Structures of two ArsR As(III)-responsive transcriptional repressors: Implications for the mechanism of derepression

AsIII Selectively Induces a Disorder-to-Order Transition in the Metalloid Binding Region of the AfArsR Protein

Arsenic is highly toxic and a significant threat to human health, but certain bacteria have developed defense mechanisms initiated by AsIII binding to AsIII-sensing proteins of the ArsR family. The transcriptional regulator AfArsR responds to AsIII and SbIII by coordinating the metalloids with three cysteines, located in a short sequence of the same monomer chain. Here, we characterize the binding of AsIII and HgII to a model peptide encompassing this fragment of the protein via solution equilibrium and spectroscopic/spectrometric techniques (pH potentiometry, UV, CD, NMR, PAC, EXAFS, and ESI-MS) combined with DFT calculations and MD simulations. Coordination of AsIII changes the peptide structure from a random-coil to a well-defined structure of the complex. A trigonal pyramidal AsS3 binding site is formed with almost exactly the same structure as observed in the crystal structure of the native protein, implying that the peptide possesses all of the features required to mimic the AsIII recognition and response selectivity of AfArsR. Contrary to this, binding of HgII to the peptide does not lead to a well-defined structure of the peptide, and the atoms near the metal binding site are displaced and reoriented in the HgII model. Our model study suggests that structural organization of the metal site by the inducer ion is a key element in the mechanism of the metalloid-selective recognition of this protein.

Increased intracellular persulfide levels attenuate HlyU-mediated hemolysin transcriptional activation in Vibrio cholerae

The vertebrate host's immune system and resident commensal bacteria deploy a range of highly reactive small molecules that provide a barrier against infections by microbial pathogens. Gut pathogens, such as Vibrio cholerae, sense and respond to these stressors by modulating the expression of exotoxins that are crucial for colonization. Here, we employ mass spectrometry-based profiling, metabolomics, expression assays, and biophysical approaches to show that transcriptional activation of the hemolysin gene hlyA in V. cholerae is regulated by intracellular forms of sulfur with sulfur-sulfur bonds, termed reactive sulfur species (RSS). We first present a comprehensive sequence similarity network analysis of the arsenic repressor superfamily of transcriptional regulators, where RSS and hydrogen peroxide sensors segregate into distinct clusters of sequences. We show that HlyU, transcriptional activator of hlyA in V. cholerae, belongs to the RSS-sensing cluster and readily reacts with organic persulfides, showing no reactivity or DNA dissociation following treatment with glutathione disulfide or hydrogen peroxide. Surprisingly, in V. cholerae cell cultures, both sulfide and peroxide treatment downregulate HlyU-dependent transcriptional activation of hlyA. However, RSS metabolite profiling shows that both sulfide and peroxide treatment raise the endogenous inorganic sulfide and disulfide levels to a similar extent, accounting for this crosstalk, and confirming that V. cholerae attenuates HlyU-mediated activation of hlyA in a specific response to intracellular RSS. These findings provide new evidence that gut pathogens may harness RSS-sensing as an evolutionary adaptation that allows them to overcome the gut inflammatory response by modulating the expression of exotoxins.

Bifunctional protein ArsRM contributes to arsenite methylation and resistance in Brevundimonas sp. M20

BACKGROUND

Arsenic (As) with various chemical forms, including inorganic arsenic and organic arsenic, is the most prevalent water and environmental toxin. This metalloid occurs worldwide and many of its forms, especially arsenite [As(III)], cause various diseases including cancer. Organification of arsenite is an effective way for organisms to cope with arsenic toxicity. Microbial communities are vital contributors to the global arsenic biocycle and represent a promising way to reduce arsenite toxicity.

METHODS

Brevundimonas sp. M20 with arsenite and roxarsone resistance was isolated from aquaculture sewage. The arsHRNBC cluster and the metRFHH operon of M20 were identified by sequencing. The gene encoding ArsR/methyltransferase fusion protein, arsRM, was amplified and expressed in Escherichia coli BL21 (DE3), and this strain showed resistance to arsenic in the present of 0.25-6 mM As(III), aresenate, or pentavalent roxarsone. The methylation activity and regulatory action of ArsRM were analyzed using Discovery Studio 2.0, and its functions were confirmed by methyltransferase activity analysis and electrophoretic mobility shift assays.

RESULTS

The minimum inhibitory concentration of the roxarsone resistant strain Brevundimonas sp. M20 to arsenite was 4.5 mM. A 3,011-bp arsenite resistance ars cluster arsHRNBC and a 5649-bp methionine biosynthesis met operon were found on the 3.315-Mb chromosome. Functional prediction analyses suggested that ArsRM is a difunctional protein with transcriptional regulation and methyltransferase activities. Expression of ArsRM in E. coli increased its arsenite resistance to 1.5 mM. The arsenite methylation activity of ArsRM and its ability to bind to its own gene promoter were confirmed. The As(III)-binding site (ABS) and S-adenosylmethionine-binding motif are responsible for the difunctional characteristic of ArsRM.

CONCLUSIONS

We conclude that ArsRM promotes arsenite methylation and is able to bind to its own promoter region to regulate transcription. This difunctional characteristic directly connects methionine and arsenic metabolism. Our findings contribute important new knowledge about microbial arsenic resistance and detoxification. Future work should further explore how ArsRM regulates the met operon and the ars cluster.

Tristhiolato Pseudopeptides Bind Arsenic(III) in an AsS3 Coordination Environment Imitating Metalloid Binding Sites in Proteins

The AsIII binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand L1) or d-penicillamine residues (ligand L2) as potential coordinating groups for soft semimetals or metal ions, was studied by experimental (UV, CD, NMR, and ESI-MS) and theoretical (DFT) methods. All of the experimental data, obtained with the variation of the AsIII:ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS3-type coordination mode. The UV-monitored titration of the ligands with arsenous acid at pH = 7.0 provided an absorbance data set that allowed for the determination of apparent stability constants of the forming species. The obtained stabilities (logK' = 5.26 (AsL1) and logK' = 3.04 (AsL2)) reflect high affinities, especially for the sterically less restricted cysteine derivative. DFT calculated structures correlate well with the spectroscopic results and, in line with the 1H NMR data, indicate a preference for the all-endo conformers resembling the AsIII environment at the semimetal binding sites in various metalloproteins.

Increased intracellular persulfide levels attenuate HlyU-mediated hemolysin transcriptional activation in Vibrio cholerae

The vertebrate host’s immune system and resident commensal bacteria deploy a range of highly reactive small molecules that provide a barrier against infections by microbial pathogens. Gut pathogens, such as Vibrio cholerae , sense and respond to these stressors by modulating the expression of exotoxins that are crucial for colonization. Here, we employ mass-spectrometry-based profiling, metabolomics, expression assays and biophysical approaches to show that transcriptional activation of the hemolysin gene hlyA in V. cholerae is regulated by intracellular reactive sulfur species (RSS), specifically sulfane sulfur. We first present a comprehensive sequence similarity network analysis of the arsenic repressor (ArsR) superfamily of transcriptional regulators where RSS and reactive oxygen species (ROS) sensors segregate into distinct clusters. We show that HlyU, transcriptional activator of hlyA in V. cholerae , belongs to the RSS-sensing cluster and readily reacts with organic persulfides, showing no reactivity and remaining DNA-bound following treatment with various ROS in vitro, including H 2 O 2 . Surprisingly, in V. cholerae cell cultures, both sulfide and peroxide treatment downregulate HlyU-dependent transcriptional activation of hlyA . However, RSS metabolite profiling shows that both sulfide and peroxide treatment raise the endogenous inorganic sulfide and disulfide levels to a similar extent, accounting for this crosstalk, and confirming that V. cholerae attenuates HlyU-mediated activation of hlyA in a specific response to intracellular RSS. These findings provide new evidence that gut pathogens may harness RSS-sensing as an evolutionary adaptation that allows them to overcome the gut inflammatory response by modulating the expression of exotoxins.

Molecular Insights into a Novel Cu(I)-Sensitive ArsR/SmtB Family Repressor in Extremophile Acidithiobacillus caldus

Acidithiobacillus caldus is a common bioleaching bacterium that is inevitably exposed to extreme copper stress in leachates. The ArsR/SmtB family of metalloregulatory repressors regulates homeostasis and resistance in bacteria by specifically responding to metals. Here, we characterized A. caldus Cu(I)-sensitive repressor (AcsR) and gained molecular insights into this new member of the ArsR/SmtB family. Transcriptional analysis indicated that the promoter (PIII) of acsR was highly active in Escherichia coli but inhibited upon AcsR binding to the PIII-acsR region. Size exclusion chromatography and circular dichroism spectra revealed that Cu^I-AcsR shared an identical assembly state with apo-AcsR, as a dimer with fewer α helices, more extended strands, and more β turns. Mutation of the cysteine site in AcsR did not affect its assembly state. Copper(I) titrations revealed that apo-AcsR bound two Cu(I) molecules per monomer in vitro with an average dissociation constant (K_D) for bicinchoninic acid competition of 2.55 × 10^-9 M. Site-directed mutation of putative Cu(I)-binding ligands in AcsR showed that replacing Cys64 with Ala reduces copper binding ability from two Cu(I) molecules per monomer to one, with an average K_D of 6.05 × 10^-9 M. Electrophoretic mobility shift assays revealed that apo-AcsR has high affinity for the 12-2-12 imperfect inverted repeats P2245 and P2270 in the acsR gene cluster and that Cu-loaded AcsR had lower affinity for DNA fragments than apo-AcsR. We developed a hypothetical working model of AcsR to better understand Cu resistance mechanisms in A. caldus. IMPORTANCE Copper (Cu) resistance among various microorganisms is attracting interest. The chemolithoautotrophic bacterium A. caldus, which can tolerate extreme copper stress (≥10 g/L Cu ions), is typically used to bioleach chalcopyrite (CuFeS₂). Understanding of Cu resistance in A. caldus is limited due to scant investigation and the absence of efficient gene manipulation tools. Here, we characterized a new member of the ArsR/SmtB family of prokaryotic metalloregulatory transcriptional proteins that repress operons linked to stress-inducing concentrations of heavy metal ions. This protein can bind two Cu(I) molecules per monomer and negatively regulate its gene cluster. Members of the ArsR/SmtB family have not been investigated in A. caldus until now. The discovery of this novel protein enriches understanding of Cu homeostasis in A. caldus.

Characterization of a novel ArsR regulates divergent ars operon in Ensifer adhaerens strain ST2

Microbes evolved resistance determinates for coping with arsenic toxicity are commonly regulated by a variety of transcriptional repressors (ArsRs). Ensifer adhaerens strain ST2 was previously shown tolerance to environmental organoarsenical methylarsenite (MAs(III)), which has been proposed to be a primordial antibiotic. In E. adhaerens strain ST2 chromosomal ars operon, two MAs(III) resistance genes, arsZ, encoding MAs(III) oxidase, and arsK, encoding MAs(III) efflux transporter, are controlled by a novel ArsR transcriptional repressor, EaArsR. It has two conserved cysteine pairs, Cys91-92 and Cys108-109. Electrophoretic mobility shift assays (EMSAs) demonstrate that EaArsR binds to two inverted-repeat sequences within the ars promoter between arsR and arsZ to repress ars operon transcription and that DNA binding is relieved upon binding of As(III) and MAs(III). Mutation of either Cys91 or Cys92 to serine (or both) abolished these mutants binding to the ars promoter. In contrast, both C108S and C109S mutants kept responsiveness to As(III) and MAs(III). These results suggest that cysteine pair Cys91-Cys92 and either Cys108 or Cys109 contribute to form arsenic binding site. Homology modeling of EaArsR indicates the binding site consisted of Cys91-Cys92 pair from one monomer and Cys108-Cys109 pair from the other monomer, which displays the diverse evolution of arsenic binding site in the ArsR metalloregulators.

Structural and functional mapping of ars gene cluster in Deinococcus indicus DR1

Deinococcus indicus DR1 is a novel Gram-negative bacterium, isolated from the Dadri wetlands in Uttar Pradesh, India. In addition to being radiation-resistant, the rod-shaped, red-pigmented organism shows extraordinary resistance to arsenic. The proteins of the corresponding ars gene cluster involved in arsenic extrusion in D. indicus DR1 have not yet been characterized. Additionally, how these proteins regulate each other providing arsenic resistance is still unclear. Here, we present a computational model of the operonic structure and the corresponding characterization of the six proteins of the ars gene cluster in D. indicus DR1. Additionally, we show the expression of the genes in the presence of arsenic using qRT-PCR. The ars gene cluster consists of two transcriptional regulators (ArsR1, ArsR2), two arsenate reductases (ArsC2, ArsC3), one metallophosphatase family protein (MPase), and a transmembrane arsenite efflux pump (ArsB). The transcriptional regulators are trans-acting repressors, and the reductases reduce arsenate (As⁵⁺) ions to arsenite (As³⁺) ions for favourable extrusion. The proteins modelled using RoseTTAFold, and their conformationally stable coordinates obtained after MD simulation indicate their various functional roles with respect to arsenic. Excluding ArsB, all the proteins belong to the α + β class of proteins. ArsB, being a membrane protein, is fully α-helical, with 12 transmembrane helices. The results show the degree of similarity or divergence of the mechanism utilized by these proteins of ars gene cluster in D. indicus DR1 to confer high levels of arsenic tolerance. This structural characterization study of the ars genes will enable new and deeper insights of arsenic tolerance.

The ArsH Protein Product of the Paracoccus denitrificans ars Operon Has an Activity of Organoarsenic Reductase and Is Regulated by a Redox-Responsive Repressor

Paracoccus denitrificans ArsH is encoded by two identical genes located in two distinct putative arsenic resistance (ars) operons. Escherichia coli-produced recombinant N-His₆-ArsH was characterized both structurally and kinetically. The X-ray structure of ArsH revealed a flavodoxin-like domain and motifs for the binding of flavin mononucleotide (FMN) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). The protein catalyzed FMN reduction by NADPH via ternary complex mechanism. At a fixed saturating FMN concentration, it acted as an NADPH-dependent organoarsenic reductase displaying ping-pong kinetics. A 1:1 enzymatic reaction of phenylarsonic acid with the reduced form of FMN (FMNH₂) and formation of phenylarsonous acid were observed. Growth experiments with P. denitrificans and E. coli revealed increased toxicity of phenylarsonic acid to cells expressing arsH, which may be related to in vivo conversion of pentavalent As to more toxic trivalent form. ArsH expression was upregulated not only by arsenite, but also by redox-active agents paraquat, tert-butyl hydroperoxide and diamide. A crucial role is played by the homodimeric transcriptional repressor ArsR, which was shown in in vitro experiments to monomerize and release from the DNA-target site. Collectively, our results establish ArsH as responsible for enhancement of organo-As(V) toxicity and demonstrate redox control of ars operon.

Structural and mechanistic basis for redox sensing by the cyanobacterial transcription regulator RexT

Organisms have a myriad of strategies for sensing, responding to, and combating reactive oxygen species, which are unavoidable consequences of aerobic life. In the heterocystous cyanobacterium Nostoc sp. PCC 7120, one such strategy is the use of an ArsR-SmtB transcriptional regulator RexT that senses H₂O₂ and upregulates expression of thioredoxin to maintain cellular redox homeostasis. Different from many other members of the ArsR-SmtB family which bind metal ions, RexT has been proposed to use disulfide bond formation as a trigger to bind and release DNA. Here, we present high-resolution crystal structures of RexT in the reduced and H₂O₂-treated states. These structures reveal that RexT showcases the ArsR-SmtB winged-helix-turn-helix fold and forms a vicinal disulfide bond to orchestrate a response to H₂O₂. The importance of the disulfide-forming Cys residues was corroborated using site-directed mutagenesis, mass spectrometry, and H₂O₂-consumption assays. Furthermore, an entrance channel for H₂O₂ was identified and key residues implicated in H₂O₂ activation were pinpointed. Finally, bioinformatics analysis of the ArsR-SmtB family indicates that the vicinal disulfide “redox switch” is a unique feature of cyanobacteria in the Nostocales order, presenting an interesting case where an ArsR-SmtB protein scaffold has been evolved to showcase peroxidatic activity and facilitate redox-based regulation.

The DNA binding and H₂O₂ sensing mechanisms are revealed for RexT, a transcriptional regulator found in cyanobacteria of the Nostocales order.

An ArsRC fusion protein enhances arsenate sensing and detoxification

Arsenical resistance (ars) operons encode genes for arsenic resistance and biotransformation. The majority are composed of individual genes, but fusion of ars genes is not uncommon, although it is not clear if the fused gene products are functional. Here we report identification of a four-gene ars operon from Paracoccus sp. SY that has two arsR-arsC gene fusions. ArsRC1 and ArsRC2 are related proteins that consist of an N-terminal ArsR arsenite (As(III))-responsive repressor with a C-terminal ArsC arsenate reductase. The other two genes in the operon are gapdh and arsJ. GAPDH, glyceraldehyde 3-phosphate dehydrogenase, forms 1-arseno-3-phosphoglycerate (1As3PGA) from 3-phosphoglyceraldehyde and arsenate (As(V)), ArsJ is an efflux permease for 1As3PGA that dissociates into extracellular As(V) and 3-phosphoglycerate. The net effect is As(V) extrusion and resistance. ArsRs are usually selective for As(III) and do not respond to As(V). However, the substrates and products of this operon are pentavalent, which would not be inducers of the operon. We propose that ArsRC fusions overcome this limitation by channelling the ArsC product into the ArsR binding site without diffusion through the cytosol, a de facto mechanism for As(V) induction. This novel mechanism for arsenate sensing can confer an evolutionary advantage for detoxification of inorganic arsenate.

Structure, dynamics, and function of SrnR, a transcription factor for nickel-dependent gene expression

Streptomyces griseus, a bacterium producing antibacterial drugs and featuring possible application in phytoremediation, expresses two metal-dependent superoxide dismutase (SOD) enzymes, containing either Fe(II) or Ni(II) in their active site. In particular, the alternative expression of the two proteins occurs in a metal-dependent mode, with the Fe(II)-enzyme gene (sodF) repressed at high intracellular Ni(II) concentrations by a two-component system (TCS). This complex involves two proteins, namely SgSrnR and SgSrnQ, which represent the transcriptional regulator and the Ni(II) sensor of the system, respectively. SgSrnR belongs to the ArsR/SmtB family of metal-dependent transcription factors; in the apo-form and in the absence of SgSrnQ, it can bind the DNA operator of sodF, upregulating gene transcription. According to a recently proposed hypothesis, Ni(II) binding to SgSrnQ would promote its interaction with SgSrnR, causing the release of the complex from DNA and the consequent downregulation of the sodF expression. SgSrnQ is predicted to be highly disordered, thus the understanding, at the molecular level, of how the SgSrnR/SgSrnQ TCS specifically responds to Ni(II) requires the knowledge of the structural, dynamic, and functional features of SgSrnR. These were investigated synergistically in this work using X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, atomistic molecular dynamics calculations, isothermal titration calorimetry, and in silico molecular docking. The results reveal that the homodimeric apo-SgSrnR binds to its operator in a two-step process that involves the more rigid globular portion of the protein and leaves its largely disordered regions available to possibly interact with the disordered SgSrnQ in a Ni-dependent process.

Design and Prototyping of Genetically Encoded Arsenic Biosensors Based on Transcriptional Regulator AfArsR

Genetically encoded biosensors based on engineered fluorescent proteins (FPs) are essential tools for monitoring the dynamics of specific ions and molecules in biological systems. Arsenic ion in the +3 oxidation state (As³⁺) is highly toxic to cells due to its ability to bind to protein thiol groups, leading to inhibition of protein function, disruption of protein–protein interactions, and eventually to cell death. A genetically encoded biosensor for the detection of As³⁺ could potentially facilitate the investigation of such toxicity both in vitro and in vivo. Here, we designed and developed two prototype genetically encoded arsenic biosensors (GEARs), based on a bacterial As³⁺ responsive transcriptional factor AfArsR from Acidithiobacillus ferrooxidans. We constructed FRET-based GEAR biosensors by insertion of AfArsR between FP acceptor/donor FRET pairs. We further designed and engineered single FP-based GEAR biosensors by insertion of AfArsR into GFP. These constructs represent prototypes for a new family of biosensors based on the ArsR transcriptional factor scaffold. Further improvements of the GEAR biosensor family could lead to variants with suitable performance for detection of As³⁺ in various biological and environmental systems.

Bacterial approaches to sensing and responding to respiration and respiration metabolites

Bacterial respiration of diverse substrates is a primary contributor to the diversity of life. Respiration also drives alterations in the geosphere and tethers ecological nodes together. It provides organisms with a means to dissipate reductants and generate potential energy in the form of an electrochemical gradient. Mechanisms have evolved to sense flux through respiratory pathways and sense the altered concentrations of respiration substrates or byproducts. These genetic regulatory systems promote efficient utilization of respiration substrates, as well as fine tune metabolism to promote cellular fitness and negate the accumulation of toxic byproducts. Many bacteria can respire one or more chemicals, and these regulatory systems promote the prioritization of high energy metabolites. Herein we focus on regulatory paradigms and discuss systems that sense the concentrations of respiration substrates and flux through respiratory pathways. This is a broad field of study, and therefore we focus on key fundamental and recent developments and highlight specific systems that capture the diversity of sensing mechanisms.

Functional and structural characterization of AntR, an Sb(III) responsive transcriptional repressor

The ant operon of the antimony-mining bacterium Comamonas testosterone JL40 confers resistance to Sb(III). The operon is transcriptionally regulated by the product of the first gene in the operon, antR. AntR is a member of ArsR/SmtB family of metal/metalloid-responsive repressors resistance. We purified and characterized C. testosterone AntR and demonstrated that it responds to metalloids in the order Sb(III) = methylarsenite (MAs(III) >> As(III)). The protein was crystallized, and the structure was solved at 2.1 Å resolution. The homodimeric structure of AntR adopts a classical ArsR/SmtB topology architecture. The protein has five cysteine residues, of which Cys103_a from one monomer and Cys113_b from the other monomer, are proposed to form one Sb(III) binding site, and Cys113_a and Cys103_b forming a second binding site. This is the first report of the structure and binding properties of a transcriptional repressor with high selectivity for environmental antimony.

Flagellin-based electrochemical sensing layer for arsenic detection in water

Regular monitoring of arsenic concentrations in water sources is essential due to the severe health effects. Our goal was to develop a rapidly responding, sensitive and stable sensing layer for the detection of arsenic. We have designed flagellin-based arsenic binding proteins capable of forming stable filament structures with high surface binding site densities. The D3 domain of Salmonella typhimurium flagellin was replaced with an arsenic-binding peptide motif of different bacterial ArsR transcriptional repressor factors. We have shown that the fusion proteins developed retain their polymerization ability and have thermal stability similar to that of wild-type filament. The strong arsenic binding capacity of the monomeric proteins was confirmed by isothermal titration calorimetry (ITC), and dissociation constants (K_d) of a few hundred nM were obtained for all three variants. As-binding fibers were immobilized on the surface of a gold electrode and used as a working electrode in cyclic voltammetry (CV) experiments to detect inorganic arsenic near the maximum allowable concentration (MAC) level. Based on these results, it can be concluded that the stable arsenic-binding flagellin variant can be used as a rapidly responding, sensitive, but simple sensing layer in a field device for the MAC-level detection of arsenic in natural waters.

Structural and Functional Investigation of the Periplasmic Arsenate-Binding Protein ArrX from Chrysiogenes arsenatis

The anaerobic bacterium Chrysiogenes arsenatis respires using the oxyanion arsenate (AsO₄^3-) as the terminal electron acceptor, where it is reduced to arsenite (AsO₃^3-) while concomitantly oxidizing various organic (e.g., acetate) electron donors. This respiratory activity is catalyzed in the periplasm of the bacterium by the enzyme arsenate reductase (Arr), with expression of the enzyme controlled by a sensor histidine kinase (ArrS) and a periplasmic-binding protein (PBP), ArrX. Here, we report for the first time, the molecular structure of ArrX in the absence and presence of bound ligand arsenate. Comparison of the ligand-bound structure of ArrX with other PBPs shows a high level of conservation of critical residues for ligand binding by these proteins; however, this suite of PBPs shows different structural alterations upon ligand binding. For ArrX and its homologue AioX (from Rhizobium sp. str. NT-26), which specifically binds arsenite, the structures of the substrate-binding sites in the vicinity of a conserved and critical cysteine residue contribute to the discrimination of binding for these chemically similar ligands.

Comamonas testosteroni antA encodes an antimonite-translocating P-type ATPase

Antimony, like arsenic, is a toxic metalloid widely distributed in the environment. Microbial detoxification of antimony has recently been identified. Here we describe a novel bacterial P_1B-type antimonite (Sb(III))-translocating ATPase from the antimony-mining bacterium Comamonas testosterone JL40 that confers resistance to Sb(III). In a comparative proteomics analysis of strain JL40, an operon (ant operon) was up-regulated by Sb(III). The ant operon includes three genes, antR, antC and antA. AntR belongs to the ArsR/SmtB family of metalloregulatory proteins that regulates expression of the ant operon. AntA belongs to the P_1B family of the P-type cation-translocating ATPases. It has both similarities to and differences from other members of the P_1B-1 subfamily and appears to be the first identified member of a distinct subfamily that we designate P_1B-8. Expression AntA in E. coli AW3110 (Δars) conferred resistance to Sb(III) and reduced the intracellular concentration of Sb(III) but not As(III) or other metals. Everted membrane vesicles from cells expressing antA accumulated Sb(III) but not As(III), where uptake in everted vesicles reflects efflux from cells. AntC is a small protein with a potential Sb(III) binding site, and co-expression of AntC with AntA increased resistance to Sb(III). We propose that AntC functions as an Sb(III) chaperone to AntA, augmenting Sb(III) efflux. The identification of a novel Sb(III)-translocating ATPase enhances our understanding of the biogeochemical cycling of environmental antimony by bacteria.

Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms

Microbially-mediated methylation of arsenic (As) plays an important role in the As biogeochemical cycle, particularly in rice paddy soils where methylated As, generated microbially, is translocated into rice grains. The presence of the arsenite (As(III)) methyltransferase gene (arsM) in soil microbes has been used as an indication of their capacity for As methylation. Here, we evaluate the ability of seven microorganisms encoding active ArsM enzymes to methylate As. Amongst those, only the aerobic species were efficient methylators. The anaerobic microorganisms presented high resistance to As exposure, presumably through their efficient As(III) efflux, but methylated As poorly. The only exception were methanogens, for which efficient As methylation was seemingly an artifact of membrane disruption. Deletion of an efflux pump gene (acr3) in one of the anaerobes, Clostridium pasteurianum, rendered the strain sensitive to As and capable of more efficiently methylating As. Our results led to the following conclusions: (i) encoding a functional ArsM enzyme does not guarantee that a microorganism will actively drive As methylation in the presence of the metalloid and (ii) there is an inverse relationship between efficient microbial As efflux and its methylation, because the former prevents the intracellular accumulation of As.

Molecular Evolution of Transition Metal Bioavailability at the Host-Pathogen Interface

The molecular evolution of the adaptive response at the host-pathogen interface has been frequently referred to as an 'arms race' between the host and bacterial pathogens. The innate immune system employs multiple strategies to starve microbes of metals. Pathogens, in turn, develop successful strategies to maintain access to bioavailable metal ions under conditions of extreme restriction of transition metals, or nutritional immunity. However, the processes by which evolution repurposes or re-engineers host and pathogen proteins to perform or refine new functions have been explored only recently. Here we review the molecular evolution of several human metalloproteins charged with restricting bacterial access to transition metals. These include the transition metal-chelating S100 proteins, natural resistance-associated macrophage protein-1 (NRAMP-1), transferrin, lactoferrin, and heme-binding proteins. We examine their coevolution with bacterial transition metal acquisition systems, involving siderophores and membrane-spanning metal importers, and the biological specificity of allosteric transcriptional regulatory proteins tasked with maintaining bacterial metallostasis. We also discuss the evolution of metallo-β-lactamases; this illustrates how rapid antibiotic-mediated evolution of a zinc metalloenzyme obligatorily occurs in the context of host-imposed nutritional immunity.