Pathways of arsenic uptake and efflux Garbinski, Luis D.; Rosen, Barry P.; Chen, Jian
Environment international,
05/2019, Letnik:
126
Journal Article
Recenzirano
Odprti dostop
Arsenic is a non-essential, environmentally ubiquitous toxic metalloid. In response to this pervasive environmental challenge, organisms evolved mechanisms to confer resistance to arsenicals. ...Inorganic pentavalent arsenate is taken into most cells adventitiously by phosphate uptake systems. Similarly, inorganic trivalent arsenite is taken into most cells adventitiously, primarily via aquaglyceroporins or sugar permeases. The most common strategy for tolerance to both inorganic and organic arsenicals is by efflux that extrude them from the cytosol. These efflux transporters span across kingdoms and belong to various families such as aquaglyceroporins, major facilitator superfamily (MFS) transporters, ATP-binding cassette (ABC) transporters and potentially novel, yet to be discovered families. This review will outline the properties and substrates of known arsenic transport systems, the current knowledge gaps in the field, and aims to provide insight into the importance of arsenic transport in the context of the global arsenic biogeocycle and human health.
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•Arsenic is a toxic metalloid that enters cells adventitiously through uptake systems for nutrients such phopshate permeases (arsenate), and or aquaglyceroporins or glucose permeases (arsenite).•Cells detoxify arsenic primarily by extrusion from cells.•Efflux permeases for arsenite include ArsB and Acr3.•Efflux permeases for methylarsenite include ArsP and ArsK.•Novel efflux proteins ArsJ and AqpS confer resistance to arsenate even though it is not the substrate of those transporters.
Summary
Environmental organoarsenicals are produced by microorganisms and are introduced anthropogenically as herbicides and antimicrobial growth promoters for poultry and swine. Nearly every ...prokaryote has an ars (arsenic resistance) operon, and some have an arsH gene encoding an atypical flavodoxin. The role of ArsH in arsenic resistance has been unclear. Here we demonstrate that ArsH is an organoarsenical oxidase that detoxifies trivalent methylated and aromatic arsenicals by oxidation to pentavalent species. Escherichia coli, which does not have an arsH gene, is very sensitive to the trivalent forms of the herbicide monosodium methylarsenate MSMA or MAs(V) and antimicrobial growth promoter roxarsone Rox(V), as well as to phenylarsenite PhAs(III), also called phenylarsine oxide or PAO. Pseudomonas putida has two chromosomally encoded arsH genes and is highly resistant to the trivalent forms of these organoarsenicals. A derivative of P. putida with both arsH genes deleted is sensitive to MAs(III), PhAs(III) or Rox(III). P. putida arsH expressed in E. coli conferred resistance to each trivalent organoarsenical. Cells expressing PpArsH oxidized the trivalent organoarsenicals. PpArsH was purified, and the enzyme in vitro similarly oxidized the trivalent organoarsenicals. These results suggest that ArsH catalyzes a novel biotransformation that confers resistance to environmental methylated and aromatic arsenicals.
ArsH is an NADPH:FMN oxidoreductase that detoxifies the trivalent forms of the herbicide sodium methylarsenate (MSMA) and the poultry antimicrobial growth promoter roxarsone by oxidation to pentavalent species. ArsH is a tetrameric dimer of dimers with two FMN molecules bound in each dimer (graphic).
Summary
Arsenic is the most ubiquitous environmental toxin. Here, we demonstrate that bacteria have evolved the ability to use arsenic to gain a competitive advantage over other bacteria at least ...twice. Microbes generate toxic methylarsenite (MAs(III)) by methylation of arsenite (As(III)) or reduction of methylarsenate (MAs(V)). MAs(III) is oxidized aerobically to MAs(V), making methylation a detoxification process. MAs(V) is continually re‐reduced to MAs(III) by other community members, giving them a competitive advantage over sensitive bacteria. Because generation of a sustained pool of MAs(III) requires microbial communities, these complex interactions are an emergent property. We show that reduction of MAs(V) by Burkholderia sp. MR1 produces toxic MAs(III) that inhibits growth of Escherichia coli in mixed culture. There are three microbial mechanisms for resistance to MAs(III). ArsH oxidizes MAs(III) to MAs(V). ArsI degrades MAs(III) to As(III). ArsP confers resistance by efflux. Cells of E. coli expressing arsI, arsH or arsP grow in mixed culture with Burkholderia sp. MR1 in the presence of MAs(V). Thus MAs(III) has antibiotic properties: a toxic organic compound produced by one microbe to kill off competitors. Our results demonstrate that life has adapted to use environmental arsenic as a weapon in the continuing battle for dominance.
Bacteria have evolved the ability to use toxic arsenic to gain a competitive advantage over other bacteria at least twice. Microbes generate toxic methylarsenite (MAs(III)) as an antibiotic, an emergent property of this complex microbial communities. We show that bacteria reduce MAs(V) to the MAs(III) antibiotic, inhibiting growth of other bacteria. There are three microbial mechanisms for resistance to MAs(III), ArsH, ArsI and ArsP. Thus, MAs(III) has antibiotic properties: a toxic organic compound produced by one microbe to kill off competitors. Our results demonstrate that life has adapted to use environmental arsenic as a weapon in the continuing battle for dominance.
Aquaporins (AQPs), members of a superfamily of transmembrane channel proteins, are ubiquitous in all domains of life. They fall into a number of branches that can be functionally categorized into two ...major sub-groups: i) orthodox aquaporins, which are water-specific channels, and ii) aquaglyceroporins, which allow the transport of water, non-polar solutes, such as urea or glycerol, the reactive oxygen species hydrogen peroxide, and gases such as ammonia, carbon dioxide and nitric oxide and, as described in this review, metalloids.
This review summarizes the key findings that AQP channels conduct bidirectional movement of metalloids into and out of cells.
As(OH)3 and Sb(OH)3 behave as inorganic molecular mimics of glycerol, a property that allows their passage through AQP channels. Plant AQPs also allow the passage of boron and silicon as their hydroxyacids, boric acid (B(OH)3) and orthosilicic acid (Si(OH)4), respectively. Genetic analysis suggests that germanic acid (GeO2) is also a substrate. While As(III), Sb(III) and Ge(IV) are toxic metalloids, borate (B(III)) and silicate (Si(IV)) are essential elements in higher plants.
The uptake of environmental metalloids by aquaporins provides an understanding of (i) how toxic elements such as arsenic enter the food chain; (ii) the delivery of arsenic and antimony containing drugs in the treatment of certain forms of leukemia and chemotherapy of diseases caused by pathogenic protozoa; and (iii) the possibility that food plants such as rice could be made safer by genetically modifying them to exclude arsenic while still accumulating boron and silicon. This article is part of a Special Issue entitled Aquaporins.
•Aquaporins (AQPs) are metalloid channels ubiquitous in all domains of life.•AQPs conduct boron, germanium, silicon, arsenic, and antimony bidirectionally.•As(III), Sb(III) and Ge(IV) are toxic metalloids, and uptake is adventitious.•B(III) and Si(IV) are nontoxic plant growth enhancers under conditions of stress.•Arsenic/antimony compounds are drugs for antimicrobial and anticancer chemotherapy.
Arsenic is the most widespread environmental toxin. Substantial amounts of pentavalent organoarsenicals have been used as herbicides, such as monosodium methylarsonic acid (MSMA), and as growth ...enhancers for animal husbandry, such as roxarsone (4-hydroxy-3-nitrophenylarsonic acid) Rox(V). These undergo environmental degradation to more toxic inorganic arsenite As(III). We previously demonstrated a two-step pathway of degradation of MSMA to As(III) by microbial communities involving sequential reduction to methylarsonous acid MAs(III) by one bacterial species and demethylation from MAs(III) to As(III) by another. In this study, the gene responsible for MAs(III) demethylation was identified from an environmental MAs(III)-demethylating isolate, Bacillus sp. MD1. This gene, termed arsenic inducible gene (arsl), is in an arsenic resistance (ars) operon and encodes a nonheme iron-dependent dioxygenase with C·As lyase activity. Heterologous expression of Arsl conferred MAs(III)-demethylating activity and MAs(III) resistance to an arsenic-hypersensitive strain of Escherichia coli, demonstrating that MAs(III) demethylation is a detoxification process. Purified Arsl catalyzes Fe2+-dependent MAs(III) demethylation. In addition, Arsl cleaves the C·As bond in trivalent roxarsone and other aromatic arsenicals. Arsl homologs are widely distributed in prokaryotes, and we propose that Arsl-catalyzed organoarsenical degradation has a significant impact on the arsenic biogeocycle. To our knowledge, this is the first report of a molecular mechanism for organoarsenic degradation by a C·As lyase.
Arsenic and selenium are metalloids found in the environment. Arsenic is considered to pose the most significant potential threat to human health based on frequency of occurrence, toxicity and human ...exposure. Selenium, on the other hand, ranks only 147th in toxicity but, in contrast to arsenic, is a required micronutrient. Whether a toxin or micronutrient, their metabolism requires that cells to accumulate these metalloids. In this review we discuss the membrane proteins that transport arsenic and selenium into cells, from bacteria to humans, as well as some of the efflux proteins involved in detoxification.
Arsenic is methylated by arsenite (As(III)) S-adenosylmethionine (SAM) methyltransferases (ArsMs). ArsM crystal structures show three domains (an N-terminal SAM binding domain (A domain), a central ...arsenic binding domain (B domain), and a C-terminal domain of unknown function (C domain)). In this study, we performed a comparative analysis of ArsMs and found a broad diversity in structural domains. The differences in the ArsM structure enable ArsMs to have a range of methylation efficiencies and substrate selectivities. Many small ArsMs with 240–300 amino acid residues have only A and B domains, represented by RpArsM from Rhodopseudomonas palustris. These small ArsMs have higher methylation activity than larger ArsMs with 320–400 residues such as Chlamydomonas reinhardtii CrArsM, which has A, B, and C domains. To examine the role of the C domain, the last 102 residues in CrArsM were deleted. This CrArsM truncation exhibited higher As(III) methylation activity than the wild-type enzyme, suggesting that the C-terminal domain has a role in modulating the rate of catalysis. In addition, the relationship of arsenite efflux systems and methylation was examined. Lower rates of efflux led to higher rates of methylation. Thus, the rate of methylation can be modulated in multiple ways.
Summary
Organoarsenicals enter the environment from biogenic and anthropogenic sources. Trivalent inorganic arsenite (As(III)) is microbially methylated to more toxic methylarsenite (MAs(III)) and ...dimethylarsenite (DMAs(III)) that oxidize in air to MAs(V) and DMAs(V). Sources include the herbicide monosodium methylarsenate (MSMA or MAs(V)), which is microbially reduced to MAs(III), and the aromatic arsenical roxarsone (3‐nitro‐4‐hydroxybenzenearsonic acid or Rox), an antimicrobial growth promoter for poultry and swine. Here we show that Sphingobacterium wenxiniae LQY‐18T, isolated from activated sludge, is resistant to trivalent MAs(III) and Rox(III). Sphingobacterium wenxiniae detoxifies MAs(III) and Rox(III) by oxidation to MAs(V) and Rox(V). Sphingobacterium wenxiniae has a novel chromosomal gene, termed arsU1. Expressed in Escherichia coli arsU1 confers resistance to MAs(III) and Rox(III) but not As(III) or pentavalent organoarsenicals. Purified ArsU1 catalyses oxidation of trivalent methylarsenite and roxarsone. ArsU1 has six conserved cysteine residues. The DNA sequence for the three C‐terminal cysteines was deleted, and the other three were mutated to serines. Only C45S and C122S lost activity, suggesting that Cys45 and Cys122 play a role in ArsU1 function. ArsU1 requires neither FMN nor FAD for activity. These results demonstrate that ArsU1 is a novel MAs(III) oxidase that contributes to S. wenxiniae tolerance to organoarsenicals.
Lake Okeechobee, FL, USA, has been subjected to intensifying cyanobacterial blooms that can spread to the adjacent St. Lucie River and Estuary via natural and anthropogenically-induced flooding ...events. In July 2016, a large, toxic cyanobacterial bloom occurred in Lake Okeechobee and throughout the St. Lucie River and Estuary, leading Florida to declare a state of emergency. This study reports on measurements and nutrient amendment experiments performed in this freshwater-estuarine ecosystem (salinity 0-25 PSU) during and after the bloom. In July, all sites along the bloom exhibited dissolved inorganic nitrogen-to-phosphorus ratios < 6, while Microcystis dominated (> 95%) phytoplankton inventories from the lake to the central part of the estuary. Chlorophyll a and microcystin concentrations peaked (100 and 34 μg L-1, respectively) within Lake Okeechobee and decreased eastwards. Metagenomic analyses indicated that genes associated with the production of microcystin (mcyE) and the algal neurotoxin saxitoxin (sxtA) originated from Microcystis and multiple diazotrophic genera, respectively. There were highly significant correlations between levels of total nitrogen, microcystin, and microcystin synthesis gene abundance across all surveyed sites (p < 0.001), suggesting high levels of nitrogen supported the production of microcystin during this event. Consistent with this, experiments performed with low salinity water from the St. Lucie River during the event indicated that algal biomass was nitrogen-limited. In the fall, densities of Microcystis and concentrations of microcystin were significantly lower, green algae co-dominated with cyanobacteria, and multiple algal groups displayed nitrogen-limitation. These results indicate that monitoring and regulatory strategies in Lake Okeechobee and the St. Lucie River and Estuary should consider managing loads of nitrogen to control future algal and microcystin-producing cyanobacterial blooms.
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