Carbon cycling in Anabaena sp. PCC 7120. Sucrose synthesis in the heterocysts and possible role in nitrogen fixation

Cyanobacteria as cell factories for the photosynthetic production of sucrose

Biofuels and other biologically manufactured sustainable goods are growing in popularity and demand. Carbohydrate feedstocks required for industrial fermentation processes have traditionally been supplied by plant biomass, but the large quantities required to produce replacement commodity products may prevent the long-term feasibility of this approach without alternative strategies to produce sugar feedstocks. Cyanobacteria are under consideration as potential candidates for sustainable production of carbohydrate feedstocks, with potentially lower land and water requirements relative to plants. Several cyanobacterial strains have been genetically engineered to export significant quantities of sugars, especially sucrose. Sucrose is not only naturally synthesized and accumulated by cyanobacteria as a compatible solute to tolerate high salt environments, but also an easily fermentable disaccharide used by many heterotrophic bacteria as a carbon source. In this review, we provide a comprehensive summary of the current knowledge of the endogenous cyanobacterial sucrose synthesis and degradation pathways. We also summarize genetic modifications that have been found to increase sucrose production and secretion. Finally, we consider the current state of synthetic microbial consortia that rely on sugar-secreting cyanobacterial strains, which are co-cultivated alongside heterotrophic microbes able to directly convert the sugars into higher-value compounds (e.g., polyhydroxybutyrates, 3-hydroxypropionic acid, or dyes) in a single-pot reaction. We summarize recent advances reported in such cyanobacteria/heterotroph co-cultivation strategies and provide a perspective on future developments that are likely required to realize their bioindustrial potential.

Current Metabolic Engineering Strategies for Photosynthetic Bioproduction in Cyanobacteria

Cyanobacteria are photosynthetic microorganisms capable of using solar energy to convert CO₂ and H₂O into O₂ and energy-rich organic compounds, thus enabling sustainable production of a wide range of bio-products. More and more strains of cyanobacteria are identified that show great promise as cell platforms for the generation of bioproducts. However, strain development is still required to optimize their biosynthesis and increase titers for industrial applications. This review describes the most well-known, newest and most promising strains available to the community and gives an overview of current cyanobacterial biotechnology and the latest innovative strategies used for engineering cyanobacteria. We summarize advanced synthetic biology tools for modulating gene expression and their use in metabolic pathway engineering to increase the production of value-added compounds, such as terpenoids, fatty acids and sugars, to provide a go-to source for scientists starting research in cyanobacterial metabolic engineering.

Flv3A facilitates O2 photoreduction and affects H2 photoproduction independently of Flv1A in diazotrophic Anabaena filaments

The model heterocyst-forming filamentous cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a typical example of a multicellular organism capable of simultaneously performing oxygenic photosynthesis in vegetative cells and O₂ -sensitive N₂ -fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O₂ (a Mehler-like reaction). Here, we performed a phenotypic and biophysical characterization of Anabaena mutants impaired in vegetative-specific Flv1A and Flv3A in order to address their physiological relevance in the bioenergetic processes occurring in diazotrophic Anabaena under variable CO₂ conditions. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler-like reaction upon a sudden increase in light intensity, which is likely important for the activation of carbon-concentrating mechanisms and CO₂ fixation. Under ambient CO₂ diazotrophic conditions, Flv3A is responsible for moderate O₂ photoreduction, independently of Flv1A, but only in the presence of Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst-specific uptake hydrogenase, which led to enhanced H₂ photoproduction under both oxic and micro-oxic conditions. These results reveal a novel regulatory network between the Mehler-like reaction and the diazotrophic metabolism, which is of great interest for future biotechnological applications.

Exploring the Diversity of the Thioredoxin Systems in Cyanobacteria

Cyanobacteria evolved the ability to perform oxygenic photosynthesis using light energy to reduce CO₂ from electrons extracted from water and form nutrients. These organisms also developed light-dependent redox regulation through the Trx system, formed by thioredoxins (Trxs) and thioredoxin reductases (TRs). Trxs are thiol-disulfide oxidoreductases that serve as reducing substrates for target enzymes involved in numerous processes such as photosynthetic CO₂ fixation and stress responses. We focus on the evolutionary diversity of Trx systems in cyanobacteria and discuss their phylogenetic relationships. The study shows that most cyanobacteria contain at least one copy of each identified Trx, and TrxA is the only one present in all genomes analyzed. Ferredoxin thioredoxin reductase (FTR) is present in all groups except Gloeobacter and Prochlorococcus, where there is a ferredoxin flavin-thioredoxin reductase (FFTR). Our data suggest that both TRs may have coexisted in ancestral cyanobacteria together with other evolutionarily related proteins such as NTRC or DDOR, probably used against oxidative stress. Phylogenetic studies indicate that they have different evolutionary histories. As cyanobacteria diversified to occupy new habitats, some of these proteins were gradually lost in some groups. Finally, we also review the physiological relevance of redox regulation in cyanobacteria through the study of target enzymes.

The Beneficial Effects of Cyanobacterial Co-Culture on Plant Growth

Cyanobacteria are ubiquitous phototrophic prokaryotes that find a wide range of applications in industry due to their broad product spectrum. In this context, the application of cyanobacteria as biofertilizers and thus as an alternative to artificial fertilizers has emerged in recent decades. The benefit is mostly based on the ability of cyanobacteria to fix elemental nitrogen and make it available to the plants in a usable form. However, the positive effects of co-cultivating plants with cyanobacteria are not limited to the provision of nitrogen. Cyanobacteria produce numerous secondary metabolites that can be useful for plants, for example, they can have growth-promoting effects or increase resistance to plant diseases. The effects of biotic and abiotic stress can as well be reduced by many secondary metabolites. Furthermore, the biofilms formed by the cyanobacteria can lead to improved soil conditions, such as increased water retention capacity. To exchange the substances mentioned, cyanobacteria form symbioses with plants, whereby the strength of the symbiosis depends on both partners, and not every plant can form symbiosis with every cyanobacterium. Not only the plants in symbiosis benefit from the cyanobacteria, but also vice versa. This review summarizes the beneficial effects of cyanobacterial co-cultivation on plants, highlighting the substances exchanged and the strength of cyanobacterial symbioses with plants. A detailed explanation of the mechanism of nitrogen fixation in cyanobacterial heterocysts is given. Finally, a summary of possible applications of co-cultivation in the (agrar-)industry is given.

Proteogenomic Analysis Provides Novel Insight into Genome Annotation and Nitrogen Metabolism in Nostoc sp. PCC 7120

Cyanobacteria, capable of oxygenic photosynthesis, play a vital role in nitrogen and carbon cycles. Nostoc sp. PCC 7120 (Nostoc 7120) is a model cyanobacterium commonly used to study cell differentiation and nitrogen metabolism. Although its genome was released in 2002, a high-quality genome annotation remains unavailable for this model cyanobacterium. Therefore, in this study, we performed an in-depth proteogenomic analysis based on high-resolution mass spectrometry (MS) data to refine the genome annotation of Nostoc 7120. We unambiguously identified 5,519 predicted protein-coding genes and revealed 26 novel genes, 75 revised genes, and 27 different kinds of posttranslational modifications in Nostoc 7120. A subset of these novel proteins were further validated at both the mRNA and peptide levels. Functional analysis suggested that many newly annotated proteins may participate in nitrogen or cadmium/mercury metabolism in Nostoc 7120. Moreover, we constructed an updated Nostoc 7120 database based on our proteogenomic results and presented examples of how the updated database could be used to improve the annotation of proteomic data. Our study provides the most comprehensive annotation of the Nostoc 7120 genome thus far and will serve as a valuable resource for the study of nitrogen metabolism in Nostoc 7120.

IMPORTANCE Cyanobacteria are a large group of prokaryotes capable of oxygenic photosynthesis and play a vital role in nitrogen and carbon cycles on Earth. Nostoc 7120 is a commonly used model cyanobacterium for studying cell differentiation and nitrogen metabolism. In this study, we presented the first comprehensive draft map of the Nostoc 7120 proteome and a wide range of posttranslational modifications. In addition, we constructed an updated database of Nostoc 7120 based on our proteogenomic results and presented examples of how the updated database could be used for system-level studies of Nostoc 7120. Our study provides the most comprehensive annotation of Nostoc 7120 genome and a valuable resource for the study of nitrogen metabolism in this model cyanobacterium.

Calm on the surface, dynamic on the inside. Molecular homeostasis of Anabaena sp. PCC 7120 nitrogen metabolism

Nitrogen sources are all converted into ammonium/ia as a first step of assimilation. It is reasonable to expect that molecular components involved in the transport of ammonium/ia across biological membranes connect with the regulation of both nitrogen and central metabolism. We applied both genetic (i.e., Δamt mutation) and environmental treatments to a target biological system, the cyanobacterium Anabaena sp PCC 7120. The aim was to both perturb nitrogen metabolism and induce multiple inner nitrogen states, respectively, followed by targeted quantification of key proteins, metabolites and enzyme activities. The absence of AMT transporters triggered a substantial whole-system response, affecting enzyme activities and quantity of proteins and metabolites, spanning nitrogen and carbon metabolisms. Moreover, the Δamt strain displayed a molecular fingerprint indicating nitrogen deficiency even under nitrogen replete conditions. Contrasting with such dynamic adaptations was the striking near-complete lack of an externally measurable altered phenotype. We conclude that this species evolved a highly robust and adaptable molecular network to maintain homeostasis, resulting in substantial internal but minimal external perturbations. This analysis provides evidence for a potential role of AMT transporters in the regulatory/signalling network of nitrogen metabolism and the existence of a novel fourth regulatory mechanism controlling glutamine synthetase activity.

Nucleotide-sugar metabolism in plants: the legacy of Luis F. Leloir

This review commemorates the 50th anniversary of the Nobel Prize in Chemistry awarded to Luis F. Leloir 'for his discovery of sugar-nucleotides and their role in the biosynthesis of carbohydrates'. He and his co-workers discovered that activated forms of simple sugars, such as UDP-glucose and UDP-galactose, are essential intermediates in the interconversion of sugars. They elucidated the biosynthetic pathways for sucrose and starch, which are the major end-products of photosynthesis, and for trehalose. Trehalose 6-phosphate, the intermediate of trehalose biosynthesis that they discovered, is now a molecule of great interest due to its function as a sugar signalling metabolite that regulates many aspects of plant metabolism and development. The work of the Leloir group also opened the doors to an understanding of the biosynthesis of cellulose and other structural cell wall polysaccharides (hemicelluloses and pectins), and ascorbic acid (vitamin C). Nucleotide-sugars also serve as sugar donors for a myriad of glycosyltransferases that conjugate sugars to other molecules, including lipids, phytohormones, secondary metabolites, and proteins, thereby modifying their biological activity. In this review, we highlight the diversity of nucleotide-sugars and their functions in plants, in recognition of Leloir's rich and enduring legacy to plant science.

A Genome-Scale Metabolic Model of Anabaena 33047 to Guide Genetic Modifications to Overproduce Nylon Monomers

Nitrogen fixing-cyanobacteria can significantly improve the economic feasibility of cyanobacterial production processes by eliminating the requirement for reduced nitrogen. Anabaena sp. ATCC 33047 is a marine, heterocyst forming, nitrogen fixing cyanobacteria with a very short doubling time of 3.8 h. We developed a comprehensive genome-scale metabolic (GSM) model, iAnC892, for this organism using annotations and content obtained from multiple databases. iAnC892 describes both the vegetative and heterocyst cell types found in the filaments of Anabaena sp. ATCC 33047. iAnC892 includes 953 unique reactions and accounts for the annotation of 892 genes. Comparison of iAnC892 reaction content with the GSM of Anabaena sp. PCC 7120 revealed that there are 109 reactions including uptake hydrogenase, pyruvate decarboxylase, and pyruvate-formate lyase unique to iAnC892. iAnC892 enabled the analysis of energy production pathways in the heterocyst by allowing the cell specific deactivation of light dependent electron transport chain and glucose-6-phosphate metabolizing pathways. The analysis revealed the importance of light dependent electron transport in generating ATP and NADPH at the required ratio for optimal N₂ fixation. When used alongside the strain design algorithm, OptForce, iAnC892 recapitulated several of the experimentally successful genetic intervention strategies that over produced valerolactam and caprolactam precursors.

Cell-cell communication through septal junctions in filamentous cyanobacteria

Septal junctions are cell-cell connections that mediate intercellular communication in filamentous cyanobacteria. The septal peptidoglycan is perforated by dozens of 20 nm-wide nanopores, through which these proteinaceous structures traverse, physically connecting adjacent cells. On each cytoplasmic side, every septal junction contains a flexible cap structure that closes the connection in a reversible manner upon stress. This gating mechanism reminds of the gap junctions from metazoans and represents a primordial control system for cell-cell communication. In this review, we summarize the knowledge about formation of the nanopore array as the framework for incorporation of cell-cell connecting septal junctions. Furthermore, the architecture of septal junctions, proteins involved in septal junction constitution and regulation of intercellular communication will be addressed.

Real-time monitoring of the in vivo redox state transition using the ratiometric redox state sensor protein FROG/B

The intracellular redox state is one of the key factors regulating various physiological phenomena in the cell. Monitoring this state is therefore important for understanding physiological homeostasis in cells. Various fluorescent sensor proteins have already been developed to monitor intracellular redox state. We also developed fluorescent redox sensor proteins named Oba-Q and Re-Q, the emissions of which are quenched under oxidized and reduced conditions, respectively. Although these sensors were useful to visualize the redox changes in the cell over time, they have the weakness that their emission signals are directly influenced by their in situ expression levels. To overcome this problem, we developed a redox sensor protein with a single excitation peak and dual variable emission peaks. This sensor protein shows green emission under oxidized conditions and blue emission under reduced conditions. We therefore named this sensor FROG/B, fluorescent protein with redox-dependent change in green/blue. By using this sensor, we successfully measured the changes in intracellular redox potentials in cyanobacterial cells quantitatively caused by light/dark transition just by calculating the ratio of emission between green and blue signals.

Proteomic Insights into Starvation of Nitrogen-Replete Cells of Nostoc sp. PCC 7120 under β-N-Methylamino-L-Alanine (BMAA) Treatment

All cyanobacteria produce a neurotoxic non-protein amino acid β-N-methylamino-L-alanine (BMAA). However, the biological function of BMAA in the regulation of cyanobacteria metabolism still remains undetermined. It is known that BMAA suppresses the formation of heterocysts in diazotrophic cyanobacteria under nitrogen starvation conditions, and BMAA induces the formation of heterocyst-like cells under nitrogen excess conditions, by causing the expression of heterocyst-specific genes that are usually “silent” under nitrogen-replete conditions, as if these bacteria receive a nitrogen deficiency intracellular molecular signal. In order to find out the molecular mechanisms underlying this unexpected BMAA effect, we studied the proteome of cyanobacterium Nostoc sp. PCC 7120 grown under BMAA treatment in nitrogen-replete medium. Experiments were performed in two experimental settings: (1) in control samples consisted of cells grown without the BMAA treatment and (2) the treated samples consisted of cells grown with addition of an aqueous solution of BMAA (20 µM). In total, 1567 different proteins of Nostoc sp. PCC 7120 were identified by LC-MS/MS spectrometry. Among them, 80 proteins belonging to different functional categories were chosen for further functional analysis and interpretation of obtained proteomic data. Here, we provide the evidence that a pleiotropic regulatory effect of BMAA on the proteome of cyanobacterium was largely different under conditions of nitrogen-excess compared to its effect under nitrogen starvation conditions (that was studied in our previous work). The most significant difference in proteome expression between the BMAA-treated and untreated samples under different growth conditions was detected in key regulatory protein PII (GlnB). BMAA downregulates protein PII in nitrogen-starved cells and upregulates this protein in nitrogen-replete conditions. PII protein is a key signal transduction protein and the change in its regulation leads to the change of many other regulatory proteins, including different transcriptional factors, enzymes and transporters. Complex changes in key metabolic and regulatory proteins (RbcL, RbcS, Rca, CmpA, GltS, NodM, thioredoxin 1, RpbD, ClpP, MinD, RecA, etc.), detected in this experimental study, could be a reason for the appearance of the “starvation” state in nitrogen-replete conditions in the presence of BMAA. In addition, 15 proteins identified in this study are encoded by genes, which are under the control of NtcA—a global transcriptional regulator—one of the main protein partners and transcriptional regulators of PII protein. Thereby, this proteomic study gives a possible explanation of cyanobacterium starvation under nitrogen-replete conditions and BMAA treatment. It allows to take a closer look at the regulation of cyanobacteria metabolism affected by this cyanotoxin.

Structure and Function of a Bacterial Gap Junction Analog

Multicellular lifestyle requires cell-cell connections. In multicellular cyanobacteria, septal junctions enable molecular exchange between sister cells and are required for cellular differentiation. The structure of septal junctions is poorly understood, and it is unknown whether they are capable of controlling intercellular communication. Here, we resolved the in situ architecture of septal junctions by electron cryotomography of cryo-focused ion beam-milled cyanobacterial filaments. Septal junctions consisted of a tube traversing the septal peptidoglycan. Each tube end comprised a FraD-containing plug, which was covered by a cytoplasmic cap. Fluorescence recovery after photobleaching showed that intercellular communication was blocked upon stress. Gating was accompanied by a reversible conformational change of the septal junction cap. We provide the mechanistic framework for a cell junction that predates eukaryotic gap junctions by a billion years. The conservation of a gated dynamic mechanism across different domains of life emphasizes the importance of controlling molecular exchange in multicellular organisms.

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The in situ architecture of septal junctions reveals cap, plug, and tube modules

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Septal junctions reversibly control cell-cell communication upon stress

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FraD is a structural element of the septal junction plug module

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Bacterial septal junctions are mechanistically analogous to metazoan gap junctions

The novel strain Desmonostoc salinum CCM-UFV059 shows higher salt and desiccation resistance compared to the model strain Nostoc sp. PCC7120

Desmonostoc salinum CCM-UFV059 (Desmonostoc) is a novel cyanobacterial strain of the order Nostocales isolated from a saline-alkaline lake. The acclimation towards salt and desiccation stress of Desmonostoc was compared to the related and well-characterized model strain Nostoc sp. PCC7120 (Nostoc). Salt-stressed cells of Desmonostoc maintained low cellular Na⁺ concentrations and accumulated high amounts of compatible solutes, mainly sucrose and to a lower extent trehalose. These features permitted Desmonostoc to grow and maintain photosynthesis at 2-fold higher salinities than Nostoc. Moreover, Desmonostoc also induced sucrose over-accumulation under desiccation, which allowed this strain to recover from this stress in contrast to Nostoc. Additional mechanisms such as the presence of highly unsaturated lipids in the membrane and an efficient ion transport system could also explain, at least partially, how Desmonostoc is able to acclimate to high salinities and to resist longer desiccation periods. Collectively, our results provide first insights into the physiological and metabolic adaptations explaining the remarkable high salt and desiccation tolerance, which qualify Desmonostoc as an attractive model for further analysis of stress acclimation among heterocystous N₂ -fixing cyanobacteria.

Thioredoxin targets are regulated in heterocysts of cyanobacterium Anabaena sp. PCC 7120 in a light-independent manner

In the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120, glucose 6-phosphate dehydrogenase (G6PDH) plays an important role in producing the power for reducing nitrogenase under light conditions. Our previous study showed that thioredoxin suppresses G6PDH by reducing its activator protein OpcA, implying that G6PDH is inactivated under light conditions because thioredoxins are reduced by the photosynthetic electron transport system in cyanobacteria. To address how Anabaena sp. PCC 7120 maintains G6PDH activity even under light conditions when nitrogen fixation occurs, we investigated the redox regulation system in vegetative cells and specific nitrogen-fixing cells named heterocysts, individually. We found that thioredoxin target proteins were more oxidized in heterocysts than in vegetative cells under light conditions. Alterations in the redox regulation mechanism of heterocysts may affect the redox states of thioredoxin target proteins, including OpcA, so that G6PDH is activated in heterocysts even under light conditions.

Freshwater Cyanobacterium Synechococcus elongatus PCC 7942 Adapts to an Environment with Salt Stress via Ion-Induced Enzymatic Balance of Compatible Solutes

Salinity is one of the most important abiotic factors in various natural habitats of microbes. Cyanobacteria are the most widely distributed family of photosynthetic microorganisms in environments with fluctuating salinity. In response to salt stress, many cyanobacteria de novo synthesize compatible solutes to maintain osmotic balance in the cell. However, the regulation of intracellular accumulation of these compounds is still not well understood. The freshwater cyanobacterium Synechococcus elongatus PCC 7942 (Syn7942) exclusively accumulates sucrose as a compatible solute upon salt stress and is thus an ideal model microorganism for studying the metabolism of compatible solute dynamics. Here, we focused on elucidating the regulatory mechanisms involved in salt-induced sucrose accumulation in Syn7942. Using a series of physiological and biochemical experiments, we showed that the ionic effect of salt stress plays an important role in inducing sucrose synthesis, whereby elevated ion concentration directly activates the sucrose-synthesizing enzyme sucrose-phosphate synthase and simultaneously inhibits the sucrose-degrading enzyme invertase, resulting in a rapid sucrose accumulation. Thus, we propose a novel mechanism for cyanobacterial adaption to salt stress and fluctuating salinity, i.e., the ion-induced synergistic modulation of the enzymes synthesizing and degrading compatible solutes. These findings greatly enhance our current understanding of microbial adaptation to salt.IMPORTANCE Most microbes de novo synthesize compatible solutes for adaptation to salt stress or fluctuating salinity environments. However, to date, one of the core questions involved in these physiological processes, i.e., the regulation of salt-induced compatible solute biosynthesis, is still not well understood. Here, this issue was systematically investigated by employing the model freshwater cyanobacterium Synechococcus elongatus PCC 7942. A novel mechanism for cyanobacterial adaption to salt stress and fluctuating salinity, i.e., the ion-induced synergistic modulation of key synthesizing and degrading enzymes of compatible solutes, is proposed. Because the ion-induced activation/inhibition of enzymes is a fast and efficient process, it may represent a common strategy of microbes for adaptation to environments with fluctuating salinity.

Spatiotemporal Gene Repression System in the Heterocyst-Forming Multicellular Cyanobacterium Anabaena sp. PCC 7120

The heterocyst-forming multicellular cyanobacterium Anabaena sp. PCC 7120 is often used as a model organism for prokaryotic cell differentiation. We recently demonstrated that heterocysts are suitable for photosynthetic production of valuable chemicals, such as ethanol, due to their active catabolism and microoxic conditions. We have developed gene regulation systems, including cell type-specific gene induction systems, to broaden this cyanobacterium's use. In the present study, a heterocyst-specific conditional gene repression system was successfully created by combining a cell type-specific gene induction system with CRISPRi technology. We targeted the gln A gene that encodes glutamine synthetase, an essential enzyme for nitrogen assimilation, to reconstruct metabolism in the multicellular cyanobacterium. Heterocyst-specific repression of gln A enhanced ethanol production. We believe that heterocyst-specific gene repression systems are useful tools for basic research on cell differentiation as well as for metabolic engineering of heterocysts.

Heterocyst Thylakoid Bioenergetics

Heterocysts are specialized cells that differentiate in the filaments of heterocystous cyanobacteria. Their role is to maintain a microoxic environment for the nitrogenase enzyme during diazotrophic growth. The lack of photosynthetic water oxidation in the heterocyst puts special constraints on the energetics for nitrogen fixation, and the electron transport pathways of heterocyst thylakoids are slightly different from those in vegetative cells. During recent years, there has been a growing interest in utilizing heterocysts as cell factories for the production of fuels and other chemical commodities. Optimization of these production systems requires some consideration of the bioenergetics behind nitrogen fixation. In this overview, we emphasize the role of photosynthetic electron transport in providing ATP and reductants to the nitrogenase enzyme, and provide some examples where heterocysts have been used as production facilities.

Transport of organic substances through the cytoplasmic membrane of cyanobacteria

Cyanobacteria are mainly known to incorporate inorganic molecules like carbon dioxide and ammonia from the environment into organic material within the cell. Nevertheless cyanobacteria do import and export organic substances through the cytoplasmic membrane and these processes are essential for all cyanobacteria. In addition understanding the mechanisms of transport of organic molecules through the cytoplasmic membrane might become very important. Genetically modified strains of cyanobacteria could serve as producers and exporters of commercially important substances. In this review we attempt to present all data of transport of organic molecules through the cytoplasmic membrane of cyanobacteria that are currently available with the transported molecules ordered according to their chemical classes.

Inactivation of invertase enhances sucrose production in the cyanobacterium Synechocystis sp. PCC 6803

Sucrose is naturally synthesized by many cyanobacteria under high salt conditions, which can be applied to produce this widely used feedstock. To improve sucrose production with the moderate halo-tolerant cyanobacterium Synechocystis sp. PCC 6803, we identified and biochemically characterized the sucrose-degrading invertase. Inactivating the invertase encoding gene sll0626 (inv) significantly increased cellular sucrose levels; interestingly sucrose over-accumulation was also observed under NaCl-free conditions. The subsequent inactivation of inv in the mutant ΔggpS, which cannot synthesize the major compatible solute glucosylglycerol, resulted in further enhanced sucrose accumulation in the presence of 1.5 % NaCl. Then, inv mutation was introduced into the previously obtained sucrose-producing strain WD25 (Du W, Liang F, Duan Y, Tan X, Lu X. Metab Eng 2013;19:17-25), which resulted in almost 40 % higher sucrose accumulation. These findings show that invertase is an interesting target in obtaining efficient sucrose production in cyanobacterial host cells.

Thioredoxin regulates G6PDH activity by changing redox states of OpcA in the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120

Glucose 6-phosphate dehydrogenase (G6PDH) catalyzes the first reaction in the oxidative pentose phosphate pathway. In green plant chloroplasts, G6PDH is a unique redox-regulated enzyme, since it is inactivated under the reducing conditions. This regulation is accomplished using a redox-active cysteine pair, which is conserved in plant G6PDH. The inactivation of this enzyme under conditions of light must be beneficial to prevent release of CO2 from the photosynthetic carbon fixation cycle. In the filamentous, heterocyst-forming, nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120), G6PDH plays a pivotal role in providing reducing power for nitrogenase, and its activity is also reported to be suppressed by reduction, though Anabaena G6PDH does not conserve the critical cysteines for regulation. Based on the thorough analyses of the redox regulation mechanisms of G6PDH from Anabaena 7120 and its activator protein OpcA, we found that m-type thioredoxin regulates G6PDH activity by changing the redox states of OpcA. Mass spectrometric analysis and mutagenesis studies indicate that Cys393 and Cys399 of OpcA are responsible for the redox regulation property of this protein. Moreover, in vivo analyses of the redox states of OpcA showed that more than half of the OpcA is present as an oxidized form, even under conditions of light, when cells are cultured under the nitrogen-fixing conditions. This redox regulation of OpcA might be necessary to provide reducing power for nitrogenase by G6PDH in heterocysts even during the day.

Multiple ABC glucoside transporters mediate sugar-stimulated growth in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120

Cyanobacteria are generally capable of photoautotrophic growth and are widely distributed on Earth. The model filamentous, heterocyst-forming strain Anabaena sp. PCC 7120 has long been considered as a strict photoautotroph but is now known to be able to assimilate fructose. We have previously described two components of ABC glucoside uptake transporters from Anabaena that are involved in uptake of the sucrose analog esculin: GlsC [a nucleotide-binding domain subunit (NBD)] and GlsP [a transmembrane component (TMD)]. Here, we created Anabaena mutants of genes encoding three further ABC transporter components needed for esculin uptake: GlsD (NBD), GlsQ (TMD) and GlsR (periplasmic substrate-binding protein). Phototrophic growth of Anabaena was significantly stimulated by sucrose, fructose and glucose. Whereas the glsC and glsD mutants were drastically hampered in sucrose-stimulated growth, the different gls mutants were generally impaired in sugar-dependent growth. Our results suggest the participation of Gls and other ABC transporters encoded in the Anabaena genome in sugar-stimulated growth. Additionally, Gls transporter components influence the function of septal junctions in the Anabaena filament. We suggest that mixotrophic growth is important in cyanobacterial physiology and may be relevant for the wide success of these organisms in diverse environments.

A Comprehensively Curated Genome-Scale Two-Cell Model for the Heterocystous Cyanobacterium Anabaena sp. PCC 7120

Anabaena sp. PCC 7120 is a nitrogen-fixing filamentous cyanobacterium. Under nitrogen-limiting conditions, a fraction of the vegetative cells in each filament terminally differentiate to nongrowing heterocysts. Heterocysts are metabolically and structurally specialized to enable O₂-sensitive nitrogen fixation. The functionality of the filament, as an association of vegetative cells and heterocysts, is postulated to depend on metabolic exchange of electrons, carbon, and fixed nitrogen. In this study, we compile and evaluate a comprehensive curated stoichiometric model of this two-cell system, with the objective function based on the growth of the filament under diazotrophic conditions. The predicted growth rate under nitrogen-replete and -deplete conditions, as well as the effect of external carbon and nitrogen sources, was thereafter verified. Furthermore, the model was utilized to comprehensively evaluate the optimality of putative metabolic exchange reactions between heterocysts and vegetative cells. The model suggested that optimal growth requires at least four exchange metabolites. Several combinations of exchange metabolites resulted in predicted growth rates that are higher than growth rates achieved by only considering exchange of metabolites previously suggested in the literature. The curated model of the metabolic network of Anabaena sp. PCC 7120 enhances our ability to understand the metabolic organization of multicellular cyanobacteria and provides a platform for further study and engineering of their metabolism.

Diazotrophic specific cytochrome c oxidase required to overcome light stress in the cyanobacterium Nostoc muscorum

Diazotrophic, filamentous and heterocystous cyanobacterium Nostoc muscorum perform photosynthesis in vegetative whereas nitrogen fixation occurs in heterocyst only. However, despite their metabolic plasticity, respiration takes place both in vegetative cells and heterocysts. The role of the respiratory electron transport system and terminal oxidases under light stress is not evident so far. As compared to the diazotrophically grown cultures, the non-diazotrophically grown cultures of the N. muscorum show a slight decrease in their growth, chlorophyll a contents and photosynthetic O2 evolution under light stress. Whereas respiratory O2 uptake under identical stress condition increases several fold. Likewise, nitrogen fixing enzyme i.e. nitrogenase over-expresses itself under light stress condition. The terminal enzyme of respiratory electron transport chain i.e. cytochrome c oxidase shows more activity under light stress, whilst light stress has no impact on Ca(++)-dependent ATPase activity. This leads to the conclusion that under light stress, cytochrome c oxidase plays a vital role in mitigating given light stress.

Sucrose in bloom-forming cyanobacteria: loss and gain of genes involved in its biosynthesis

Bloom-forming cyanobacteria are widely distributed in freshwater ecosystems. To cope with salinity fluctuations, cyanobacteria synthesize compatible solutes, such as sucrose, to maintain the intracellular osmotic balance. The screening of cyanobacterial genomes revealed that homologues to sucrose metabolism-related genes only occur in few bloom-forming strains, mostly belonging to Nostocales and Stigonematales orders. Remarkably, among Chroococcales and Oscillatoriales strains, homologues were only found in M. aeruginosa PCC 7806 and Leptolyngbya boryana PCC 6306, suggesting a massive loss of sucrose metabolism in bloom-forming strains of these orders. After a complete functional characterization of sucrose genes in M. aeruginosa PCC 7806, we showed that sucrose metabolism depends on the expression of a gene cluster that defines a transcriptional unit, unique among all sucrose-containing cyanobacteria. It was also demonstrated that the expression of the encoding genes of sucrose-related proteins is stimulated by salt. In view of its ancestral origin in cyanobacteria, the fact that most bloom-forming strains lack sucrose metabolism indicates that the genes involved might have been lost during evolution. However, in a particular strain, like M. aeruginosa PCC 7806, sucrose synthesis genes were probably regained by horizontal gene transfer, which could be hypothesized as a response to salinity fluctuations.

Requirement of Fra proteins for communication channels between cells in the filamentous nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120

The filamentous nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 differentiates specialized cells, heterocysts, that fix atmospheric nitrogen and transfer the fixed nitrogen to adjacent vegetative cells. Reciprocally, vegetative cells transfer fixed carbon to heterocysts. Several routes have been described for metabolite exchange within the filament, one of which involves communicating channels that penetrate the septum between adjacent cells. Several fra gene mutants were isolated 25 y ago on the basis of their phenotypes: inability to fix nitrogen and fragmentation of filaments upon transfer from N+ to N- media. Cryopreservation combined with electron tomography were used to investigate the role of three fra gene products in channel formation. FraC and FraG are clearly involved in channel formation, whereas FraD has a minor part. Additionally, FraG was located close to the cytoplasmic membrane and in the heterocyst neck, using immunogold labeling with antibody raised to the N-terminal domain of the FraG protein.

The Crystal Structure of Nitrosomonas europaea Sucrose Synthase Reveals Critical Conformational Changes and Insights into Sucrose Metabolism in Prokaryotes

In this paper we report the first crystal structure of a prokaryotic sucrose synthase from the nonphotosynthetic bacterium Nitrosomonas europaea. The obtained structure was in an open form, whereas the only other available structure, from the plant Arabidopsis thaliana, was in a closed conformation. Comparative structural analysis revealed a "hinge-latch" combination, which is critical to transition between the open and closed forms of the enzyme. The N. europaea sucrose synthase shares the same fold as the GT-B family of the retaining glycosyltransferases. In addition, a triad of conserved homologous catalytic residues in the family was shown to be functionally critical in the N. europaea sucrose synthase (Arg567, Lys572, and Glu663). This implies that sucrose synthase shares not only a common origin with the GT-B family but also a similar catalytic mechanism. The enzyme preferred transferring glucose from ADP-glucose rather than UDP-glucose like the eukaryotic counterparts. This predicts that these prokaryotic organisms have a different sucrose metabolic scenario from plants. Nucleotide preference determines where the glucose moiety is targeted after sucrose is degraded.

IMPORTANCE

We obtained biochemical and structural evidence of sucrose metabolism in nonphotosynthetic bacteria. Until now, only sucrose synthases from photosynthetic organisms have been characterized. Here, we provide the crystal structure of the sucrose synthase from the chemolithoautotroph N. europaea. The structure supported that the enzyme functions with an open/close induced fit mechanism. The enzyme prefers as the substrate adenine-based nucleotides rather than uridine-based like the eukaryotic counterparts, implying a strong connection between sucrose and glycogen metabolism in these bacteria. Mutagenesis data showed that the catalytic mechanism must be conserved not only in sucrose synthases but also in all other retaining GT-B glycosyltransferases.

Identification of sucrose synthase in nonphotosynthetic bacteria and characterization of the recombinant enzymes

Sucrose synthase (SuSy) catalyzes the reversible conversion of sucrose and a nucleoside diphosphate into fructose and nucleotide (NDP)-glucose. To date, only SuSy's from plants and cyanobacteria, both photosynthetic organisms, have been characterized. Here, four prokaryotic SuSy enzymes from the nonphotosynthetic organisms Nitrosomonas Europaea (SuSyNe), Acidithiobacillus caldus (SuSyAc), Denitrovibrio acetiphilus (SusyDa), and Melioribacter roseus (SuSyMr) were recombinantly expressed in Escherichia coli and thoroughly characterized. The purified enzymes were found to display high-temperature optima (up to 80 °C), high activities (up to 125 U/mg), and high thermostability (up to 15 min at 60 °C). Furthermore, SuSyAc, SuSyNe, and SuSyDa showed a clear preference for ADP as nucleotide, as opposed to plant SuSy's which prefer UDP. A structural and mutational analysis was performed to elucidate the difference in NDP preference between eukaryotic and prokaryotic SuSy's. Finally, the physiological relevance of this enzyme specificity is discussed in the context of metabolic pathways and genomic organization.

Differential transcriptional regulation of orthologous dps genes from two closely related heterocyst-forming cyanobacteria

In cyanobacteria, DNA-binding proteins from starved cells (Dps) play an important role in the cellular response to oxidative and nutritional stresses. In this study, we have characterized the cell-type specificity and the promoter regions of two orthologous dps genes, Npun_R5799 in Nostoc punctiforme and alr3808 in Anabaena sp. PCC 7120. A transcriptional start site (TSS), identical in location to the previously identified proximal TSS of alr3808, was identified for Npun_R5799 under both combined nitrogen supplemented and N2-fixing growth conditions. However, only alr3808 was also transcribed from a second distal TSS. Sequence homologies suggest that the promoter region containing the distal TSS is not conserved upstream of orthologous genes among heterocyst-forming cyanobacteria. The analysis of promoter GFP-reporter strains showed a different role in governing cell-type specificity between the proximal and distal promoter of alr3808. We here confirmed the heterocyst specificity of the distal promoter of alr3808 and described a very early induction of its expression during proheterocyst differentiation. In contrast, the complete promoters of both genes were active in all cells. Even though Npun_R5799 and alr3808 are orthologs, the regulation of their respective expression differs, indicating distinctions in the function of these cyanobacterial Dps proteins depending on the strain and cell type.

Sucrose in cyanobacteria: from a salt-response molecule to play a key role in nitrogen fixation

In the biosphere, sucrose is mainly synthesized in oxygenic photosynthetic organisms, such as cyanobacteria, green algae and land plants, as part of the carbon dioxide assimilation pathway. Even though its central position in the functional biology of plants is well documented, much less is known about the role of sucrose in cyanobacteria. In those prokaryotes, sucrose accumulation has been associated with salt acclimation, and considered as a compatible solute in low-salt tolerant strains. In the last years, functional characterizations of sucrose metabolizing enzymes, metabolic control analysis, cellular localization of gene expressions, and reverse genetic experiments have revealed that sucrose metabolism is crucial in the diazotrophic growth of heterocystic strains, and besides, that it can be connected to glycogen synthesis. This article briefly summarizes the current state of knowledge of sucrose physiological functions in modern cyanobacteria and how they might have evolved taking into account the phylogenetic analyses of sucrose enzymes.

Sucrose synthesis in the nitrogen-fixing Cyanobacterium Anabaena sp. strain PCC 7120 is controlled by the two-component response regulator OrrA

The filamentous, nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 accumulates sucrose as a compatible solute against salt stress. Sucrose-phosphate synthase activity, which is responsible for the sucrose synthesis, is increased by salt stress, but the mechanism underlying the regulation of sucrose synthesis remains unknown. In the present study, a response regulator, OrrA, was shown to control sucrose synthesis. Expression of spsA, which encodes a sucrose-phosphate synthase, and susA and susB, which encode sucrose synthases, was induced by salt stress. In the orrA disruptant, salt induction of these genes was completely abolished. The cellular sucrose level of the orrA disruptant was reduced to 40% of that in the wild type under salt stress conditions. Moreover, overexpression of orrA resulted in enhanced expression of spsA, susA, and susB, followed by accumulation of sucrose, without the addition of NaCl. We also found that SigB2, a group 2 sigma factor of RNA polymerase, regulated the early response to salt stress under the control of OrrA. It is concluded that OrrA controls sucrose synthesis in collaboration with SigB2.

Functional characterization of Synechococcus amylosucrase and fructokinase encoding genes discovers two novel actors on the stage of cyanobacterial sucrose metabolism

Plants and most cyanobacteria metabolize sucrose (Suc) with a similar set of enzymes. In Synechococcus sp. PCC 7002, a marine cyanobacterium strain, genes involved in Suc synthesis (spsA and sppA) have been characterized; however, its breakdown was still unknown. Indeed, neither invertase nor sucrose synthase genes, usually found in plants and cyanobacteria, were found in that Synechococcus genome. In the present study, we functionally characterized the amsA gene that codes for an amylosucrase (AMS), a glycoside-hydrolase family 13 enzyme described in bacteria, which may catabolyze Suc in Synechococcus. Additionally, we identified and characterized the frkA gene that codes for a fructokinase (FRK), enzyme that yields fructose-6P, one of the substrates for Suc synthesis. Interestingly, we demonstrate that spsA, sppA, frkA and amsA are grouped in a transcriptional unit that were named Suc cluster, whose expression is increased in response to a salt treatment. This is the first report on the characterization of an AMS and FRK in an oxygenic photosynthetic microorganism, which could be associated with Suc metabolism.

Identification and characterization of UDP-glucose pyrophosphorylase in cyanobacteria Anabaena sp. PCC 7120

Exopolysaccharides produced by photosynthetic cyanobacteria have received considerable attention in recent years for their potential applications in the production of renewable biofuels. Particularly, cyanobacterial cellulose is one of the most promising products because it is extracellularly secreted as a non-crystalline form, which can be easily harvested from the media and converted into glucose units. In cyanobacteria, the production of UDP-glucose, the cellulose precursor, is a key step in the cellulose synthesis pathway. UDP-glucose is synthesized from UTP and glucose-1-phosphate (Glc-1P) by UDP-glucose pyrophosphorylase (UGPase), but this pathway in cyanobacteria has not been well characterized. Therefore, to elucidate the overall cellulose biosynthesis pathway in cyanobacteria, we studied the putative UGPase All3274 and seven other putative NDP-sugar pyrophosphorylases (NSPases), All4645, Alr2825, Alr4491, Alr0188, Alr3400, Alr2361, and Alr3921 of Anabaena sp. PCC 7120. Assays using the purified recombinant proteins revealed that All3274 exhibited UGPase activity, All4645, Alr2825, Alr4491, Alr0188, and Alr3921 exhibited pyrophosphorylase activities on ADP-glucose, CDP-glucose, dTDP-glucose, GDP-mannose, and UDP-N-acetylglucosamine, respectively. Further characterization of All3274 revealed that the kcat for UDP-glucose formation was one or two orders lower than those of other known UGPases. The activity and dimerization tendency of All3274 increased at higher enzyme concentrations, implying catalytic activation by dimerization. However, most interestingly, All3274 dimerization was inhibited by UTP and Glc-1P, but not by UDP-glucose. This study presents the first in vitro characterization of a cyanobacterial UGPase, and provides insights into biotechnological attempts to utilize the photosynthetic production of cellulose from cyanobacteria.

Cell-specific gene expression in Anabaena variabilis grown phototrophically, mixotrophically, and heterotrophically

BACKGROUND

When the filamentous cyanobacterium Anabaena variabilis grows aerobically without combined nitrogen, some vegetative cells differentiate into N2-fixing heterocysts, while the other vegetative cells perform photosynthesis. Microarrays of sequences within protein-encoding genes were probed with RNA purified from extracts of vegetative cells, from isolated heterocysts, and from whole filaments to investigate transcript levels, and carbon and energy metabolism, in vegetative cells and heterocysts in phototrophic, mixotrophic, and heterotrophic cultures.

RESULTS

Heterocysts represent only 5% to 10% of cells in the filaments. Accordingly, levels of specific transcripts in vegetative cells were with few exceptions very close to those in whole filaments and, also with few exceptions (e.g., nif1 transcripts), levels of specific transcripts in heterocysts had little effect on the overall level of those transcripts in filaments. In phototrophic, mixotrophic, and heterotrophic growth conditions, respectively, 845, 649, and 846 genes showed more than 2-fold difference (p < 0.01) in transcript levels between vegetative cells and heterocysts. Principal component analysis showed that the culture conditions tested affected transcript patterns strongly in vegetative cells but much less in heterocysts. Transcript levels of the genes involved in phycobilisome assembly, photosynthesis, and CO2 assimilation were high in vegetative cells in phototrophic conditions, and decreased when fructose was provided. Our results suggest that Gln, Glu, Ser, Gly, Cys, Thr, and Pro can be actively produced in heterocysts. Whether other protein amino acids are synthesized in heterocysts is unclear. Two possible components of a sucrose transporter were identified that were upregulated in heterocysts in two growth conditions. We consider it likely that genes with unknown function represent a larger fraction of total transcripts in heterocysts than in vegetative cells across growth conditions.

CONCLUSIONS

This study provides the first comparison of transcript levels in heterocysts and vegetative cells from heterocyst-bearing filaments of Anabaena. Although the data presented do not give a complete picture of metabolism in either type of cell, they provide a metabolic scaffold on which to build future analyses of cell-specific processes and of the interactions of the two types of cells.

Bifunctional sucrose phosphate synthase/phosphatase is involved in the sucrose biosynthesis by Methylobacillus flagellatus KT

The aerobic obligate methylotroph Methylobacillus flagellatus KT was shown to synthesize sucrose in the presence of 0.5-2% NaCl in the growth medium. In the genome of this bacterium, an open reading frame (ORF) encoding a predicted 84-kD polypeptide homologous to the plant and cyanobacterial sucrose phosphate synthases (SPSs) was found. Using heterologous expression of the putative sps gene in Escherichia coli, followed by affinity chromatography, pure recombinant protein SPS-His6 was obtained. The enzyme catalyzed two reactions: conversion of fructose 6-phosphate and UDP-glucose into sucrose 6-phosphate and hydrolysis of sucrose 6-phosphate to sucrose. The bifunctional sucrose phosphate synthase/phosphatase (SPS/SPP) was a 340 kDa homotetrameric Mg(2+) -dependent enzyme activated by fructose 1,6-phosphate2 and ATP but inhibited by glucose 6-phosphate, fructose 1-phosphate, AMP and inorganic phosphate. The amino acid sequence of the protein had a C-terminal domain homologous to SPPs. This correlated with the absence of the spp gene in the M. flagellatus chromosome. The ORFs homologous to the M. flagellatus SPS were found in the genomes of another obligate methylotroph Methylovorus glucosetrophus as well as the lithoautotrophic bacteria Acidithiobacillus ferrooxidans, Nitrosomonas europaea and Nitrosospira multiformis whose genomes lacked the spp genes. Thus, data extending the knowledge of biochemical properties of bacterial SPSs have been obtained.

Starch biosynthesis, its regulation and biotechnological approaches to improve crop yields

Structurally composed of the glucose homopolymers amylose and amylopectin, starch is the main storage carbohydrate in vascular plants, and is synthesized in the plastids of both photosynthetic and non-photosynthetic cells. Its abundance as a naturally occurring organic compound is surpassed only by cellulose, and represents both a cornerstone for human and animal nutrition and a feedstock for many non-food industrial applications including production of adhesives, biodegradable materials, and first-generation bioethanol. This review provides an update on the different proposed pathways of starch biosynthesis occurring in both autotrophic and heterotrophic organs, and provides emerging information about the networks regulating them and their interactions with the environment. Special emphasis is given to recent findings showing that volatile compounds emitted by microorganisms promote both growth and the accumulation of exceptionally high levels of starch in mono- and dicotyledonous plants. We also review how plant biotechnologists have attempted to use basic knowledge on starch metabolism for the rational design of genetic engineering traits aimed at increasing starch in annual crop species. Finally we present some potential biotechnological strategies for enhancing starch content.

Insights into the physiology and ecology of the brackish-water-adapted Cyanobacterium Nodularia spumigena CCY9414 based on a genome-transcriptome analysis

Nodularia spumigena is a filamentous diazotrophic cyanobacterium that dominates the annual late summer cyanobacterial blooms in the Baltic Sea. But N. spumigena also is common in brackish water bodies worldwide, suggesting special adaptation allowing it to thrive at moderate salinities. A draft genome analysis of N. spumigena sp. CCY9414 yielded a single scaffold of 5,462,271 nucleotides in length on which genes for 5,294 proteins were annotated. A subsequent strand-specific transcriptome analysis identified more than 6,000 putative transcriptional start sites (TSS). Orphan TSSs located in intergenic regions led us to predict 764 non-coding RNAs, among them 70 copies of a possible retrotransposon and several potential RNA regulators, some of which are also present in other N2-fixing cyanobacteria. Approximately 4% of the total coding capacity is devoted to the production of secondary metabolites, among them the potent hepatotoxin nodularin, the linear spumigin and the cyclic nodulapeptin. The transcriptional complexity associated with genes involved in nitrogen fixation and heterocyst differentiation is considerably smaller compared to other Nostocales. In contrast, sophisticated systems exist for the uptake and assimilation of iron and phosphorus compounds, for the synthesis of compatible solutes, and for the formation of gas vesicles, required for the active control of buoyancy. Hence, the annotation and interpretation of this sequence provides a vast array of clues into the genomic underpinnings of the physiology of this cyanobacterium and indicates in particular a competitive edge of N. spumigena in nutrient-limited brackish water ecosystems.

Enhancing sucrose synthase activity results in increased levels of starch and ADP-glucose in maize (Zea mays L.) seed endosperms

Sucrose synthase (SuSy) is a highly regulated cytosolic enzyme that catalyzes the conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose. In cereal endosperms, it is widely assumed that the stepwise reactions of SuSy, UDPglucose pyrophosphorylase and ADPglucose (ADPG) pyrophosphorylase (AGP) take place in the cytosol to convert sucrose into ADPG necessary for starch biosynthesis, although it has also been suggested that SuSy may participate in the direct conversion of sucrose into ADPG. In this study, the levels of the major primary carbon metabolites, and the activities of starch metabolism-related enzymes were assessed in endosperms of transgenic maize plants ectopically expressing StSUS4, which encodes a potato SuSy isoform. A total of 29 fertile lines transformed with StSUS4 were obtained, five of them containing a single copy of the transgene that was still functional after five generations. The number of seeds per ear of the five transgenic lines containing a single StSUS4 copy was comparable with that of wild-type (WT) control seeds. However, transgenic seeds accumulated 10-15% more starch at the mature stage, and contained a higher amylose/amylopectin balance than WT seeds. Endosperms of developing StSUS4-expressing seeds exhibited a significant increase in SuSy activity, and in starch and ADPG contents when compared with WT endosperms. No significant changes could be detected in the transgenic seeds in the content of soluble sugars, and in activities of starch metabolism-related enzymes when compared with WT seeds. A suggested metabolic model is presented wherein both AGP and SuSy are involved in the production of ADPG linked to starch biosynthesis in maize endosperm cells.