Impact of Carbon Fixation, Distribution and Storage on the Production of Farnesene and Limonene in Synechocystis PCC 6803 and Synechococcus PCC 7002.

Terpenes are high-value chemicals which can be produced by engineered cyanobacteria from sustainable resources, solar energy, water and CO2. We previously reported that the euryhaline unicellular cyanobacteria Synechocystis sp. PCC 6803 (S.6803) and Synechococcus sp. PCC 7002 (S.7002) produce farnesene and limonene, respectively, more efficiently than other terpenes. In the present study, we attempted to enhance farnesene production in S.6803 and limonene production in S.7002. Practically, we tested the influence of key cyanobacterial enzymes acting in carbon fixation (RubisCO, PRK, CcmK3 and CcmK4), utilization (CrtE, CrtR and CruF) and storage (PhaA and PhaB) on terpene production in S.6803, and we compared some of the findings with the data obtained in S.7002. We report that the overproduction of RubisCO from S.7002 and PRK from Cyanothece sp. PCC 7425 increased farnesene production in S.6803, but not limonene production in S.7002. The overexpression of the crtE genes (synthesis of terpene precursors) from S.6803 or S.7002 did not increase farnesene production in S.6803. In contrast, the overexpression of the crtE gene from S.6803, but not S.7002, increased farnesene production in S.7002, emphasizing the physiological difference between these two model cyanobacteria. Furthermore, the deletion of the crtR and cruF genes (carotenoid synthesis) and phaAB genes (carbon storage) did not increase the production of farnesene in S.6803. Finally, as a containment strategy of genetically modified strains of S.6803, we report that the deletion of the ccmK3K4 genes (carboxysome for CO2 fixation) did not affect the production of limonene, but decreased the production of farnesene in S.6803.

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).

Exploring the Potential of the Model Cyanobacteria Synechococcus PCC 7002 and PCC 7942 for the Photoproduction of High-Value Terpenes: A Comparison with Synechocystis PCC 6803.

We have performed the first comparative analysis of the potential of two physiologically-diverse model cyanobacteria, Synechococcus PCC 7002 (S.7002) and Synechococcus elongatus PCC 7942 (S.7942), for the photosynthetic production of four chemically-different high-value terpenes: two monoterpenes limonene and pinene, and two sesquiterpenes bisabolene and farnesene. We showed, for the first time, that S.7002 and S.7942 can produce farnesene and bisabolene, respectively. Both cyanobacteria produced farnesene (S.7942 produced more efficiently than S.7002) more efficiently than the other tested terpenes (especially pinene, the weakest produced terpene). S.7002 produced limonene more efficiently than bisabolene, whereas S.7942 produced bisabolene more efficiently than limonene. These findings suggest that S.7942 is better suited to produce sesquiterpenes than monoterpenes. Interestingly, higher levels of terpenes were produced by S.7942 and S.7002 expressing a terpene-synthase gene from both an RSF1010-derived replicating plasmid and a neutral chromosomal site, as compared to either the plasmid alone or the chromosome alone. These results suggest that in both cyanobacteria, the production of terpenes is more limited by the activity of terpene synthases than the abundance of terpene precursors. Finally, higher levels of terpenes were produced by S.7002 growing on urea (a frequent pollutant) as compared to nitrate or ammonium, the standard nitrogen sources for cyanobacteria.

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.

Exploring the potential of the model cyanobacterium Synechocystis PCC 6803 for the photosynthetic production of various high-value terpenes.

BACKGROUND: The robust model cyanobacterium Synechocystis PCC 6803 is increasingly explored for its potential to use solar energy, water and atmospheric CO2 for the carbon-neutral production of terpenes, the high-value chemicals that can be used for the production of drugs, flavors, fragrances and biofuels. However, as terpenes are chemically diverse, it is extremely difficult to predict whether Synechocystis is a suitable chassis for the photosynthetic production of various terpenes or only a few of them. RESULTS: We have performed the first-time engineering and comparative analysis of the best-studied cyanobacterium Synechocystis PCC 6803 for the photosynthetic production of five chemically diverse high-value terpenes: two monoterpenes (C10H16) limonene (cyclic molecule) and pinene (bicyclic), and three sesquiterpenes (C15H24) bisabolene (cyclic), farnesene (linear) and santalene (cyclic). All terpene producers appeared to grow well and to be genetically stable, as shown by the absence of changes in their production levels during the 5-9-month periods of their sub-cultivation under photoautotrophic conditions). We also found that Synechocystis PCC 6803 can efficiently and stably produce farnesene and santalene, which had never been produced before by this model organism or any other cyanobacteria, respectively. Similar production levels were observed for cells growing on nitrate (the standard nitrogen source for cyanobacteria) or urea (cheaper than nitrate). Furthermore, higher levels of farnesene were produced by cloning the heterologous farnesene synthase gene in a RSF1010-derived replicating plasmid as compared to the well-used slr0168 neutral cloning site of the chromosome. CONCLUSIONS:  Altogether, the present results indicate that Synechocystis PCC 6803 is better suited to produce sesquiterpenes (particularly farnesene, the most highly produced terpene of this study) than monoterpenes (especially pinene).

First in vivo analysis of the regulatory protein CP12 of the model cyanobacterium Synechocystis PCC 6803: Biotechnological implications.

We report the first in vivo analysis of a canonical CP12 regulatory protein, namely the unique CP12 of the model cyanobacterium Synechocystis PCC 6803, which has the advantage of being able to grow photoautotrophically, photomixotrophically, and photoheterotrophically. The data showed that CP12 is dispensable to cell growth under standard (continuous) light and light/dark cycle, whereas it is essential for the catabolism of exogenously added glucose that normally sustains cell growth in absence of photosynthesis. Furthermore, to be active in glucose catabolism, CP12 requires its three conserved features: its AWD_VEEL motif and its two pairs of cysteine residues. Also interestingly, CP12 was found to regulate the redox equilibrium of NADPH, an activity involving its AWD_VEEL motif and its C-ter cysteine residues, but not its N-ter cysteine residues. This finding is important because NADPH powers up the methylerythritol 4-phosphate (MEP) pathway that synthesizes the geranyl-diphosphate (GPP) and farnesyl-diphosphate (FPP) metabolites, which can be transformed into high-value terpenes by recombinant cyanobacteria producing plant terpene synthase enzymes. Therefore, we have introduced into the Δcp12 mutant and the wild-type (control) strain our replicative plasmids directing the production of the monoterpene limonene and the sesquiterpene bisabolene. The photosynthetic production of both bisabolene and limonene appeared to be increased (more than two-fold) in the Δcp12 mutant as compared to the WT strain. Furthermore, the level of bisabolene production was also higher to those previously reported for various strains of Synechocystis PCC 6803 growing under standard (non-optimized) photoautotrophic conditions. Hence, the presently described Δcp12 strain with a healthy photoautotrophic growth and an increased capability to produce terpenes, is an attractive cell chassis for further gene manipulations aiming at engineering cyanobacteria for high-level photoproduction of terpenes.

A New Gene Family Diagnostic for Intracellular Biomineralization of Amorphous Ca Carbonates by Cyanobacteria.

Cyanobacteria have massively contributed to carbonate deposition over the geological history. They are traditionally thought to biomineralize CaCO3 extracellularly as an indirect byproduct of photosynthesis. However, the recent discovery of freshwater cyanobacteria-forming intracellular amorphous calcium carbonates (iACC) challenges this view. Despite the geochemical interest of such a biomineralization process, its molecular mechanisms and evolutionary history remain elusive. Here, using comparative genomics, we identify a new gene (ccyA) and protein family (calcyanin) possibly associated with cyanobacterial iACC biomineralization. Proteins of the calcyanin family are composed of a conserved C-terminal domain, which likely adopts an original fold, and a variable N-terminal domain whose structure allows differentiating four major types among the 35 known calcyanin homologs. Calcyanin lacks detectable full-length homologs with known function. The overexpression of ccyA in iACC-lacking cyanobacteria resulted in an increased intracellular Ca content. Moreover, ccyA presence was correlated and/or colocalized with genes involved in Ca or HCO3- transport and homeostasis, supporting the hypothesis of a functional role of calcyanin in iACC biomineralization. Whatever its function, ccyA appears as diagnostic of intracellular calcification in cyanobacteria. By searching for ccyA in publicly available genomes, we identified 13 additional cyanobacterial strains forming iACC, as confirmed by microscopy. This extends our knowledge about the phylogenetic and environmental distribution of cyanobacterial iACC biomineralization, especially with the detection of multicellular genera as well as a marine species. Moreover, ccyA was probably present in ancient cyanobacteria, with independent losses in various lineages that resulted in a broad but patchy distribution across modern cyanobacteria.

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.

A Genetic Toolbox for the New Model Cyanobacterium Cyanothece PCC 7425: A Case Study for the Photosynthetic Production of Limonene.

Cyanobacteria, the largest phylum of prokaryotes, perform oxygenic photosynthesis and are regarded as the ancestors of the plant chloroplast and the purveyors of the oxygen and biomass that shaped the biosphere. Nowadays, cyanobacteria are attracting a growing interest in being able to use solar energy, H2O, CO2 and minerals to produce biotechnologically interesting chemicals. This often requires the introduction and expression of heterologous genes encoding the enzymes that are not present in natural cyanobacteria. However, only a handful of model strains with a well-established genetic system are being studied so far, leaving the vast biodiversity of cyanobacteria poorly understood and exploited. In this study, we focused on the robust unicellular cyanobacterium Cyanothece PCC 7425 that has many interesting attributes, such as large cell size; capacity to fix atmospheric nitrogen (under anaerobiosis) and to grow not only on nitrate but also on urea (a frequent pollutant) as the sole nitrogen source; capacity to form CO2-sequestrating intracellular calcium carbonate granules and to produce various biotechnologically interesting products. We demonstrate for the first time that RSF1010-derived plasmid vectors can be used for promoter analysis, as well as constitutive or temperature-controlled overproduction of proteins and analysis of their sub-cellular localization in Cyanothece PCC 7425. These findings are important because no gene manipulation system had been developed for Cyanothece PCC 7425, yet, handicapping its potential to serve as a model host. Furthermore, using this toolbox, we engineered Cyanothece PCC 7425 to produce the high-value terpene, limonene which has applications in biofuels, bioplastics, cosmetics, food and pharmaceutical industries. This is the first report of the engineering of a Cyanothece strain for the production of a chemical and the first demonstration that terpene can be produced by an engineered cyanobacterium growing on urea as the sole nitrogen source.

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

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.IMPORTANCE 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.

Recent Advances in the Photoautotrophic Metabolism of Cyanobacteria: Biotechnological Implications.

Cyanobacteria constitute the only phylum of oxygen-evolving photosynthetic prokaryotes that shaped the oxygenic atmosphere of our planet. Over time, cyanobacteria have evolved as a widely diverse group of organisms that have colonized most aquatic and soil ecosystems of our planet and constitute a large proportion of the biomass that sustains the biosphere. Cyanobacteria synthesize a vast array of biologically active metabolites that are of great interest for human health and industry, and several model cyanobacteria can be genetically manipulated. Hence, cyanobacteria are regarded as promising microbial factories for the production of chemicals from highly abundant natural resources, e.g., solar energy, CO2, minerals, and waters, eventually coupled to wastewater treatment to save costs. In this review, we summarize new important discoveries on the plasticity of the photoautotrophic metabolism of cyanobacteria, emphasizing the coordinated partitioning of carbon and nitrogen towards growth or compound storage, and the importance of these processes for biotechnological perspectives. We also emphasize the importance of redox regulation (including glutathionylation) on these processes, a subject which has often been overlooked.

From Cyanobacteria to Human, MAPEG-Type Glutathione-S-Transferases Operate in Cell Tolerance to Heat, Cold, and Lipid Peroxidation.

The MAPEG2 sub-family of glutathione-S-transferase proteins (GST) has been poorly investigated in vivo, even in prokaryotes such as cyanobacteria the organisms that are regarded as having developed glutathione-dependent enzymes to protect themselves against the reactive oxygen species (ROS) often produced by their powerful photosynthesis. We report the first in vivo analysis of a cyanobacterial MAPEG2-like protein (Sll1147) in the model cyanobacterium Synechocystis PCC 6803. While Sll1147 is dispensable to cell growth in standard photo-autotrophic conditions, it plays an important role in the resistance to heat and cold, and to n-tertbutyl hydroperoxide (n-tBOOH) that induces lipid peroxidation. These findings suggest that Sll1147 could be involved in membrane fluidity, which is critical for photosynthesis. Attesting its sensitivity to these stresses, the Δsll1147 mutant lacking Sll1147 challenged by heat, cold, or n-tBOOH undergoes transient accumulation of peroxidized lipids and then of reduced and oxidized glutathione. These results are welcome because little is known concerning the signaling and/or protection mechanisms used by cyanobacteria to cope with heat and cold, two inevitable environmental stresses that limit their growth, and thus their production of biomass for our food chain and of biotechnologically interesting chemicals. Also interestingly, the decreased resistance to heat, cold and n-tBOOH of the Δsll1147 mutant could be rescued back to normal (wild-type) levels upon the expression of synthetic MAPEG2-encoding human genes adapted to the cyanobacterial codon usage. These synthetic hmGST2 and hmGST3 genes were also able to increase the Escherichia coli tolerance to heat and n-tBOOH. Collectively, these finding indicate that the activity of the MAPEG2 proteins have been conserved, at least in part, during evolution from (cyano)bacteria to human.

Genomics of Urea Transport and Catabolism in Cyanobacteria: Biotechnological Implications.

Cyanobacteria are widely-diverse prokaryotes that colonize our planet. They use solar energy to assimilate huge amounts of atmospheric CO2 and produce a large part of the biomass and oxygen that sustain most life forms. Cyanobacteria are therefore increasingly studied for basic research objectives, as well as for the photosynthetic production of chemicals with industrial interests. One potential approach to reduce the cost of future bioproduction processes is to couple them with wastewater treatment, often polluted with urea, which in any case is cheaper than nitrate. As of yet, however, research has mostly focused on a very small number of model cyanobacteria growing on nitrate. Thus, the genetic inventory of the cyanobacterial phylum is still insufficiently employed to meaningfully select the right host for the right purpose. This review reports what is known about urea transport and catabolism in cyanobacteria, and what can be inferred from the comparative analysis of the publicly available genome sequence of the 308 cyanobacteria. We found that most cyanobacteria mostly harbor the genes encoding the urea catabolytic enzymes urease (ureABCDEFG), but not systematically, together with the urea transport (urtABCDE). These findings are consistent with the capacity of the few tested cyanobacteria that grow on urea as the sole nitrogen source. They also indicate that urease is important for the detoxification of internally generated urea (re-cycling its carbon and nitrogen). In contrast, several cyanobacteria have urtABCDE but not ureABCDEFG, suggesting that urtABCDE could operate in the transport of not only urea but also of other nutrients. Only four cyanobacteria appeared to have the genes encoding the urea carboxylase (uc) and allophanate hydrolase (ah) enzymes that sequentially catabolize urea. Three of these cyanobacteria belongs to the genera Gloeobacter and Gloeomargarita that have likely diverged early from other cyanobacteria, suggesting that the urea carboxylase and allophanate hydrolase enzymes appeared in cyanobacteria before urease.

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.

Overproduction of the cyanobacterial hydrogenase and selection of a mutant thriving on urea, as a possible step towards the future production of hydrogen coupled with water treatment.

Using a combination of various types of genetic manipulations (promoter replacement and gene cloning in replicating plasmid expression vector), we have overproduced the complex hydrogenase enzyme in the model cyanobacterium Synechocystis PCC6803. This new strain overproduces all twelve following proteins: HoxEFUYH (hydrogen production), HoxW (maturation of the HoxH subunit of hydrogenase) and HypABCDEF (assembly of the [NiFe] redox center of HoxHY hydrogenase). This strain when grown in the presence of a suitable quantities of nickel and iron used here exhibits a strong (25-fold) increase in hydrogenase activity, as compared to the WT strain growing in the standard medium. Hence, this strain can be very useful for future analyses of the cyanobacterial [NiFe] hydrogenase to determine its structure and, in turn, improve its tolerance to oxygen with the future goal of increasing hydrogen production. We also report the counterintuitive notion that lowering the activity of the Synechocystis urease can increase the photoproduction of biomass from urea-polluted waters, without decreasing hydrogenase activity. Such cyanobacterial factories with high hydrogenase activity and a healthy growth on urea constitute an important step towards the future development of an economical industrial processes coupling H2 production from solar energy and CO2, with wastewater treatment (urea depollution).

Cyanobacteria: photosynthetic factories combining biodiversity, radiation resistance, and genetics to facilitate drug discovery.

Cyanobacteria are ancient, abundant, and widely diverse photosynthetic prokaryotes, which are viewed as promising cell factories for the ecologically responsible production of chemicals. Natural cyanobacteria synthesize a vast array of biologically active (secondary) metabolites with great potential for human health, while a few genetic models can be engineered for the (low level) production of biofuels. Recently, genome sequencing and mining has revealed that natural cyanobacteria have the capacity to produce many more secondary metabolites than have been characterized. The corresponding panoply of enzymes (polyketide synthases and non-ribosomal peptide synthases) of interest for synthetic biology can still be increased through gene manipulations with the tools available for the few genetically manipulable strains. In this review, we propose to exploit the metabolic diversity and radiation resistance of cyanobacteria, and when required the genetics of model strains, for the production and radioactive (14C) labeling of bioactive products, in order to facilitate the screening for new drugs.

Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications.

Cyanobacteria are fascinating photosynthetic prokaryotes that are regarded as the ancestors of the plant chloroplast; the purveyors of oxygen and biomass for the food chain; and promising cell factories for an environmentally friendly production of chemicals. In colonizing most waters and soils of our planet, cyanobacteria are inevitably challenged by environmental stresses that generate DNA damages. Furthermore, many strains engineered for biotechnological purposes can use DNA recombination to stop synthesizing the biotechnological product. Hence, it is important to study DNA recombination and repair in cyanobacteria for both basic and applied research. This review reports what is known in a few widely studied model cyanobacteria and what can be inferred by mining the sequenced genomes of morphologically and physiologically diverse strains. We show that cyanobacteria possess many E. coli-like DNA recombination and repair genes, and possibly other genes not yet identified. E. coli-homolog genes are unevenly distributed in cyanobacteria, in agreement with their wide genome diversity. Many genes are extremely well conserved in cyanobacteria (mutMS, radA, recA, recFO, recG, recN, ruvABC, ssb, and uvrABCD), even in small genomes, suggesting that they encode the core DNA repair process. In addition to these core genes, the marine Prochlorococcus and Synechococcus strains harbor recBCD (DNA recombination), umuCD (mutational DNA replication), as well as the key SOS genes lexA (regulation of the SOS system) and sulA (postponing of cell division until completion of DNA reparation). Hence, these strains could possess an E. coli-type SOS system. In contrast, several cyanobacteria endowed with larger genomes lack typical SOS genes. For examples, the two studied Gloeobacter strains lack alkB, lexA, and sulA; and Synechococcus PCC7942 has neither lexA nor recCD. Furthermore, the Synechocystis PCC6803 lexA product does not regulate DNA repair genes. Collectively, these findings indicate that not all cyanobacteria have an E. coli-type SOS system. Also interestingly, several cyanobacteria possess multiple copies of E. coli-like DNA repair genes, such as Acaryochloris marina MBIC11017 (2 alkB, 3 ogt, 7 recA, 3 recD, 2 ssb, 3 umuC, 4 umuD, and 8 xerC), Cyanothece ATCC51142 (2 lexA and 4 ruvC), and Nostoc PCC7120 (2 ssb and 3 xerC).

Oxidative-stress detoxification and signalling in cyanobacteria: the crucial glutathione synthesis pathway supports the production of ergothioneine and ophthalmate.

Using genetics and metabolomics we investigated the synthesis (gshA and gshB genes) and catabolism (ggt) of the conserved antioxidant glutathione in the model cyanobacterium Synechocystis PCC6803. These three genes are crucial to Synechocystis, in agreement with the proposed invention of glutathione by ancient cyanobacteria to protect themselves against the toxicity of oxygen they produced through photosynthesis. Consistent with their indispensability, gshA and gshB also operate in the production of another antioxidant, ergothioneine, as well as of the glutathione analogues ophthalmate and norophthalmate. Furthermore, we show that glutathione, ophthalmate and norophthalmate are accumulated in cells stressed by glucose, and that the two glutathione-dependent glyoxalase enzymes operate in the protection against glucose and its catabolite methylglyoxal. These findings are interesting because ophthalmate and norophthalmate were observed only in mammals so far, where ophthalmate is regarded as a biomarker of glutathione depletion. Instead, our data suggest that ophthalmate and norophthalmate are stress-induced markers of cysteine depletion triggered by its accelerated incorporation into glutathione, to face its increased demand for detoxification purposes. Hence, Synechocystis is an attractive model for the analysis of the role of glutathione, ergothioneine, ophthalmate and norophthalmate, in signalling and detoxification of oxidants and metabolic by-products.

The challenge of studying TiO2 nanoparticle bioaccumulation at environmental concentrations: crucial use of a stable isotope tracer.

The ecotoxicity of nanoparticles (NPs) is a growing area of research with many challenges ahead. To be relevant, laboratory experiments must be performed with well-controlled and environmentally realistic (i.e., low) exposure doses. Moreover, when focusing on the intensively manufactured titanium dioxide (TiO2) NPs, sample preparations and chemical analysis are critical steps to meaningfully assay NP's bioaccumulation. To deal with these imperatives, we synthesized for the first time TiO2 NPs labeled with the stable isotope (47)Ti. Thanks to the (47)Ti labeling, we could detect the bioaccumulation of NPs in zebra mussels (Dreissena polymorpha) exposed for 1 h at environmental concentrations via water (7-120 µg/L of (47)TiO2 NPs) and via their food (4-830 µg/L of (47)TiO2 NPs mixed with 1 × 10(6) cells/mL of cyanobacteria) despite the high natural Ti background, which varied in individual mussels. The assimilation efficiency (AE) of TiO2 NPs by mussels from their diet was very low (AE = 3.0 ± 2.7%) suggesting that NPs are mainly captured in mussel gut, with little penetration in their internal organs. Thus, our methodology is particularly relevant in predicting NP's bioaccumulation and investigating the factors influencing their toxicokinetics in conditions mimicking real environments.

First proteomic study of S-glutathionylation in cyanobacteria.

Glutathionylation, the reversible post-translational formation of a mixed disulfide between a cysteine residue and glutathione (GSH), is a crucial mechanism for signal transduction and regulation of protein function. Until now this reversible redox modification was studied mainly in eukaryotic cells. Here we report a large-scale proteomic analysis of glutathionylation in a photosynthetic prokaryote, the model cyanobacterium Synechocystis sp. PCC6803. Treatment of acellular extracts with N,N-biotinyl glutathione disulfide (BioGSSG) induced glutathionylation of numerous proteins, which were subsequently isolated by affinity chromatography on streptavidin columns and identified by nano LC-MS/MS analysis. Potential sites of glutathionylation were also determined for 125 proteins following tryptic cleavage, streptavidin-affinity purification, and mass spectrometry analysis. Taken together the two approaches allowed the identification of 383 glutathionylatable proteins that participate in a wide range of cellular processes and metabolic pathways such as carbon and nitrogen metabolisms, cell division, stress responses, and H2 production. In addition, the glutathionylation of two putative targets, namely, peroxiredoxin (Sll1621) involved in oxidative stress tolerance and 3-phosphoglycerate dehydrogenase (Sll1908) acting on amino acids metabolism, was confirmed by biochemical studies on the purified recombinant proteins. These results suggest that glutathionylation constitutes a major mechanism of global regulation of the cyanobacterial metabolism under oxidative stress conditions.