Characterization of a Highly pH Stable Chi-Class Glutathione S-Transferase from Synechocystis PCC 6803

Two novel glutathione S-transferase (GST) genes in the toxic marine dinoflagellate Alexandrium pacificum and their transcriptional responses to environmental contaminants

The present study identified two novel glutathione S-transferase (GST) genes from the toxic dinoflagellate Alexandrium pacificum and examined their molecular characteristics and transcriptional responses to algicides and environmental contaminants. Bioinformatic analysis revealed that both ApGSTs are cytosolic, belonging to the chi-like class (ApGST1) and an undefined class (ApGST2). The overall expression of ApGSTs showed similar patterns depending on the exposed contaminants, while they were differently regulated by polychlorinated biphenyl (PCB). Copper treatments (CuCl2 and CuSO4) did not significantly induce the expression of ApGSTs. The highest up-regulations of ApGST1 and ApGST2 were under 6-h treatments of 0.10 and 0.50 mg L-1 NaOCl. Interestingly, only ApGST1 increased significantly after 0.10, 0.50, and 1.00 mg L-1 of PCB exposure (6 h). Intracellular reactive oxygen species (ROS) increased considerably under NaOCl; however, it was not significantly higher in the PCB-treated cells. GST activity was increased by NaOCl and PCB treatments, but only PCB caused apoptosis. These results suggest that GSTs are involved in the first line of phase II detoxification, protecting dinoflagellate cells against oxidative damage.

In Vitro Enzyme Kinetics and NMR-Based Product Elucidation for Glutathione S-Conjugation of the Anticancer Unsymmetrical Bisacridine C-2028 in Liver Microsomes and Cytosol: Major Role of Glutathione S-Transferase M1-1 Isoenzyme

This work is the next step in studying the interplay between C-2028 (anticancer-active unsymmetrical bisacridine developed in our group) and the glutathione S-transferase/glutathione (GST/GSH) system. Here, we analyzed the concentration- and pH-dependent GSH conjugation of C-2028 in rat liver microsomes and cytosol. We also applied three recombinant human GST isoenzymes, which altered expression was found in various tumors. The formation of GSH S-conjugate of C-2028 in liver subfractions followed Michaelis-Menten kinetics. We found that C-2028 was conjugated with GSH preferentially by GSTM1-1, revealing a sigmoidal kinetic model. Using a colorimetric assay (MTT test), we initially assessed the cellular GST/GSH-dependent biotransformation of C-2028 in relation to cytotoxicity against Du-145 human prostate cancer cells in the presence or absence of the modulator of GSH biosynthesis. Pretreatment of cells with buthionine sulfoximine resulted in a cytotoxicity decrease, suggesting a possible GSH-mediated bioactivation process. Altogether, our results confirmed the importance of GSH conjugation in C-2028 metabolism, which humans must consider when planning a treatment strategy. Finally, nuclear magnetic resonance spectroscopy elucidated the structure of the GSH-derived product of C-2028. Hence, synthesizing the compound standard necessary for further advanced biological and bioanalytical investigations will be achievable.

The Glutathione System: A Journey from Cyanobacteria to Higher Eukaryotes

From bacteria to plants and humans, the glutathione system plays a pleiotropic role in cell defense against metabolic, oxidative and metal stresses. Glutathione (GSH), the γ-L-glutamyl-L-cysteinyl-glycine nucleophile tri-peptide, is the central player of this system that acts in redox homeostasis, detoxification and iron metabolism in most living organisms. GSH directly scavenges diverse reactive oxygen species (ROS), such as singlet oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, nitric oxide and carbon radicals. It also serves as a cofactor for various enzymes, such as glutaredoxins (Grxs), glutathione peroxidases (Gpxs), glutathione reductase (GR) and glutathione-S-transferases (GSTs), which play crucial roles in cell detoxication. This review summarizes what is known concerning the GSH-system (GSH, GSH-derived metabolites and GSH-dependent enzymes) in selected model organisms (Escherichia coli, Saccharomyces cerevisiae, Arabidopsis thaliana and human), emphasizing cyanobacteria for the following reasons. Cyanobacteria are environmentally crucial and biotechnologically important organisms that are regarded as having evolved photosynthesis and the GSH system to protect themselves against the ROS produced by their active photoautotrophic metabolism. Furthermore, cyanobacteria synthesize the GSH-derived metabolites, ergothioneine and phytochelatin, that play crucial roles in cell detoxication in humans and plants, respectively. Cyanobacteria also synthesize the thiol-less GSH homologs ophthalmate and norophthalmate that serve as biomarkers of various diseases in humans. Hence, cyanobacteria are well-suited to thoroughly analyze the role/specificity/redundancy of the players of the GSH-system using a genetic approach (deletion/overproduction) that is hardly feasible with other model organisms (E. coli and S. cerevisiae do not synthesize ergothioneine, while plants and humans acquire it from their soil and their diet, respectively).

Physiological Responses of a Diazotrophic Cyanobacterium to Acidification of Paddy Floodwater: N2 Fixation, Photosynthesis, and Oxidative-Antioxidative Characteristics

Long-term of excessive fertilization using nitrogen (N) chemical fertilizer caused the acidification of paddy soils. Presently, the impacts of soil acidification on physiological characteristics of diazotrophic cyanobacteria remain unknown. In order to elucidate this issue, the effects of paddy floodwater acidification on activities of respiration, photosynthetic oxygen evolution, and N₂ fixation of a paddy diazotrophic cyanobacterium Aliinostoc sp. YYLX235 were investigated in this study. In addition, the origination and quenching of intracellular reactive oxygen species (ROS) were analyzed. The acidification of paddy floodwater decreased intracellular pH and interfered in energy flux from light-harvesting chlorophyll antenna to the reaction center of photosystem II (PS II). Activities of respiration, photosynthetic oxygen evolution, and N₂ fixation were decreased by the acidification of paddy floodwater. Accompanied with an increase in ROS, the level of antioxidative system increased. Superoxide dismutase (SOD) and catalase (CAT) were the main enzymatic ROS scavengers in the cells of YYLX235; reduced glutathione (GSH) was the main non-enzymatic antioxidant. Antioxidants and oxidants in the cells of YYLX235 lost balance when the pH of paddy floodwater fell to 5.0 and 4.0, and lipid oxidative damage happened. The results presented in this study suggest that the acidification of paddy soil severely interfered in the photosynthesis of diazotrophic cyanobacteria and induced the production of ROS, which in turn resulted in oxidative damage on diazotrophic cyanobacteria and a decrease in cell vitality.

Biochemical and Structural Characterization of Chi-Class Glutathione Transferases: A Snapshot on the Glutathione Transferase Encoded by sll0067 Gene in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Glutathione transferases (GSTs) constitute a widespread superfamily of enzymes notably involved in detoxification processes and/or in specialized metabolism. In the cyanobacterium Synechocsytis sp. PCC 6803, SynGSTC1, a chi-class GST (GSTC), is thought to participate in the detoxification process of methylglyoxal, a toxic by-product of cellular metabolism. A comparative genomic analysis showed that GSTCs were present in all orders of cyanobacteria with the exception of the basal order Gloeobacterales. These enzymes were also detected in some marine and freshwater noncyanobacterial bacteria, probably as a result of horizontal gene transfer events. GSTCs were shorter of about 30 residues compared to most cytosolic GSTs and had a well-conserved SRAS motif in the active site (¹⁰SRAS¹³ in SynGSTC1). The crystal structure of SynGSTC1 in complex with glutathione adopted the canonical GST fold with a very open active site because the α4 and α5 helices were exceptionally short. A transferred multipolar electron-density analysis allowed a fine description of the solved structure. Unexpectedly, Ser10 did not have an electrostatic influence on glutathione as usually observed in serinyl-GSTs. The S10A variant was only slightly less efficient than the wild-type and molecular dynamics simulations suggested that S10 was a stabilizer of the protein backbone rather than an anchor site for glutathione.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

Molecular and functional insights into glutathione S-transferase genes associated with salt stress in Halothece sp. PCC7418

Evolution and function of glutathione S-transferase (GST) in primordial oxygenic phototrophs such as cyanobacteria are poorly understood. In this study, we identified and functionally characterized the GST gene family in the halotolerant cyanobacterium Halothece sp. PCC7418. Four putative Halothece-GSTs had very low homology, which implies evolutionary divergence. Of these, H0647, H0729 and H3557 were differentially expressed by oxidative stress whereas H3557 was highly and specifically upregulated under salt stress. In vitro analysis revealed that the recombinant H3557 exhibited GST activity toward 1-chloro-2, 4-dinitrobenzene (CDNB) and glutathione (GSH). H3557 displayed a broad range of activity at pH 6.5-10.5. Kinetic parameters showed the apparent K_m for CDNB and GSH was 0.14 and 0.75 mM, respectively. H3557 remained catalytically active in the presence of NaCl. Structural modelling supported that H3557 is salt-adaptive enzyme with highly acidic residues on the protein surface. The vital function of H3557 in heterologous expression system was evaluated. The H3557-expressing cells were more tolerant to H₂ O₂ -induced oxidative stress compared with other GST-expressing cells and conferred salt tolerance. Taken together, the findings of this study provide insights into the molecular and cellular functions of GST in cyanobacteria, particularly under salt stress, which is less understood compared with other species.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Methylglyoxal Detoxification Revisited: Role of Glutathione Transferase in Model Cyanobacterium Synechocystis sp. Strain PCC 6803

In most organisms, methylglyoxal (MG), a toxic metabolite by-product that causes diabetes in humans, is predominantly detoxified by the glyoxalase enzymes. This process begins with the so-called “spontaneous” conjugation of MG with the cytoprotectant metabolite glutathione (GSH). In this study, we unravel a logical, but as yet unsuspected, link between MG detoxification and a (prokaryotic) representative of the ubiquitous glutathione transferase (GST) enzymes. We show that a GST of a model cyanobacterium plays a prominent role in the detoxification of MG in catalyzing its conjugation with GSH. This finding is important because this reaction, always regarded as nonenzymatic, could exist in plants and/or human and thus have an impact on agriculture and/or human health.

Methylglyoxal (MG) is a detrimental metabolic by-product that threatens most organisms (in humans MG causes diabetes). MG is predominantly detoxified by the glyoxalase pathway. This process begins with the conjugation of MG with glutathione (GSH), yielding a hemithioacetal product that is subsequently transformed by the glyoxalase enzymes into d-lactate and GSH. MG has been overlooked in photosynthetic organisms, although they inevitably produce it not only by the catabolism of sugars, lipids, and amino acids, as do heterotrophic organisms, but also by their active photoautotrophic metabolism. This is especially true for cyanobacteria that are regarded as having developed photosynthesis and GSH-dependent enzymes to detoxify the reactive oxygen species produced by their photosynthesis (CO₂ assimilation) and respiration (glucose catabolism), which they perform in the same cell compartment. In this study, we used a combination of in vivo and in vitro approaches to characterize a logical, but as yet never described, link between MG detoxification and a (prokaryotic) representative of the evolutionarily conserved glutathione transferase (GST) detoxification enzymes. We show that the Sll0067 GST of the model cyanobacterium Synechocystis sp. strain PCC 6803 plays a prominent role in MG tolerance and detoxification, unlike the other five GSTs of this organism. Sll0067 catalyzes the conjugation of MG with GSH to initiate its elimination driven by glyoxalases. These results are novel because the conjugation of MG with GSH is always described as nonenzymatic. They will certainly stimulate the analysis of Sll0067 orthologs from other organisms with possible impacts on human health (development of biomarkers or drugs) and/or agriculture.

Glutathione S-transferase: a versatile protein family

Glutathione-S transferase (GST) is a most ancient protein superfamily of multipurpose roles and evolved principally from gene duplication of an ancestral GSH binding protein. They have implemented in diverse plant functions such as detoxification of xenobiotic, secondary metabolism, growth and development, and majorly against biotic and abiotic stresses. The vital structural features of GSTs like highly divergent functional topographies, conserved integrated architecture with separate binding pockets for substrates and ligand, the stringent structural fidelity with high Tm values (50º-60º), and stress-responsive cis-regulatory elements in the promoter region offer this protein as most flexible plant protein for plant breeding approaches, biotechnological applications, etc. This review article summarizes the recent information of GST evolution, and their distribution and structural features with emphasis on the assorted roles of Ser and Cys GSTs with the signature motifs in their active sites, alongside their recent biotechnological application in the area of agriculture, environment, and nanotechnology have been highlighted.

Diversity of Glutathione S-Transferases (GSTs) in Cyanobacteria with Reference to Their Structures, Substrate Recognition and Catalytic Functions

Glutathione S-Transferases (GSTs) comprise a diverse group of protein superfamily involved in cellular detoxification of various harmful xenobiotics and endobiotics. Cyanobacteria, being the primordial photosynthetic prokaryotes, served as an origin for the evolution of GSTs with diversity in their structures, substrate recognition, and catalytic functions. This study analysed the diversity of GSTs in cyanobacteria for the first time. Based on the sequence alignment and phylogenetic tree analysis, 12 GST classes were identified, which are distributed variedly within cyanobacterial orders such as four in Pleurocapsales, eight in Chroococcales, seven in Oscillatoriales, five in Stigonematales, and nine in Nostocales. Detailed evolutionary analysis of cyanobacterial GSTs suggested that the order Pleurocapsales served as the ancestry for GST evolution. The analysis also identified a conserved motif S[GLNTARS][ADE]I[LAI] with signature residues, cysteine, serine, and tyrosine at the N-terminal end that serves as the initiating residue for detoxification. Alternatively, the grouping of cyanobacterial GSTs and their unique signature residues were located, which serve as a possible discriminating factor. The study also described the mode of glutathione binding between the identified cyanobacterial GST groups highlighting the differences among the GST classes. New GST sequence data may improve further our understanding on GST evolution and other possible divergences in cyanobacteria.

Molecular characterization and functional analysis of glutathione S-transferase kappa 1 (GSTκ1) from the big belly seahorse (Hippocampus abdominalis): Elucidation of its involvement in innate immune responses

Glutathione S-transferases (GSTs) are essential enzymes for the bioactivation of xenobiotics through the conjugation of the thiol group of glutathione (GSH). In this study, a kappa class of GST was identified from the big belly seahorse (Hippocampus abdominalis) (HaGSTκ1) and its biochemical and functional properties were analyzed. HaGSTκ1 has 231 amino acids encoded by a 696 bp open reading frame (ORF). The protein has a predicted molecular mass of 26.04 kDa and theoretical isoelectric point (pI) of 8.28. It comprised a thioredoxin domain, disulfide bond formation protein A (DsbA) general fold, and Ser15 catalytic site as well as GSH-binding and polypeptide-binding sites. Phylogenetic analysis revealed that HaGSTκ1 is closely clustered with the kappa class of GSTs from teleost fishes. The recombinant (rHaGSTκ1) protein exhibited activity toward 1-chloro-2,4-dinitrobenzene (CDNB), 4-nitrobenzyl (4-NBC), and 4-nitrophenethyl bromide (4-NPB) but not 1,2-dichloro-4-nitrobenzene (DCNB). The optimum pH and temperature were 8 and 30 °C, respectively, for the catalysis of CDNB and the universal substrate of GSTs. The rHaGSTκ1 activity was efficiently inhibited in the presence of Cibacron blue (CB) as compared with hematin. Most prominent expression of HaGSTκ1 was observed in the liver and kidney among the fourteen different tissues of normal seahorse. After challenge with lipopolysaccharide (LPS), polyinosinic-polycytidylic (poly I:C), gram-negative Edwardsiella tarda, and gram-positive Streptococcus iniae, HaGSTκ1 expression was significantly modulated in the liver and blood tissues. Altogether, our study proposes the plausible important role of HaGSTκ1 in innate immunity and detoxification of harmful xenobiotics.

First in vivo Evidence That Glutathione-S-Transferase Operates in Photo-Oxidative Stress in Cyanobacteria

Although glutathione (GSH) and GSH-dependent enzymes, such as glutathione transferases (GSTs), are thought to have been developed by cyanobacteria to cope with the reactive oxygen species (ROS) that they massively produced by their active photosynthesis, there had been no in vivo analysis of the role of GSTs in cyanobacteria so far. Consequently, we have analyzed two of the six GSTs of the model cyanobacterium Synechocystis PCC 6803, namely Sll1545 (to extend its in vitro study) and Slr0236 (because it is the best homolog to Sll1545). We report that Sll1545 is essential to cell growth in standard photo-autotrophic conditions, whereas Slr0236 is dispensable. Furthermore, both Sll1545 and Slr0236 operate in the protection against stresses triggered by high light, H₂O₂, menadione and methylene blue. The absence of Slr0236 and the depletion of Sll1545 decrease the tolerance to methylene blue in a cumulative way. Similarly, the combined absence of Slr0236 and depletion of Sll1545 decrease the resistance to high light. Attesting their sensitivity to high-light or methylene blue, these Δslr0236-sll1545 cells transiently accumulate ROS, and then reduced and oxidized glutathione in that order. In contrast, the absence of Slr0236 and the depletion of Sll1545 increase the tolerance to menadione in a cumulative way. This increased menadione resistance is due, at least in part, to the higher level of catalase and/or peroxidase activity of these mutants. Similarly, the increased H₂O₂ resistance of the Δslr0236-sll1545 cells is due, at least in part, to its higher level of peroxidase activity.

Cold-Adapted Glutathione S-Transferases from Antarctic Psychrophilic Bacterium Halomonas sp. ANT108: Heterologous Expression, Characterization, and Oxidative Resistance

Glutathione S-transferases are one of the most important antioxidant enzymes to protect against oxidative damage induced by reactive oxygen species. In this study, a novel gst gene, designated as hsgst, was derived from Antarctic sea ice bacterium Halomonas sp. ANT108 and expressed in Escherichia coli (E. coli) BL21. The hsgst gene was 603 bp in length and encoded a protein of 200 amino acids. Compared with the mesophilic EcGST, homology modeling indicated HsGST had some structural characteristics of cold-adapted enzymes, such as higher frequency of glycine residues, lower frequency of proline and arginine residues, and reduced electrostatic interactions, which might be in relation to the high catalytic efficiency at low temperature. The recombinant HsGST (rHsGST) was purified to apparent homogeneity with Ni-affinity chromatography and its biochemical properties were investigated. The specific activity of the purified rHsGST was 254.20 nmol/min/mg. The optimum temperature and pH of enzyme were 25 °C and 7.5, respectively. Most importantly, rHsGST retained 41.67% of its maximal activity at 0 °C. 2.0 M NaCl and 0.2% H₂O₂ had no effect on the enzyme activity. Moreover, rHsGST exhibited its protective effects against oxidative stresses in E. coli cells. Due to its high catalytic efficiency and oxidative resistance at low temperature, rHsGST may be a potential candidate as antioxidant in low temperature health foods.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Comprehensive analysis of the catalytic and structural properties of a mu-class glutathione s-transferase from Fasciola gigantica

Glutathione S‒transferases (GSTs) play an important role in the detoxification of xenobiotics. They catalyze the nucleophilic addition of glutathione (GSH) to nonpolar compounds, rendering the products water-soluble. In the present study, we investigated the catalytic and structural properties of a mu-class GST from Fasciola gigantica (FgGST1). The purified recombinant FgGST1 formed a homodimer composed of 25 kDa subunit. Kinetic analysis revealed that FgGST1 displays broad substrate specificity and shows high GSH conjugation activity toward 1-chloro-2,4-dinitrobenzene, 4-nitroquinoline-1-oxide, and trans-4-phenyl-3-butene-2-one and peroxidase activity towards trans-2-nonenal and hexa-2,4-dienal. The FgGST1 was highly sensitive to inhibition by cibacron blue. The cofactor (GSH) and inhibitor (cibacron blue) were docked, and binding sites were identified. The molecular dynamics studies and principal component analysis indicated the stability of the systems and the collective motions, respectively. Unfolding studies suggest that FgGST1 is a highly cooperative molecule because, during GdnHCl-induced denaturation, a simultaneous unfolding of the protein without stabilization of any partially folded intermediate is observed. The protein is stabilized with a conformational free energy of about 10 ± 0.3 kcal mol-1. Additionally, the presence of conserved Pro-53 and structural motifs such as N-capping box and hydrophobic staple, further aided in the stability and proper folding of FgGST1.

Tolerance of cyanobacteria to the toxicity of BDE-47 and their removal ability

Polybrominated diphenyl ethers are ubiquitous and toxic contaminants in aquatic environments. The effect of polybrominated diphenyl ether BDE-47 on five species of cyanobacteria, along with their removal ability was investigated. Four species, namely Synechocystis sp., Oscillatoria planctonica, Microcystis flos-aquae and Nostoc sp., were exposed to BDE-47 at concentrations ranging from 0.05 to 1.0 mg L^-1 for 14 days, while the exposure time for Pseudanabaena sp. was 30 days. The first four species were very tolerant to BDE-47 while growth and photosynthesis of Pseudanabaena were significantly inhibited by BDE-47 at concentrations over 0.1 mg L^-1. However, this species could recover from the toxicity of high concentrations of BDE-47 after 30 days of exposure, indicating the development of some "resistance" after pre-exposure to 1.0 mg L^-1 BDE-47. The "resistant" cells had a higher growth rate, photosynthesis and glutathione S-transferase activity than normal Pseudanabaena cells. The sensitivity of Pseudanabaena to BDE-47 toxicity was affected by its initial filament density, with cultures having a low filament density (2.3 × 10⁶ filaments mL^-1) being up to 14-15 times more sensitive than cultures with a high filament density (13 × 10⁶ filaments mL^-1). All cyanobacteria could remove 70-82% of BDE-47 in their media, with more than 60% of BDE-47 accumulated in cells. This is the first study showing the high tolerance of different cyanobacteria species to BDE-47 toxicity and their removal ability. The study also revealed that the sensitive Pseudanabaena could acquire a "resistance" to BDE-47, which was transferred to the next generation.

SaxA-Mediated Isothiocyanate Metabolism in Phytopathogenic Pectobacteria

Pectobacteria are devastating plant pathogens that infect a large variety of crops, including members of the family Brassicaceae. To infect cabbage crops, these plant pathogens need to overcome the plant's antibacterial defense mechanisms, where isothiocyanates are liberated by hydrolysis of glucosinolates. Here, we found that a Pectobacterium isolate from the gut of cabbage root fly larvae was particularly resistant to isothiocyanate and even seemed to benefit from the abundant Brassica root metabolite 2-phenylethyl isothiocyanate as a nitrogen source in an ecosystem where nitrogen is scarce. The Pectobacterium isolate harbored a naturally occurring mobile plasmid that contained a sax operon. We hypothesized that SaxA was the enzyme responsible for the breakdown of 2-phenylethyl isothiocyanate. Subsequently, we heterologously produced and purified the SaxA protein and characterized the recombinant enzyme. It hydrolyzed 2-phenylethyl isothiocyanate to yield the products carbonyl sulfide and phenylethylamine. It was also active toward another aromatic isothiocyanate but hardly toward aliphatic isothiocyanates. It belongs to the class B metal-dependent beta-lactamase fold protein family but was not, however, able to hydrolyze beta-lactam antibiotics. We discovered that several copies of the saxA gene are widespread in full and draft Pectobacterium genomes and therefore hypothesize that SaxA might be a new pathogenicity factor of the genus Pectobacterium, possibly compromising food preservation strategies using isothiocyanates.