes likely originated from cyanobacteria. This study reveals a key step in arsenosugar biosynthesis and also a new paradigm in radical SAM chemistry, highlighting the catalytic diversity of this superfamily of enzymes.

Transformation pathways of arsenic species: SRB mediated mechanism and seasonal patterns

Sulfate reducing bacteria (SRB) mediated reduction plays a key role in the biological cycling of As, which inherently associates with the transformation of As species. However, the potential pathways of As species transformation, the predominant driving process and their explanatory factors regulating seasonal As mobility mediated by SRB remains poorly understood. This study explored the possible pathways of seasonal As species transformation mediated by SRB, and identified the predominant driving process and key environmental factors in response to As mobilization in different seasons. SRB-mediated reduction governed the seasonal mobilization of As, significantly promoted reduction of As (V) and endogenous release of As, and exhibited strong seasonal variability. The flux of As(III) and TAs in group SRB in summer were 1.92-3.53 times higher than those during the ice-bound period. The results showed two distinct stages namely release and re-immobilization both in summer and ice-bound period. While As was easier to be gradually transformed into a more stable state in SRB reduction process during ice-bound period. Both in summer and ice-bound period, SRB presented significant regulating effects on As behavior by influencing loosely adsorbed As, pyrite and As sulfides in sediments as well as the formation of sulfide during the process of SRB reduction. The main effecting pathways on As mobilization were the direct effects of SRB, S^2- and Fe²⁺ in summer, but IP was also an important pathway affecting As mobility during ice-bound period. This work provides new insights into mechanisms responsible for seasonal As mobilization.

The response of Pyropia haitanensis to inorganic arsenic under laboratory culture

Up to now, complicated organoarsenicals were mainly identified in marine organisms, suggesting that these organisms play a critical role in arsenic biogeochemical cycling because of low phosphate and relatively high arsenic concentration in the marine environment. However, the response of marine macroalgae to inorganic arsenic remains unknown. In this study, Pyropia haitanensis were exposed to arsenate [As(V)] (0.1, 1, 10, 100 μM) or arsenite [As(III)] (0.1, 1, 10 μM) under laboratory conditions for 3 d. The species of water-soluble arsenic, the total concentration of lipid-soluble and cell residue arsenic of the algae cells was analyzed. As(V) was mainly transformed into oxo-arsenosugar-phosphate, with other arsenic compounds such as monomethylated, As(III), demethylated arsenic and oxo-arsenosugar-glycerol being likely the intermediates of arsenosugar synthesis. When high concentration of As(III) was toxic to P. haitanensis, As(III) entered into the cells and was transformed into less toxic organoarsenicals and As(V). Transcriptome results showed genes involved in DNA replication, mismatch repair, base excision repair, and nucleotide excision repair were up-regulated in the algae cells exposed to 10 μM As(V), and multiple genes involved in glutathione metabolism and photosynthetic were up-regulated by 1 μM As(III). A large number of ABC transporters were down-regulated by As(V) while ten genes related to ABC transporters were up-regulated by As(III), indicating that ABC transporters were involved in transporting As(III) to vacuoles in algae cells. These results indicated that P. haitanensis detoxifies inorganic arsenic via transforming them into organoarsenicals and enhancing the isolation of highly toxic As(III) in vacuoles.

Bioaccumulation and biotransformation of arsenic in Leptolyngbya boryana

Leptolyngbya boryana (L. boryana) is a typical filamentous cyanobacterium that is widely distributed in aquatic ecosystems and is considered to play an important role in the arsenic biogeochemical cycle. Our results showed that L. boryana resisted arsenite (As(III)) and arsenate (As(V)) concentrations up to 0.25 mM and 5 mM, respectively. When exposed to 100 μM As(III) or As(V) for 4 weeks, L. boryana accumulated as much arsenic as 570.0 mg kg^-1 and 268.5 mg kg^-1, respectively. After the 4-week exposure to As(III) and As(V), organoarsenicals including dimethylarsenate (DMAs(V)) and oxo-arsenosugar-phosphate (Oxo-PO₄) were detected in the cells of L. boryana, while inorganic arsenic, especially As(V), was still the main species in both the cells and medium. Furthermore, arsenic oxidation was observed to be solely caused by L. boryana cells and was considered the dominant detoxification pathway. In conclusion, due to its powerful arsenic accumulation, biotransformation, and detoxification abilities, L. boryana might play an important role in arsenic remediation in aquatic environments.

Arsenic transformation mediated by gut microbiota affects the fecundity of Caenorhabditis elegans

Arsenic biotransformation has been discovered in guts of soil invertebrates. Reproduction of invertebrates is sensitive to arsenic contamination in soils. However, little is known about the impact of gut microbe-mediated arsenic biotransformation on the fecundity of invertebrates. Here, Caenorhabditis elegans was firstly pre-fed with Escherichia coli BL21 possessing the capability of reducing arsenate [As(V)] or BL21M having the ability to reduce As(V) and methylate arsenite [As(III)], then inoculated worms were transferred to inactive E. coli AW3110 (harboring no arsenic transformation gene)-seeded plates treated with As(V) at different concentrations. Quantification of gut microbes showed that both E. coli BL21 and BL21M stably colonized in the guts after worms were cultured on inactive E. coli AW3110-seeded plates for 72 h. The analysis of arsenic species indicated that there was As(III) in C. elegans guts colonized with E. coli BL21, As(III) and dimethylarsinic acid [DMAs(V)] in C. elegans guts with E. coli BL21M exposed to As(V) for 6 h. After treatment of 100 μM As(V), decrease in brood sizes was observed for worms that were colonized with E. coli BL21 or BL21M compared to that with AW3110 in the guts. The levels of vitellogenin (VTG), glutathione S-transferases (GST) and superoxide dismutase (SOD), closely linked to reproduction and antioxidation-linked indicators, were the highest in worms whose guts colonized with E. coli BL21, followed by worms colonized with E. coli BL21M and worms colonized with inactive E. coli AW3110 exposed to As(V). Our results indicated the toxic impact of As(III) and DMAs(V) produced by gut microbes on reproduction of C. elegans. The work provides novel insight into the interplay between arsenic biotransformation mediated by gut microbes and the host fecundity in soils.

Arsenic and the gastrointestinal tract microbiome

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

Arsenic Toxicity: Molecular Targets and Therapeutic Agents

High arsenic (As) levels in food and drinking water, or under some occupational conditions, can precipitate chronic toxicity and in some cases cancer. Millions of people are exposed to unacceptable amounts of As through drinking water and food. Highly exposed individuals may develop acute, subacute, or chronic signs of poisoning, characterized by skin lesions, cardiovascular symptoms, and in some cases, multi-organ failure. Inorganic arsenite(III) and organic arsenicals with the general formula R-As²⁺ are bound tightly to thiol groups, particularly to vicinal dithiols such as dihydrolipoic acid (DHLA), which together with some seleno-enzymes constitute vulnerable targets for the toxic action of As. In addition, R-As²⁺-compounds have even higher affinity to selenol groups, e.g., in thioredoxin reductase that also possesses a thiol group vicinal to the selenol. Inhibition of this and other ROS scavenging seleno-enzymes explain the oxidative stress associated with arsenic poisoning. The development of chelating agents, such as the dithiols BAL (dimercaptopropanol), DMPS (dimercapto-propanesulfonate) and DMSA (dimercaptosuccinic acid), took advantage of the fact that As had high affinity towards vicinal dithiols. Primary prevention by reducing exposure of the millions of people exposed to unacceptable As levels should be the prioritized strategy. However, in acute and subacute and even some cases with chronic As poisonings chelation treatment with therapeutic dithiols, in particular DMPS appears promising as regards alleviation of symptoms. In acute cases, initial treatment with BAL combined with DMPS should be considered.

Unique diversity and functions of the arsenic-methylating microorganisms from the tailings of Shimen Realgar Mine

Microbial arsenic (As) methylation plays important roles in the As biogeochemical cycle. However, little is known about the diversity and functions of As-methylating microorganisms from the tailings of a Realgar Mine, which is characterized as containing extremely high concentrations of As. To address this issue, we collected five samples (T1-T5) from the tailings of Shimen Realgar Mine. Microcosm assays without addition of exogenous As and carbon indicated that all the five samples possess significant As-methylating activities, producing 0.8-5.7 μg/L DMAs^V, and 1.1-10.7 μg/L MMAs^V with an exception of T3, from which MMAs^V was not detectable after 14.0 days of incubation. In comparison, addition of 20.0 mM lactate to the microcosms significantly enhanced the activities of these samples; the produced DMAs^V and MMAs^V are 8.0-39.7 μg/L and 5.8-38.3 μg/L, respectively. The biogenic DMAs^V shows significant positive correlations with the Fe concentrations and negative correlations with the total nitrogen concentrations in the environment. A total of 63 different arsM genes were identified from the five samples, which code for new or new-type ArsM proteins, suggesting that a unique diversity of As-methylating microbes are present in the environment. The microbial community structures of the samples were significantly shaped by the environmental total organic carbon, total As contents and NO₃^- contents. These data help to better understand the microorganisms-catalyzed As methylation occurred in the environment with extremely high contents of As.

Effects of Arsenic on Gut Microbiota and Its Biotransformation Genes in Earthworm Metaphire sieboldi

Arsenic biotransformation mediated by gut microbiota can affect arsenic bioavailability and microbial community. Arsenic species, arsenic biotransformation genes (ABGs), and the composition of gut microbial community were characterized after the earthworm Metaphire sieboldi was cultured in soils spiked with different arsenic concentrations. Arsenite (As(III)) was the major component in the earthworm gut, whereas arsenate (As(V)) was predominant in the soil. A total of 16 ABGs were quantified by high-throughput quantitative polymerase chain reaction (HT-qPCR). Genes involved in arsenic redox and efflux were predominant in all samples, and the abundance of ABGs involved in arsenic methylation and demethylation in the gut was very low. These results reveal that the earthworm gut can be a reservoir of microbes with the capability of reducing As(V) and extruding As(III) but with little methylation of arsenic. Moreover, gut microbial communities were dominated by Actinobacteria, Firmicutes, and Proteobacteria at the phylum level and were considerably different from those in the surrounding soil. Our work demonstrates that exposure to As(V) disturbs the gut microbiota of earthworms and provides some insights into arsenic biotransformation in the earthworm gut.

Identification of Steps in the Pathway of Arsenosugar Biosynthesis

Arsenosugars are arsenic-containing ribosides that play a substantial role in arsenic biogeochemical cycles. Arsenosugars were identified more than 30 years ago, and yet their mechanism of biosynthesis remains unknown. In this study we report identification of the arsS gene from the cyanobacterium Synechocystis sp. PCC 6803 and show that it is involved in arsenosugar biosynthesis. In the Synechocystis sp. PCC 6803 ars operon, arsS is adjacent to the arsM gene that encodes an As(III) S-adenosylmethionine (SAM) methyltransferase. The gene product, ArsS, contains a characteristic CX3CX2C motif which is typical for the radical SAM superfamily. The function of ArsS was identified from a combination of arsS disruption in Synechocystis sp. PCC 6803 and heterologous expression of arsM and arsS in Escherichia coli. Both genes are necessary, indicating a multistep pathway of arsenosugar biosynthesis. In addition, we demonstrate that ArsS orthologs from three other freshwater cyanobacteria and one picocyanobacterium are involved in arsenosugar biosynthesis in those microbes. This study represents the identification of the first two steps in the pathway of arsenosugar biosynthesis. Our discovery expands the catalytic repertoire of the diverse radical SAM enzyme superfamily and provides a basis for studying the biogeochemistry of complex organoarsenicals.

Two facets of world arsenic problem solution: crop poisoning restriction and enforcement of phytoremediation

This review provides insights into As toxicity in plants with focus on photosynthesis and sugar metabolism as important arsenic targets and simultaneously defence tools against accompanying oxidative stress. Heavy metal contamination is a great problem all over the world. Arsenic, a metalloid occurring naturally in the Earth's crust, also massively spreads out in the environment by human activities. Its accumulation in crops poses a severe health risk to humans and animals. Besides the restriction of human-caused contamination, there are two basic ways how to cope with the problem: first, to limit arsenic accumulation in harvestable parts of the crops; second, to make use of some arsenic hyperaccumulating plants for phytoremediation of contaminated soils and waters. Progress in the use of both strategies depends strongly on the level of our knowledge on the physiological and morphological processes resulting from arsenic exposure. Arsenic uptake is mediated preferentially by P and Si transporters and its accumulation substantially impairs plant metabolism at numerous levels including damages through oxidative stress. Rice is a predominantly studied crop where substantial progress has been made in understanding of the mechanisms of arsenic uptake, distribution, and detoxification, though many questions still remain. Full exploitation of plant potential for soil and water phytoremediations also requires deep understanding of the plant response to this toxic metalloid. The aim of this review is to summarize data regarding the effect of arsenic on plant physiology with a focus on mechanisms providing increased arsenic tolerance and/or hyperaccumulation. The emphasis is placed on the topic unjustifiably neglected in the previous reviews - i.e., carbohydrate metabolism, tightly connected to photosynthesis, and beside others involved in plant ability to cope with arsenic-induced oxidative and nitrosative stresses.

Arsenolipid biosynthesis by the unicellular alga Dunaliella tertiolecta is influenced by As/P ratio in culture experiments

Culture experiments exposing unicellular algae to varying arsenate/phosphate regimes and determining their arsenometallomes by HPLC–MS shows the interconnection of arsenolipids and water-soluble arsenicals.

Linking Genes to Microbial Biogeochemical Cycling: Lessons from Arsenic

The biotransformation of arsenic is highly relevant to the arsenic biogeochemical cycle. Identification of the molecular details of microbial pathways of arsenic biotransformation coupled with analyses of microbial communities by meta-omics can provide insights into detailed aspects of the complexities of this biocycle. Arsenic transformations couple to other biogeochemical cycles, and to the fate of both nutrients and other toxic environmental contaminants. Microbial redox metabolism of iron, carbon, sulfur, and nitrogen affects the redox and bioavailability of arsenic species. In this critical review we illustrate the biogeochemical processes and genes involved in arsenic biotransformations. We discuss how current and future metagenomic-, metatranscriptomic-, metaproteomic-, and metabolomic-based methods will help to decipher individual microbial arsenic transformation processes, and their connections to other biogeochemical cycle. These insights will allow future use of microbial metabolic capabilities for new biotechnological solutions to environmental problems. To understand the complex nature of inorganic and organic arsenic species and the fate of environmental arsenic will require integrating systematic approaches with biogeochemical modeling. Finally, from the lessons learned from these studies of arsenic biogeochemistry, we will be able to predict how the environment changes arsenic, and, in response, how arsenic biotransformations change the environment.