Microbial metallothioneins

OaAEP1 Ligase-Assisted Chemoenzymatic Synthesis of Full Cysteine-Rich Metal-Binding Cyanobacterial Metallothionein SmtA

Among all approaches used for the semisynthesis of natural or chemically modified products, enzyme-assisted ligation is among the most promising and dynamically developing approaches. Applying an efficient C247A mutant of Oldenlandia affinis plant ligase OaAEP1 and solid-phase peptide synthesis chemistry, we present the chemoenzymatic synthesis of a complete sequence of the cysteine-rich and metal-binding cyanobacterial metallothionein Synechococcus metallothionein A (SmtA). Zn(II) and Cd(II) binding to the newly synthesized SmtA showed identical properties to the protein expressed in Escherichia coli. The presented approach is the first example of the use of OaAEP1 mutant for total protein synthesis of metallothionein, which occurs in mild conditions preventing cysteine thiol oxidation. The recognition motif of the applied enzyme could naturally occur in the protein structure or be synthetically or genetically incorporated in some loops or secondary structure elements. Therefore, we envision that this strategy can be used for efficiently obtaining SmtA and for a wide range of proteins and their derivatives.

A comprehensive assessment of Yarrowia lipolytica and its interactions with metals: Current updates and future prospective

The non-conventional yeast Yarrowia lipolytica has been popular as a model system for understanding biological processes such as dimorphism and lipid accumulation. The organism can efficiently utilize hydrophobic substrates (hydrocarbons and triglycerides) thereby rendering it relevant in bioremediation of oil polluted environments. The current review focuses on the interactions of this fungus with metal pollutants and its potential application in bioremediation of metal contaminated locales. This fungus is intrinsically equipped with a variety of physiological and biochemical features that enable it to tide over stress conditions induced by the presence of metals. Production of enzymes such as phosphatases, reductases and superoxide dismutases are worth a special mention. In the presence of metals, levels of inherently produced metal binding proteins (metallothioneins) and the pigment melanin are seen to be elevated. Morphological alterations with respect to biofilm formation and dimorphic transition from yeast to mycelial form are also induced by certain metals. The biomass of Y. lipolytica is inherently important as a biosorbent and cell surface modification, process optimization or whole cell immobilization techniques have aided in improving this capability. In the presence of metals such as mercury, cadmium, copper and uranium, the culture forms nanoparticulate deposits. In addition, on account of its intrinsic reductive ability, Y. lipolytica is being exploited for synthesizing nanoparticles of gold, silver, cadmium and selenium with applications as antimicrobial compounds, location agents for bioimaging and as feed supplements. This versatile organism thus has great potential in interacting with various metals and addressing problems related to their pollutant status.

Pseudomonas sp. TCd-1 significantly alters the rhizosphere bacterial community of rice in Cd contaminated paddy field

Cadmium (Cd) pollution of paddy soils is one of the main concerns causing food security and environmental problems. Microbial bioremediation is an effective and eco-friendly measure that uses microbes to reduce Cd accumulation in crops. Additionally, rhizosphere bacterial communities also act essential roles in crop tolerance of heavy metals. However, the effects of inoculations with Cd resistant bacteria on crop rhizosphere bacterial communities under Cd exposure are largely unknown. In this study, we used high-throughput 16S rRNA gene sequencing technologies to explore the community structure and co-occurrence network of the rhizosphere bacterial communities associated with the rice crop under different Cd treatments and the application of Cd-tolerant strain Pseudomonas sp. TCd-1. We found that the strain TCd-1 both significantly reduced the rhizobacterial alpha diversity and changed the beta diversity. PERMANOVA and NMDS analysis showed that Cd stress and TCd-1 strain could act as strong environmental filters resulting in observable differentiation of rhizobacterial community composition among different groups. In addition, RDA results indicated that the rhizosphere pH, root Cd content, catalase (CAT), urease (URE), gibberellic acid (GA3) exert significant association with rhizosphere bacterial assembly. PICRUSt analysis revealed that the TCd-1 strain improved the metabolic capacity of rhizosphere bacteria under Cd stress. Furthermore, co-occurrence network topological features and keystone taxa also varied among different groups. This study could provide necessary insights into developing an efficient bioremediation and safe production of rice crops in Cd contaminated paddy fields with the application of Pseudomonas sp. TCd-1 strain, as well as advance our understanding of the principles of rhizosphere bacterial community assembly under Cd stress.

Comprehensive and functional analyses reveal the genomic diversity and potential toxicity of Microcystis

Microcystis is a cyanobacteria that is widely distributed across the world. It has attracted great attention because it produces the hepatotoxin microcystin (MC) that can inhibit eukaryotic protein phosphatases and pose a great risk to animal and human health. Due to the high diversity of morphospecies and genomes, it is still difficult to classify Microcystis species. In this study, we investigated the pangenome of 23 Microcystis strains to detect the genetic diversity and evolutionary dynamics. Microcystis revealed an open pangenome containing 22,009 gene families and exhibited different functional constraints. The core-genome phylogenetic analysis accurately differentiated the toxic and nontoxic strains and could be used as a taxonomic standard at the genetic level. We also investigated the functions of HGT events, of which were mostly conferred from cyanobacteria and closely related species. In order to detect the potential toxicity of Microcystis, we searched and characterized MC biosynthetic gene clusters and other secondary metabolite gene clusters. Our work provides insights into the genetic diversity, evolutionary dynamics, and potential toxicity of Microcystis, which could benefit the species classification and development of new methods for drinking water quality control and management of bloom formation in the future.

Sometimes less is more-the impact of the number of His residues on the stability of Zn(II)-SmtB and BigR4 α-5 domain complexes

The increasing number of antibiotic-resistant pathogens has become one of the major health problems of modern times, including infections caused by Mycobacterium tuberculosis. One of the possible mammalian immune system responses to mycobacterial infection is the increase of the zinc(ii) concentration in phagosomes to a toxic level. The mycobacterial SmtB protein belongs to the family of ArsR/SmtB transcription regulators. In the presence of high concentrations of metals, SmtB dissociates from DNA and activates the expression of metal efflux proteins. In this work, we focus on the α5 zinc(ii) binding domains of SmtB/BigR4 proteins (the latter being the SmtB homolog from non-pathogenic M. smegmatis) and two mutants of BigR4. We will be taking a closer look at the coordination modes and thermodynamic properties of their zinc(ii) complexes. The study points out the specificity of metal-ligand interactions and describes the effect of mutations on the coordination properties of the studied systems. The stabilities of the zinc(ii) complexes were determined by potentiometry. The coordination sites were determined by NMR experiments and DFT calculations. The comparison of complex stabilities reveals that the Zn(ii)-BigR4 species are more stable than the Zn(ii)-SmtB complexes. His mutations strongly affect the stability of the complexes and the coordination modes of the metal ion. Exchanging one of the histidines for alanine causes, surprisingly, an increase in the stability of zinc(ii) complexes with the studied domain. This was confirmed by potentiometric and DFT methods. This work can be considered as a bioinorganic introduction to the discovery of new strategies in M. tuberculosis infection treatment based on zinc(ii)-sensitive mechanisms.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Transition metal transporters in rhizobia: tuning the inorganic micronutrient requirements to different living styles

A group of bacteria known as rhizobia are key players in symbiotic nitrogen fixation (SNF) in partnership with legumes. After a molecular exchange, the bacteria end surrounded by a plant membrane forming symbiosomes, organelle-like structures, where they differentiate to bacteroids and fix nitrogen. This symbiotic process is highly dependent on dynamic nutrient exchanges between the partners. Among these are transition metals (TM) participating as inorganic and organic cofactors of fundamental enzymes. While the understanding of how plant transporters facilitate TMs to the very near environment of the bacteroid is expanding, our knowledge on how bacteroid transporters integrate to TM homeostasis mechanisms in the plant host is still limited. This is significantly relevant considering the low solubility and scarcity of TMs in soils, and the in crescendo gradient of TM bioavailability rhizobia faces during the infection and bacteroid differentiation processes. In the present work, we review the main metal transporter families found in rhizobia, their role in free-living conditions and, when known, in symbiosis. We focus on discussing those transporters which could play a significant role in TM-dependent biochemical and physiological processes in the bacteroid, thus paving the way towards an optimized SNF.

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 aminoglycoside resistance-promoting AmgRS envelope stress-responsive two-component system in Pseudomonas aeruginosa is zinc-activated and protects cells from zinc-promoted membrane damage

Exposure of wild-type (WT) Pseudomonas aeruginosa PAO1 to ZnCl2 (Zn) yielded a concentration-dependent increase in depolarization of the cytoplasmic membrane (CM), an indication that this metal is membrane-damaging. Consistent with this, Zn activated the AmgRS envelope stress-responsive two-component system (TCS) that was previously shown to be activated by and to protect P. aeruginosa from the membrane-damaging effects of aminoglycoside (AG) antibiotics. A mutant lacking amgR showed enhanced Zn-promoted CM perturbation and was Zn-sensitive, an indication that the TCS protected cells from the CM-damaging effects of this metal. In agreement with this, a mutant carrying an AmgRS-activating amgS mutation was less susceptible to Zn-promoted CM perturbation and more tolerant of elevated levels of Zn than WT. AG activation of AmgRS is known to drive expression of the AG resistance-promoting mexXY multidrug efflux operon, and while Zn similarly induced mexXY expression this was independent of AmgRS and reliant on a second TCS implicated in mexXY regulation, ParRS. MexXY did not, however, contribute to Zn resistance or protection from Zn-promoted CM damage. Despite its activation of AmgRS and induction of mexXY, Zn had a minimal impact on the AG resistance of WT P. aeruginosa although, given that Zn-tolerant AmgRS-activated amgS mutant strains are AG resistant, there is still the prospect of this metal promoting AG resistance development in this organism.

Enhancement of the adaptability of anammox granules to zinc shock by appropriate organic carbon treatment

Heavy metals, which are commonly present in high ammonia-containing wastewater, can cause inhibitory effects to anammox reaction. This study proposes a novel approach to enhance the adaptability of anammox granules to heavy metal [Zn(II)] shock by organic carbon (sodium acetate, NaAc) treatment, paying special attention to optimization of the treatment dosage and duration. For granules treated with 200 mg chemical oxygen demand (COD)/L NaAc for 2 d, the activity recovery (six cycles) efficiency after Zn(II) (40 mg/L) shock reached 127.4%. The extracellular polymeric substance (EPS) production increased by 168% and heterotrophic bacteria mildly proliferated (increased by 14%) in such granules compared with the control. The dramatic recovery capacity was likely due to the entrapment and barrier function of EPS and the outer-layer proliferated heterotrophic bacteria. This finding offers a useful process to enable maximum adaptability of anammox granules from heavy metals shocks, allowing anammox technology to be more widely applied.

Metallothionein: a Potential Link in the Regulation of Zinc in Nutritional Immunity

Nutritional immunity describes mechanisms for withholding essential transition metals as well as directing the toxicity of these metals against infectious agents. Zinc is one of these transition elements that are essential for both humans and microbial pathogens. At the same time, Zn can be toxic both for man and microbes if its concentration is higher than the tolerance limit. Therefore a "delicate" balance of Zn must be maintained to keep the immune cells surveilling while making the level of Zn either to starve or to intoxicate the pathogens. On the other hand, the invading pathogens will exploit the host Zn pool for its survival and replication. Apparently, different sets of protein in human and bacteria are involved to maintain their Zn need. Metallothionein (MT)-a group of low molecular weight proteins, is well known for its Zn-binding ability and is expected to play an important role in that Zn balance at the time of active infection. However, the differences in structural, functional, and molecular control of biosynthesis between human and bacterial MT might play an important role to determine the proper use of Zn and the winning side. The current review explains the possible involvement of human and bacterial MT at the time of infection to control and exploit Zn for their need.

Zinc depletion promotes apoptosis-like death in drug-sensitive and antimony-resistance Leishmania donovani

Micronutrients are essential for survival and growth for all the organisms including pathogens. In this manuscript, we report that zinc (Zn) chelator N,N,N’,N’-tetrakis(2-pyridinylmethyl)-1,2-ethylenediamine (TPEN) affects growth and viability of intracellular pathogen Leishmania donovani (LD) by a concentration and time dependent manner. Simultaneous addition of zinc salt reverses the effect of TPEN. Further experiments provide evidence of apoptosis-like death of the parasite due to Zn-depletion. TPEN treatment enhances caspase-like activity suggesting increase in apoptosis-like events in LD. Specific inhibitors of cathepsin B and Endoclease G block TPEN-induced leishmanial death. Evidences show involvement of reactive oxygen species (ROS) potentially of extra-mitochondrial origin in TPEN-induced LD death. Pentavalent antimonials remained the prime source of treatment against leishmaniasis for several decades; however, antimony-resistant Leishmania is now common source of the disease. We also reveal that Zn-depletion can promote apoptosis-like death in antimony-resistant parasites. In summary, we present a new finding about the role of zinc in the survival of drug sensitive and antimony-resistant LD.

Origin, Function, and Fate of Metallothionein in Human Blood

Toxic heavy metals, toxic organic compounds, reactive oxygen species (ROS), infections, and temperature are well-known metallothionein (MT) inducers in human blood. The current review aims to summarize synthesis, function, and fate of human blood MT in response to the known MT inducers. Part of the MTs that are synthesized in different organs such as the liver, kidney, and spleen is transported and stored in different blood cells and in plasma. Cells of the circulatory system also synthesize MT. From the circulation, MT returns to the kidney where the metal-bound MTs are degraded to release the metal ion that in turn induces MT expression therein. The blood MTs play important roles in metal detoxification, transportation, and storage. By neutralizing ROS, MTs protect blood cells from oxidative stress-induced cytotoxicity and genotoxicity. Arguably, MTs are also involved in immune suppression. Given the permeating distribution of blood MT throughout the body as well as its diverse role in the protection against harmful environmental factors and in metal homeostasis, MT could be better recognized as a major public health protein.

Potential use of algae for heavy metal bioremediation, a critical review

Algae have several industrial applications that can lower the cost of biofuel co-production. Among these co-production applications, environmental and wastewater bioremediation are increasingly important. Heavy metal pollution and its implications for public health and the environment have led to increased interest in developing environmental biotechnology approaches. We review the potential for algal biosorption and/or neutralization of the toxic effects of heavy metal ions, primarily focusing on their cellular structure, pretreatment, modification, as well as potential application of genetic engineering in biosorption performance. We evaluate pretreatment, immobilization, and factors affecting biosorption capacity, such as initial metal ion concentration, biomass concentration, initial pH, time, temperature, and interference of multi metal ions and introduce molecular tools to develop engineered algal strains with higher biosorption capacity and selectivity. We conclude that consideration of these parameters can lead to the development of low-cost micro and macroalgae cultivation with high bioremediation potential.

Bio-rescue of marine environments: On the track of microbially-based metal/metalloid remediation

The recent awareness of the huge relevance of marine resources and ecological services is driving regulatory demands for their protection from overwhelming contaminants, such as metals/metalloids. These contaminants enter and accumulate in different marine niches, hence deeply compromising their quality and integrity. Bioremediation has been flourishing to counteract metal/metalloid impacts, since it provides cost-effective and sustainable options by relying on ecology-based technologies. The potential of marine microbes for metal/metalloid bioremediation is the core of many studies, due to their high plasticity to overcome successive environmental hurdles. However, any thorough review on the advances of metal/metalloid bioremediation in marine environments was so far unveiled. This review is designed to (i) outline the characteristics and potential of marine microbes for metal/metalloid bioremediation, (ii) describe the underlying pathways of resistance and detoxification, as well as useful methodologies for their characterization, (iii) identify major bottlenecks on metal/metalloid bioremediation with marine microbes, (iv) present alternative strategies based on microbial consortia and engineered microbes for enhanced bioremediation, and (v) propose key research avenues to keep pace with a changing society, science and economy in a sustainable manner.

Assessment of toxicity using dehydrogenases activity and mathematical modeling

Dehydrogenase activity is frequently used to assess the general condition of microorganisms in soil and activated sludge. Many studies have investigated the inhibition of dehydrogenase activity by various compounds, including heavy metal ions. However, the time after which the measurements are carried out is often chosen arbitrarily. Thus, it can be difficult to estimate how the toxic effects of compounds vary during the reaction and when the maximum of the effect would be reached. Hence, the aim of this study was to create simple and useful mathematical model describing changes in dehydrogenase activity during exposure to substances that inactivate enzymes. Our model is based on the Lagergrens pseudo-first-order equation, the rate of chemical reactions, enzyme activity, and inactivation and was created to describe short-term changes in dehydrogenase activity. The main assumption of our model is that toxic substances cause irreversible inactivation of enzyme units. The model is able to predict the maximum direct toxic effect (MDTE) and the time to reach this maximum (TMDTE). In order to validate our model, we present two examples: inactivation of dehydrogenase in microorganisms in soil and activated sludge. The model was applied successfully for cadmium and copper ions. Our results indicate that the predicted MDTE and TMDTE are more appropriate than EC50 and IC50 for toxicity assessments, except for long exposure times.

The Tetrahymena metallothionein gene family: twenty-one new cDNAs, molecular characterization, phylogenetic study and comparative analysis of the gene expression under different abiotic stressors

Background

Ciliate metallothioneins (MTs) are included in family 7 of the MT superfamily. This family has been divided into two main subfamilies: 7a or CdMTs and 7b or CuMTs. All ciliate MTs reported have been isolated from different Tetrahymena species and present unique features with regard to standard MTs. Likewise, an expression analysis has been carried out on some of MT genes under metal stress, corroborating their classification into two subfamilies.

Results

We isolated 21 new cDNAs from different Tetrahymena species to obtain a wider view of the biodiversity of these conserved genes. Structural analysis (cysteine patterns) and an updated phylogenetic study both corroborated the previous classification into two subfamilies. A new CuMT from a Tetrahymena-related species Ichthyophthirius multifiliis was also included in this general analysis. We detected a certain tendency towards the presentation of a CdMT tri-modular structure in Borealis group species with respect to Australis group. We report for the first time a semi-complete paralog duplication of a CdMT gene originating a new CdMT gene isoform in T. malaccensis. An asymmetry of the codon usage for glutamine residues was detected between Cd- and CuMTs, and the phylogenetic implications are discussed. A comparative gene expression analysis of several MT genes by qRT-PCR revealed differential behavior among them under different abiotic stressors in the same Tetrahymena species.

Conclusions

The Tetrahymena metallothionein family represents a quite conserved protein structure group with unique features with respect to standard MTs. Both Cd- and CuMT subfamilies present very defined and differentiated characteristics at several levels: cysteine patterns, modular structure, glutamine codon usage and gene expression under metal stress, among others. Gene duplication through evolution seems to be the major genetic mechanism for creating new MT gene isoforms and increasing their functional diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2658-6) contains supplementary material, which is available to authorized users.

Construction of a self-luminescent cyanobacterial bioreporter that detects a broad range of bioavailable heavy metals in aquatic environments

A self-luminescent bioreporter strain of the unicellular cyanobacterium Synechococcus sp. PCC 7942 was constructed by fusing the promoter region of the smt locus (encoding the transcriptional repressor SmtB and the metallothionein SmtA) to luxCDABE from Photorhabdus luminescens; the sensor smtB gene controlling the expression of smtA was cloned in the same vector. The bioreporter performance was tested with a range of heavy metals and was shown to respond linearly to divalent Zn, Cd, Cu, Co, Hg, and monovalent Ag. Chemical modeling was used to link bioreporter response with metal speciation and bioavailability. Limits of Detection (LODs), Maximum Permissive Concentrations (MPCs) and dynamic ranges for each metal were calculated in terms of free ion concentrations. The ranges of detection varied from 11 to 72 pM for Hg²⁺ (the ion to which the bioreporter was most sensitive) to 1.54–5.35 μM for Cd²⁺ with an order of decreasing sensitivity as follows: Hg²⁺ >> Cu²⁺ >> Ag⁺ > Co²⁺ ≥ Zn²⁺ > Cd²⁺. However, the maximum induction factor reached 75-fold in the case of Zn²⁺ and 56-fold in the case of Cd²⁺, implying that Zn²⁺ is the preferred metal in vivo for the SmtB sensor, followed by Cd²⁺, Ag⁺ and Cu²⁺ (around 45–50-fold induction), Hg²⁺ (30-fold) and finally Co²⁺ (20-fold). The bioreporter performance was tested in real environmental samples with different water matrix complexity artificially contaminated with increasing concentrations of Zn, Cd, Ag, and Cu, confirming its validity as a sensor of free heavy metal cations bioavailability in aquatic environments.

Metals in cyanobacteria: analysis of the copper, nickel, cobalt and arsenic homeostasis mechanisms

Traces of metal are required for fundamental biochemical processes, such as photosynthesis and respiration. Cyanobacteria metal homeostasis acquires an important role because the photosynthetic machinery imposes a high demand for metals, making them a limiting factor for cyanobacteria, especially in the open oceans. On the other hand, in the last two centuries, the metal concentrations in marine environments and lake sediments have increased as a result of several industrial activities. In all cases, cells have to tightly regulate uptake to maintain their intracellular concentrations below toxic levels. Mechanisms to obtain metal under limiting conditions and to protect cells from an excess of metals are present in cyanobacteria. Understanding metal homeostasis in cyanobacteria and the proteins involved will help to evaluate the use of these microorganisms in metal bioremediation. Furthermore, it will also help to understand how metal availability impacts primary production in the oceans. In this review, we will focus on copper, nickel, cobalt and arsenic (a toxic metalloid) metabolism, which has been mainly analyzed in model cyanobacterium Synechocystis sp. PCC 6803.

Bacillithiol, a new role in buffering intracellular zinc

Zinc is a catalytic or structural cofactor of numerous proteins but can also be toxic if cells accumulate too much of this essential metal. Therefore, mechanisms of zinc homeostasis are needed to maintain a low but adequate amount of free zinc so that newly translated zinc-dependent proteins can bind their cofactor without confounding issues of toxicity. These mechanisms include the regulation of uptake and efflux transporters and buffering of the free metal concentration by low-molecular-weight ligands in the cytosol. While many of the transporters involved in zinc homeostasis have been discovered in recent years, the molecules that buffer zinc have remained largely a mystery. In the new report highlighted by this commentary, Ma et al. (2014) provide convincing evidence that bacillithiol, the major low-molecular-weight thiol compound in Bacillus subtilis, serves as an important zinc buffer in those cells. Their discovery provides an important piece to the puzzle of how zinc buffering occurs in a large number of microbes and provides new clues about the role and relative importance of zinc buffering in all organisms.

Proteomic responses of oceanic Synechococcus WH8102 to phosphate and zinc scarcity and cadmium additions

Synechococcus sp. WH 8102 is a motile marine cyanobacterium isolated originally from the Sargasso Sea. To test the response of this organism to cadmium (Cd), generally considered a toxin, cultures were grown in a matrix of high and low zinc (Zn) and phosphate (PO₄³⁻) and were then exposed to an addition of 4.4 pM free Cd²⁺ at mid-log phase and harvested after 24 h. Whereas Zn and PO₄³⁻ had little effect on overall growth rates, in the final 24 h of the experiment three growth effects were noticed: (i) low PO₄³⁻ treatments showed increased growth rates relative to high PO₄³⁻ treatments, (ii) the Zn/high PO₄³⁻ treatment appeared to enter stationary phase, and (iii) Cd increased growth rates further in both the low PO₄³⁻ and Zn treatments. Global proteomic analysis revealed that: (i) Zn appeared to be critical to the PO₄³⁻ response in this organism, (ii) bacterial metallothionein (SmtA) appears correlated with PO₄³⁻ stress-associated proteins, (iii) Cd has the greatest influence on the proteome at low PO₄³⁻ and Zn, (iv) Zn buffered the effects of Cd, and (v) in the presence of both replete PO₄³⁻ and added Cd the proteome showed little response to the presence of Zn. Similar trends in alkaline phosphate (ALP) and SmtA suggest the possibility of a Zn supply system to provide Zn to ALP that involves SmtA. In addition, proteome results were consistent with a previous transcriptome study of PO₄³⁻ stress (with replete Zn) in this organism, including the greater relative abundance of ALP (PhoA), ABC phosphate binding protein (PstS) and other proteins. Yet with no Zn in this proteome experiment the PO₄³⁻ response was quite different including the greater relative abundance of five hypothetical proteins with no increase in PhoA or PstS, suggesting that Zn nutritional levels are connected to the PO₄³⁻ response in this cyanobacterium. Alternate ALP PhoX (Ca) was found to be a low abundance protein, suggesting that PhoA (Zn, Mg) may be more environmentally relevant than PhoX.

How prokaryotes deal with arsenic(†)

Arsenic is a notorious poison classified as a carcinogen, a teratogen and a clastogen that ranks number one on the Environmental Protection Agency's priority list of drinking water contaminants. It is ubiquitous and relatively abundant in the Earth's crust. Its mobilization in waters by weathering, volcanic, anthropogenic or biological activities represents a major hazard to public health, exemplified in India and Bangladesh where 50 million people are acutely at risk. Since basically the origin of life, microorganisms have been exposed to this toxic compound and have evolved a variety of resistance mechanisms, such as extracellular precipitation, chelation, intracellular sequestration, active extrusion from the cell or biochemical transformation (redox or methylation). Arsenic efflux systems are widespread and are found in nearly all organisms. Some microorganisms are also able to utilize this metalloid as a metabolic energy source through either arsenite oxidation or arsenate reduction. The energy metabolism involving redox reactions of arsenic has been suggested to have evolved during early life on Earth. This review highlights the different systems evolved by prokaryotes to cope with arsenic and how they participate in its biogeochemical cycle.

Mining genomes of marine cyanobacteria for elements of zinc homeostasis

Zinc is a recognized essential element for the majority of organisms, and is indispensable for the correct function of hundreds of enzymes and thousands of regulatory proteins. In aquatic photoautotrophs including cyanobacteria, zinc is thought to be required for carbonic anhydrase and alkaline phosphatase, although there is evidence that at least some carbonic anhydrases can be cambialistic, i.e., are able to acquire in vivo and function with different metal cofactors such as Co²⁺ and Cd²⁺. Given the global importance of marine phytoplankton, zinc availability in the oceans is likely to have an impact on both carbon and phosphorus cycles. Zinc concentrations in seawater vary over several orders of magnitude, and in the open oceans adopt a nutrient-like profile. Most studies on zinc handling by cyanobacteria have focused on freshwater strains and zinc toxicity; much less information is available on marine strains and zinc limitation. Several systems for zinc homeostasis have been characterized in the freshwater species Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803, but little is known about zinc requirements or zinc handling by marine species. Comparative metallo-genomics has begun to explore not only the putative zinc proteome, but also specific protein families predicted to have an involvement in zinc homeostasis, including sensors for excess and limitation (SmtB and its homologs as well as Zur), uptake systems (ZnuABC), putative intracellular zinc chaperones (COG0523) and metallothioneins (BmtA), and efflux pumps (ZiaA and its homologs).

Chemical and structural status of copper associated with oxygenic and anoxygenic phototrophs and heterotrophs: possible evolutionary consequences

Copper adsorption on the surface and intracellular uptake inside the cells of four representative taxons of soil and aquatic micro-organisms: aerobic rhizospheric heterotrophs (Pseudomonas aureofaciens), anoxygenic (Rhodovulum steppense) and oxygenic (cyanobacteria Gloeocapsa sp. and freshwater diatoms Navicula minima) phototrophs were studied in a wide range of pH, copper concentration, and time of exposure. Chemical status of adsorbed and assimilated Cu was investigated using in situ X-ray absorption spectroscopy. In case of adsorbed copper, XANES spectra demonstrated significant fractions of Cu(I) likely in the form of tri-coordinate complexes with O/N and/or S ligands. Upon short-term reversible adsorption at all four studied micro-organisms' cell surface, Cu(II) is coordinated by 4.0 ± 0.5 planar oxygens at an average distance of 1.97 ± 0.02 Å, which is tentatively assigned to the carboxylate groups. The atomic environment of copper incorporated into diatoms and cyanobacteria during long-term growth is similar to that of the adsorbed metal with slightly shorter distances to the first O/N neighbor (1.95 Å). In contrast to the common view of Cu status in phototrophic micro-organisms, XAFS failed to detect sulfur in the nearest atomic environment of Cu assimilated by freshwater plankton (cyanobacteria) and periphyton (diatoms). The appearance of S in Cu 1st coordination shell at 2.27-2.32 Å was revealed only after long-term interaction of Cu with anoxygenic phototrophs (and Cu uptake by soil heterotrophs), suggesting Cu scavenging in the form of sulfhydryl, histidine/carboxyl or a mixture of carboxylate and sulfhydryl complexes. These new structural constraints suggest that adsorbed Cu(II) is partially reduced to Cu(I) already at the cell surface, where as intracellular Cu uptake and storage occur in the form of both Cu(I)-S linked proteins and Cu(II) carboxylates. Obtained results allow to better understand how, in the course of biological evolution, micro-organisms elaborated various mechanisms of Cu uptake and storage, from passive adsorption and uptake to active, protein-controlled surface reduction, and intracellular storage.

Characterization of MtnE, the fifth metallothionein member in Drosophila

Metallothioneins (MTs) constitute a family of cysteine-rich, low molecular weight metal-binding proteins which occur in almost all forms of life. They bind physiological metals, such as zinc and copper, as well as nonessential, toxic heavy metals, such as cadmium, mercury, and silver. MT expression is regulated at the transcriptional level by metal-regulatory transcription factor 1 (MTF-1), which binds to the metal-response elements (MREs) in the enhancer/promoter regions of MT genes. Drosophila was thought to have four MT genes, namely, MtnA, MtnB, MtnC, and MtnD. Here we characterize a new fifth member of Drosophila MT gene family, coding for metallothionein E (MtnE). The MtnE transcription unit is located head-to-head with the one of MtnD. The intervening sequence contains four MREs which bind, with different affinities, to MTF-1. Both of the divergently transcribed MT genes are completely dependent on MTF-1, whereby MtnE is consistently more strongly transcribed. MtnE expression is induced in response to heavy metals, notably copper, mercury, and silver, and is upregulated in a genetic background where the other four MTs are missing.

Responses of environmental Amycolatopsis strains to copper stress

Copper is a redox-active metal, which acts as a catalyst in the formation of Reactive Oxygen Species (ROS) encouraging oxidative stress. Protection against oxidants is intrinsic to every living cell; however, in stress conditions, cells are forced to increase and expand their antioxidative network. In this work, the novel copper-resistant strain Amycolatopsis tucumanensis and the copper-sensitive Amycolatopsis eurytherma were grown under copper increasing concentrations in order to elucidate the dissimilar effects of the metal on the strains viability, mainly on morphology and antioxidant capacity. Although biosorbed copper encouraged ROS production in a dose-dependent manner in both strains, the increase in ROS production from the basal level to the stress conditions in A. tucumanensis is lesser than in the copper-sensitive strain; likewise, in presence of copper A. eurytherma suffered inexorable morphological alteration while A. tucumanensis was not affected. The levels of antioxidant enzymes and metallothioneins (MT) were all greater in A. tucumanensis than in A. eurytherma; in addition MT levels as well as superoxide dismutase and thioredoxin reductase activities in A. tucumanensis, were higher as higher the concentration of copper in the culture medium. This work has given evidence that an efficient antioxidant defense system might aid microorganisms to survive in copper-stress conditions; besides it constitutes the first report of oxidative stress study in the genus Amycolatopsis and contributes to enlarge the knowledge on the copper-resistance mechanisms of A. tucumanensis.

In silico prediction of potential metallothioneins and metallohistins in actinobacteria

Metallothioneins and metallohistins are short peptides with a high cysteine and/or histidine content able to coordinate metals intracellularly, thereby increasing the tolerance against elevated concentrations of metals. Because of their features, they can be detected by in silico prediction from proteomes annotated from sequenced genomes. Here, we analyzed 73 sequenced actinobacterial genomes for peptides (≤ 100 amino acids) with a high content of cysteine and histidine (≥ 15%) and identified 103 putative metallothioneins and metallohistins. For 45 of these peptides, we found similarities to metal binding protein domains, including zinc fingers, heavy metal transporters or eukaryotic metallothioneins, which can serve as proof-of-principle in underscoring a potential function as metal binding peptides. An evolutionary origin from metal containing domains of enzymes is discussed and metallohistins not containing cysteine are described for the first time for bacteria.

Metallomics: lessons for metalliferous soil remediation

The term metallomics has been established for the investigation of transcriptome, proteome, and metabolome changes induced by metals. The mechanisms allowing the organisms to cope with metals in the environment, metal resistance factors, will in turn change biogeochemical cycles of metals in soil, coupling the metal pool with the root system of plants. This makes microorganisms key players in introducing metals into food webs, as well as for bioremediation strategies. Research on physiological and metabolic responses of microorganisms on metal stress in soil is thus essential for the selection of optimized consortia applicable in bioremediation strategies such as bioaugmentation or microbially enhanced phytoextraction. The results of metallomics studies will help to develop applications including identification of biomarkers for ecotoxicological studies, bioleaching, in situ soil regeneration, and microbially assisted phytoremediation of contaminated land. This review will therefore focus on the molecular understanding of metal resistance in bacteria and fungi, as can be derived from metallomics studies.

Equilibrium and kinetic approaches for studying oligomeric protein folding

This chapter describes the approaches and considerations necessary for extension of current protein folding methods to the equilibrium and kinetic reactions of oligomeric proteins, using dimers as the primary example. Spectroscopic and transport methods to monitor folding and unfolding transitions are summarized. The data collection and analyses to determine protein stability and kinetic folding mechanisms are discussed in the context of the additional dimension of complexity that arises in higher order folding processes, compared to first order monomeric proteins. As a case study to illustrate the data analysis process, equilibrium, and kinetic data are presented for SmtB, a homodimeric DNA-binding protein from Synechococcus PCC7942.

Regulation of Metallothionein Gene Expression

Organisms from bacteria to humans use elaborate systems to regulate levels of bioavailable zinc, copper, and other essential metals. An excess of them, or even traces of non-essential metals such as cadmium and mercury, can be highly toxic. Metallothioneins (MTs), short, cysteine-rich proteins, play pivotal roles in metal homeostasis and detoxification. With their sulfhydryl groups they avidly bind toxic metals and also play a role in cellular redox balance and radical scavenging. The intracellular concentration of MTs is adjusted to cellular demand primarily via regulated transcription. Especially upon heavy metal load, metallothionein gene transcription is strongly induced. From insects to mammals, the major regulator of MT transcription is MTF-1 (metal-responsive transcription factor 1), a zinc finger protein that binds to specific DNA sequence motifs (MREs) in the promoters of MT genes and other metal-regulated genes. This chapter provides an overview of our current knowledge on the expression and regulation of MT genes in higher eukaryotes, with some reference also to fungi which apparently have independently evolved their own regulatory systems.

Zinc-handling in cyanobacteria: an update

Zinc is a constituent of all six classes of enzymes, plays important roles in gene regulation, and is thought to be essential for most organisms. Despite initial discoveries of cyanobacterial metallothioneins, zinc efflux pumps and uptake systems, and zinc sensors, our knowledge of the zinc requirements, uptake, and detoxification mechanisms of cyanobacteria is still limited. Although cyanobacteria occupy extremely diverse habitats, most available data pertains to freshwater species, and almost no studies of zinc-handling mechanisms have been conducted in marine species. The current report highlights what is known about zinc homeostasis in cyanobacteria, and presents an analysis of the 40 sequenced cyanobacterial genomes.

Cyanobacteria Metal Interactions: Requirements, Toxicity, and Ecological Implications

The environmental health-related relevance of cyanobacteria is primarily related to their ability to produce a wide range of toxins, which are known to be hazardous to many organisms, including human beings. The occurrence of cyanobacterial blooms has been related to eutrophic surface water. In the bloom-forming process the levels of phosphorus and nitrogen have been well documented but information regarding concentrations of other chemicals (inorganic, organo-metallic, and organic) is still incipient. Several contaminants, like trace metals, elicit a variety of acute and chronic toxicity effects, but cyanobacteria also have the capability to accumulate, detoxify, or metabolize such substances, to some extent. The role of cyanobacterial exudates has been proved a means of both nutrient acquisition and detoxification. In addition, cyanobacteria are effective biological metal sorbents, representing an important sink for metals in aquatic environment. Understanding the fundamental physicochemical mechanisms of trace metal bio-uptake by cyanobacteria in natural systems is a step towards identifying under what conditions cyanobacterial growth is favored and to ascertain the mechanisms by which blooms (and toxin production) are triggered. In this review the cyanobacterial interactions with metals will be discussed, focusing on freshwater systems.

Identification of a copper-binding metallothionein in pathogenic mycobacteria

A screen of a genomic library from Mycobacterium tuberculosis (Mtb) identified a small, unannotated open reading frame (MT0196) that encodes a 4.9-kDa, cysteine-rich protein. Despite extensive nucleotide divergence, the amino acid sequence is highly conserved among mycobacteria that are pathogenic in vertebrate hosts. We synthesized the protein and found that it preferentially binds up to six Cu(I) ions in a solvent-shielded core. Copper, cadmium and compounds that generate nitric oxide or superoxide induced the gene's expression in Mtb up to 1,000-fold above normal expression. The native protein bound copper within Mtb and partially protected Mtb from copper toxicity. We propose that the product of the MT0196 gene be named mycobacterial metallothionein (MymT). To our knowledge, MymT is the first metallothionein of a Gram-positive bacterium with a demonstrated function.

Two new members of the Tetrahymena multi-stress-inducible metallothionein family: characterization and expression analysis of T. rostrata Cd/Cu metallothionein genes

We report the cloning and characterization of two new metallothionein (MT) genes (TrosMTT1 and TrosMTT2), isolated as cDNAs, from the ciliated protozoa Tetrahymena rostrata. The TrosMTT1 inferred protein has been identified as a CdMT and included into the 7a subfamily of Tetrahymena MTs, while TrosMTT2 has been identified as a CuMT (including it into 7b subfamily), due to its similarity to TpigMT-2 and its significant induction by copper. TrosMTT1 protein sequence reveals a remarkably regular and hierarchical modular organization, as it is known for other Tetrahymena CdMTs, showing a bi-modular structure. TrosMTT2 presents a structural organization based on CKCX(2-5)CKC repeats, like it occurs in other Tetrahymena CuMTs, indicating that an evolutionary history based on intra-gene duplications might be also possible. Both are also multi-stress-inducible genes because they are induced by other heavy metals and stressors, as it has been shown by quantitative real-time RT-PCR. It is the first time that the gene expression of a putative Tetrahymena CuMT is analyzed by quantitative PCR, confirming it as a CuMT. These two new Tetrahymena MTs complete, at present, the actual view of this protein superfamily, and corroborate the unique features of ciliate MTs. Furthermore, both, a comparative analysis of relative gene expression values obtained by quantitative RT-PCR on other Tetrahymena MT genes and an analysis of the different Tetrahymena MTs based on the different Cys clusters of these proteins are carried out, which show an update view of Tetrahymena MT gene family.

Metallothioneins with unusual residues: histidines as modulators of zinc affinity and reactivity

For many years, paradigms regarding metallothioneins comprised the exclusive metal coordination by thiolates from cysteine residues and the absence of aromatic residues. As more sequence and in vitro data on metallothioneins, in particular from non-vertebrate organisms, has become available, both the occurrence of and metal coordination by histidine residues in metallothioneins is emerging as a more frequent feature than expected. We discuss the general implications of histidines versus cysteines in zinc binding sites, and review some recent results from literature and our own lab. We conclude that histidines can stabilise metallothionein clusters by reducing the overall charge, offering the ability to help with structural organisation by supplying H-bond donor and acceptor properties, reducing the likelihood for disulfide bond formation, whilst maintaining a high affinity towards metal ions, in particular the borderline zinc ion.

Quantification of changing Pseudomonas aeruginosa sodA, htpX and mt gene abundance in response to trace metal toxicity: a potential in situ biomarker of environmental health

Sediment-dwelling prokaryotes play a vital role in determining the fate and speciation of metals, yet are also susceptible to the biological effects of trace metals. In this article, optimized DNA extraction and purification techniques and species-specific primers are used to assess the genetic incidence and abundance of metal detoxification and general stress genes of Pseudomonas aeruginosa to complement chemical analysis in inferring the severity of metal-contaminated sites along the Clark Fork River, Montana. Results show the highest incidence of candidate genes related to bacterial stress at the most polluted site, while multiple regression analysis demonstrated significant correlations (P<0.05, r(2)=0.9) between in situ metal concentrations (As, Cu and Zn), total gene incidence, and the incidence of metal detoxification genes. Furthermore, principal components plotting the incidence of genes related to metal resistance show clear separation of sites giving clear clusters on the basis of contamination. Quantification of three genes (sodA, htpX and mt) from surveyed sites found significantly higher (anova, P<0.05) copy numbers at the more contaminated sites compared with reference sites. The development of rapid microbial biomarker tools represents a significant advance in the field of environmental biomonitoring and the prediction of metal bioavailability.

Tetrahymena metallothioneins fall into two discrete subfamilies

BACKGROUND

Metallothioneins are ubiquitous small, cysteine-rich, multifunctional proteins which can bind heavy metals.

METHODOLOGY/PRINCIPAL FINDINGS

We report the results of phylogenetic and gene expression analyses that include two new Tetrahymena thermophila metallothionein genes (MTT3 and MTT5). Sequence alignments of all known Tetrahymena metallothioneins have allowed us to rationalize the structure of these proteins. We now formally subdivide the known metallothioneins from the ciliate genus Tetrahymena into two well defined subfamilies, 7a and 7b, based on phylogenetic analysis, on the pattern of clustering of Cys residues, and on the pattern of inducibility by the heavy metals Cd and Cu. Sequence alignment also reveals a remarkably regular, conserved and hierarchical modular structure of all five subfamily 7a MTs, which include MTT3 and MTT5. The former has three modules, while the latter has only two. Induction levels of the three T. thermophila genes were determined using quantitative real time RT-PCR. Various stressors (including heavy metals) brought about dramatically different fold-inductions for each gene; MTT5 showed the highest fold-induction. Conserved DNA motifs with potential regulatory significance were identified, in an unbiased way, upstream of the start codons of subfamily 7a MTs. EST evidence for alternative splicing in the 3' UTR of the MTT5 mRNA with potential regulatory activity is reported.

CONCLUSION/SIGNIFICANCE

The small number and remarkably regular structure of Tetrahymena MTs, coupled with the experimental tractability of this model organism for studies of in vivo function, make it an attractive system for the experimental dissection of the roles, structure/function relationships, regulation of gene expression, and adaptive evolution of these proteins, as well as for the development of biotechnological applications for the environmental monitoring of toxic substances.

Intracellular sequestration of manganese and phosphorus in a metal-resistant fungus Cladosporium cladosporioides from deep-sea sediment

A heavy metal resistant fungus was isolated from the sediment of Pacific Ocean, and identified to be Cladosporium cladosporioides. It grew normally in a medium containing 60 mM Mn(2+) and could endure 1,200 mM as the highest concentration tested. Quantification analysis confirmed a high accumulation of Mn which was 58 mg/g in dried biomass. Under transmission electron microscope, many intracellular crystals were observed in the cytoplasm of the hypha cells grown in a Mn-rich medium, and varied from a few nanometers to 200 nm in length. Energy dispersive X-ray (EDX) analysis showed that the crystals were composed of manganese and phosphorus in atomic ratio of 1.6:1 (Mn/P). Further, factors which might influence the resistance of this fungus were investigated. As a result, its high resistance to Mn(2+) was found dependent on the presence of Mg(2+), and could be further enhanced by phosphate. However, the effect of phosphate was not observed without the presence of Mg(2+). In addition, the resistance was also influenced by pH of the medium, which was lost above pH 8. This is the first report on a fungus which showed a hyper resistance to manganese by forming a large quantity of intracellular Mn/P crystals.

Structural diversity of bacterial communities in a heavy metal mineralized granite outcrop

This laboratory study of a variably mineralized and hydrothermally altered granite outcrop investigated the influences of rock-surface chemistry and heavy metal content on resident bacterial populations. Results indicated that elevated heavy metal concentrations had a profound impact on bacterial community structure, with strong relationships found between certain ribotypes and particular chemical/heavy metal elements. Automated ribosomal intergenic sequence analysis (ARISA) was used to assess the nature and extent of bacterial diversity, and this was combined with chemical analysis and multivariate statistics to identify the main geochemical factors influencing bacterial community structure. A randomization test revealed significant changes in bacterial structure between samples, while canonical correspondence analysis (CCA) related each individual ARISA profile to linear combinations of the chemical variables (mineralogy, major element and heavy metal concentrations) revealing the geochemical factors that correlated with changes in the ARISA data. anova was performed to further explore interactions between individual ribotypes and chemical/heavy metal composition, and revealed that a high proportion of ribotypes correlated significantly with heavy metals.

Characterization of a mercury-reducing Bacillus cereus strain isolated from the Pulicat Lake sediments, south east coast of India

Pulicat Lake sediments are often severely polluted with the toxic heavy metal mercury. Several mercury-resistant strains of Bacillus species were isolated from the sediments and all the isolates exhibited broad spectrum resistance (resistance to both organic and inorganic mercuric compounds). Plasmid curing assay showed that all the isolated Bacillus strains carry chromosomally borne mercury resistance. Polymerase chain reaction and southern hybridization analyses using merA and merB3 gene primers/probes showed that five of the isolated Bacillus strains carry sequences similar to known merA and merB3 genes. Results of multiple sequence alignment revealed 99% similarity with merA and merB3 of TnMERI1 (class II transposons). Other mercury resistant Bacillus species lacking homology to these genes were not able to volatilize mercuric chloride, indicating the presence of other modes of resistance to mercuric compounds.

Diamide stress induces a metallothionein BmtA through a repressor BxmR and is modulated by Zn-inducible BmtA in the cyanobacterium Oscillatoria brevis

A cysteine-rich metal binding protein MT (metallothionein) (named BmtA) is induced upon exposure to multiple heavy metal ions in the freshwater cyanobacterium Oscillatoria brevis. The SmtB/ArsR family repressor BxmR from O. brevis represses the expression of an operon encoding bmtA and bxmR. In the present study, the expression of bmtA was induced in vivo by diamide, a specific thiol oxidant, in O. brevis cells. In vitro electrophoretic gel mobility shift experiments revealed that the incubation with diamide induces disassembly of the BxmR-bxmR/bmtA operator (O)/promoter (P) complex [multiple resolvable complexes of BxmR with oligonucleotide (named P5) containing a single 12-2-12 inverted repeat derived from the O/P region of bxmR/bmtA]. Thus, the exposure to diamide induces MT mRNA in O. brevis, and this induction is associated with diamide-mediated inhibition of BxmR-P5 complex. BxmR is more sensitive to diamide than to H(2)O(2). Furthermore, pretreatment of O. brevis with Zn decreased intracellular peroxidation products caused by diamide. Thus, these results imply that MT induced by Zn-pretreatment functions to protect O. brevis cells against diamide stress.

Rv2358 and FurB: two transcriptional regulators from Mycobacterium tuberculosis which respond to zinc

In a previous work, we demonstrated that the Mycobacterium tuberculosis Rv2358-furB operon is induced by zinc. In this study, the orthologous genes from Mycobacterium smegmatis mc(2)155 were inactivated and mutants analyzed. Rv2358 protein was purified and found to bind upstream of the Rv2358 gene. Binding was inhibited by Zn(2+) ions.