A major facilitator superfamily protein, HepP, is involved in formation of the heterocyst envelope polysaccharide in the cyanobacterium Anabaena sp. strain PCC 7120

Effects of Cold Plasma Pretreatment on the Synthesis of Polysaccharide from Pleurotus ostreatus

Fungal polysaccharides have attracted wide attention because of their medical pharmaceutical and health care value. So far, many efforts have been made in strain improvement to produce polysaccharides on a large scale at low cost. Here, a novel cold plasma-induced strain improvement technology was employed to pretreat Pleurotus ostreatus CGMCC 5.374 by radio-frequency (RF) low-vacuum cold plasma (LVCP) for the purpose of obtaining a high-yield polysaccharide strain. The optimum pretreatment conditions including discharge power, treatment time, and working pressure were determined by single factor and orthogonal experiment in succession. Furthermore, transcriptome analysis was conducted to study the effects of RF-LVCP on cell metabolism and proliferation. Results showed that under the optimal condition of discharge power of 130 W, treatment time of 25 s and working pressure of 140 Pa, polysaccharide content in mycelium was increased by 3.16% after 6 days in comparison to the original strain. Transcriptome analysis showed that RF-LVCP is helpful for specific gene transcription profiles, Gene Ontology (GO) and KEGG pathways, of which the differentially expressed genes (DEGs) were mainly involve with the up-regulation of polysaccharide transport, physiology, synthesis and metabolism, as well as the down-regulation of polysaccharide hydrolysis and macromolecular degradation.

Evaluation of self-sustaining cyanobacterial biofilms for technical applications

Cyanobacteria are potent microorganisms for sustainable photo-biotechnological production processes, as they are depending mainly on water, light, and carbon dioxide. Persisting challenges preventing their application include low biomass, as well as insufficient process stability and productivity. Here, we evaluate different cyanobacteria to be applied in a novel capillary biofilm reactor. Cultivated as biofilms, the organisms self-immobilize to the reactor walls, reach high biomass and enable long and robust production processes. As ‘best performer’ Tolypothrix sp. PCC 7712 emerged from this study. It reached the highest biomass in the reactors with 62.6 ± 6.34 g_BDWL⁻¹, produced 0.14 μmole H₂ mg_{Chl a}⁻¹h⁻¹ under N₂-fixing conditions, showed optimal surface coverage of the available growth surface, and only minor detachment in contrast to other tested species, highlighting its potential for photobiotechnology in the near future.

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Testing biofilm forming ability of selected cyanobacterial strains in a technical setting.

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Tolypothrix sp. PCC 7712 shows the highest biomass, 62.6 ± 6.34 g_BDWL⁻¹, the lowest detachment, and high surface coverage.

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Continuous H₂ production under N₂ fixation conditions.

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Tolypothrix sp. PCC 7712 as new photo-biotechnology workhorse.

Iron deficiency influences NtcA-dependent regulation of fatty acid desaturation and heterocyte envelop formation in Anabaena sp. PCC 7120

In Anabaena sp. PCC 7120, iron is an essential trace element and its availability determines proper functioning of several kinds of metabolisms. Iron deficiency leads to several unavoidable consequences including membrane damage. In the present study, we dealt with the impact of iron deficiency on NtcA (global nitrogen regulator)-dependent regulation of two important processes, i.e. fatty acid desaturation and heterocyte envelop formation in cyanobacterium Anabaena sp. PCC 7120. In Anabaena sp. PCC 7120, NtcA regulates fatty acid desaturation by regulating enzyme fatty acid desaturases. The NtcA-based regulation of fatty acid desaturation may be direct or indirect. Furthermore, the expression of genes involved in the heterocyte envelope polysaccharide (HEP) layer formation (hepABCK) and heterocyte-specific glycolipids (HGLs) synthesis (devH, hglE_A , prpJ and devB) were also under the control of NtcA and reduced under iron deficiency background. The enhanced expression of furA and early downregulation of ntcA under iron deficiency is responsible for reduction in fatty acid desaturation as well as decrease in the expression of genes involved in HEP layer formation and HGL synthesis. Overall results confirmed that iron deficiency influences the NtcA-based regulation of fatty acid desaturation and heterocyte envelop formation in Anabaena sp. PCC 7120.

Catabolic pathway of arginine in Anabaena involves a novel bifunctional enzyme that produces proline from arginine

Arginine participates widely in metabolic processes. The heterocyst-forming cyanobacterium Anabaena catabolizes arginine to produce proline and glutamate, with concomitant release of ammonium, as major products. Analysis of mutant Anabaena strains showed that this catabolic pathway is the product of two genes, agrE (alr4995) and putA (alr0540). The predicted PutA protein is a conventional, bifunctional proline oxidase that produces glutamate from proline. In contrast, AgrE is a hitherto unrecognized enzyme that contains both an N-terminal α/β propeller domain and a unique C-terminal domain of previously unidentified function. In vitro analysis of the proteins expressed in Escherichia coli or Anabaena showed arginine dihydrolase activity of the N-terminal domain and ornithine cyclodeaminase activity of the C-terminal domain, overall producing proline from arginine. In the diazotrophic filaments of Anabaena, β-aspartyl-arginine dipeptide is transferred from the heterocysts to the vegetative cells, where it is cleaved producing aspartate and arginine. Both agrE and putA were found to be expressed at higher levels in vegetative cells than in heterocysts, implying that arginine is catabolized by the AgrE-PutA pathway mainly in the vegetative cells. Expression in Anabaena of a homolog of the C-terminal domain of AgrE obtained from Methanococcus maripaludis enabled us to identify an archaeal ornithine cyclodeaminase.

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.

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.

On the cradle of CCM research: discovery, development, and challenges ahead

Herein, 40 years after its discovery, I briefly and critically survey the development of ideas that propelled research on CO2-concentrating mechanisms (CCMs; a term proposed by Dean Price) of phytoplankton, mainly focusing on cyanobacteria. This is not a comprehensive review on CCM research, but a personal view on the past developments and challenges that lie ahead.

Specific Glucoside Transporters Influence Septal Structure and Function in the Filamentous, Heterocyst-Forming Cyanobacterium Anabaena sp. Strain PCC 7120

When deprived of combined nitrogen, some filamentous cyanobacteria contain two cell types: vegetative cells that fix CO₂ through oxygenic photosynthesis and heterocysts that are specialized in N₂ fixation. In the diazotrophic filament, the vegetative cells provide the heterocysts with reduced carbon (mainly in the form of sucrose) and heterocysts provide the vegetative cells with combined nitrogen. Septal junctions traverse peptidoglycan through structures known as nanopores and appear to mediate intercellular molecular transfer that can be traced with fluorescent markers, including the sucrose analog esculin (a coumarin glucoside) that is incorporated into the cells. Uptake of esculin by the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 was inhibited by the α-glucosides sucrose and maltose. Analysis of Anabaena mutants identified components of three glucoside transporters that move esculin into the cells: GlsC (Alr4781) and GlsP (All0261) are an ATP-binding subunit and a permease subunit of two different ABC transporters, respectively, and HepP (All1711) is a major facilitator superfamily (MFS) protein that was shown previously to be involved in formation of the heterocyst envelope. Transfer of fluorescent markers (especially calcein) between vegetative cells of Anabaena was impaired by mutation of glucoside transporter genes. GlsP and HepP interact in bacterial two-hybrid assays with the septal junction-related protein SepJ, and GlsC was found to be necessary for the formation of a normal number of septal peptidoglycan nanopores and for normal subcellular localization of SepJ. Therefore, beyond their possible role in nutrient uptake in Anabaena, glucoside transporters influence the structure and function of septal junctions.IMPORTANCE Heterocyst-forming cyanobacteria have the ability to perform oxygenic photosynthesis and to assimilate atmospheric CO₂ and N₂ These organisms grow as filaments that fix these gases specifically in vegetative cells and heterocysts, respectively. For the filaments to grow, these types of cells exchange nutrients, including sucrose, which serves as a source of reducing power and of carbon skeletons for the heterocysts. Movement of sucrose between cells in the filament takes place through septal junctions and has been traced with a fluorescent sucrose analog, esculin, that can be taken up by the cells. Here, we identified α-glucoside transporters of Anabaena that mediate uptake of esculin and, notably, influence septal structure and the function of septal junctions.

Structural and enzymatic analyses of a glucosyltransferase Alr3699/HepE involved in Anabaena heterocyst envelop polysaccharide biosynthesis

Formation of the heterocyst envelope polysaccharide (HEP) is a key process for cyanobacterial heterocyst differentiation. The maturation of HEP in Anabaena sp. strain PCC 7120 is controlled by a gene cluster termed HEP island in addition to an operon alr3698-alr3699, which encodes two putative proteins termed Alr3698/HepD and Alr3699/HepE. Here we report the crystal structures of HepE in the apo-form and three complex forms that bind to UDP-glucose (UDPG), UDP&glucose, and UDP, respectively. The overall structure of HepE displays a typical GT-B fold of glycosyltransferases, comprising two separate β/α/β Rossmann-fold domains that form an inter-domain substrate-binding crevice. Structural analyses combined with enzymatic assays indicate that HepE is a glucosyltransferase using UDPG as a sugar donor. Further site-directed mutageneses enable us to assign the key residues that stabilize the sugar donor and putative acceptor. Based on the comparative structural analyses, we propose a putative catalytic cycle of HepE, which undergoes "open-closed-open" conformational changes upon binding to the substrates and release of products. These findings provide structural and catalytic insights into the first enzyme involved in the HEP biosynthesis pathway.

Inactivation of agmatinase expressed in vegetative cells alters arginine catabolism and prevents diazotrophic growth in the heterocyst-forming cyanobacterium Anabaena

Arginine decarboxylase produces agmatine, and arginase and agmatinase are ureohydrolases that catalyze the production of ornithine and putrescine from arginine and agmatine, respectively, releasing urea. In the genome of the filamentous, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120, ORF alr2310 putatively encodes an ureohydrolase. Cells of Anabaena supplemented with [¹⁴C]arginine took up and catabolized this amino acid generating a set of labeled amino acids that included ornithine, proline, and glutamate. In an alr2310 deletion mutant, an agmatine spot appeared and labeled glutamate increased with respect to the wild type, suggesting that Alr2310 is an agmatinase rather than an arginase. As determined in cell-free extracts, agmatinase activity could be detected in the wild type but not in the mutant. Thus, alr2310 is the Anabaena speB gene encoding agmatinase. The Δalr2310 mutant accumulated large amounts of cyanophycin granule polypeptide, lacked nitrogenase activity, and did not grow diazotrophically. Growth tests in solid media showed that agmatine is inhibitory for Anabaena, especially under diazotrophic conditions, suggesting that growth of the mutant is inhibited by non-metabolized agmatine. Measurements of incorporation of radioactivity from [¹⁴C]leucine into macromolecules showed, however, a limited inhibition of protein synthesis in the Δalr2310 mutant. Analysis of an Anabaena strain producing an Alr2310-GFP (green fluorescent protein) fusion showed expression in vegetative cells but much less in heterocysts, implying compartmentalization of the arginine decarboxylation pathway in the diazotrophic filaments of this heterocyst-forming cyanobacterium.

Compartmentalized cyanophycin metabolism in the diazotrophic filaments of a heterocyst-forming cyanobacterium

Heterocyst-forming cyanobacteria are multicellular organisms in which growth requires the activity of two metabolically interdependent cell types, the vegetative cells that perform oxygenic photosynthesis and the dinitrogen-fixing heterocysts. Vegetative cells provide the heterocysts with reduced carbon, and heterocysts provide the vegetative cells with fixed nitrogen. Heterocysts conspicuously accumulate polar granules made of cyanophycin [multi-L-arginyl-poly (L-aspartic acid)], which is synthesized by cyanophycin synthetase and degraded by the concerted action of cyanophycinase (that releases β-aspartyl-arginine) and isoaspartyl dipeptidase (that produces aspartate and arginine). Cyanophycin synthetase and cyanophycinase are present at high levels in the heterocysts. Here we created a deletion mutant of gene all3922 encoding isoaspartyl dipeptidase in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. The mutant accumulated cyanophycin and β-aspartyl-arginine, and was impaired specifically in diazotrophic growth. Analysis of an Anabaena strain bearing an All3922-GFP (green fluorescent protein) fusion and determination of the enzyme activity in specific cell types showed that isoaspartyl dipeptidase is present at significantly lower levels in heterocysts than in vegetative cells. Consistently, isolated heterocysts released substantial amounts of β-aspartyl-arginine. These observations imply that β-aspartyl-arginine produced from cyanophycin in the heterocysts is transferred intercellularly to be hydrolyzed, producing aspartate and arginine in the vegetative cells. Our results showing compartmentalized metabolism of cyanophycin identify the nitrogen-rich molecule β-aspartyl-arginine as a nitrogen vehicle in the unique multicellular system represented by the heterocyst-forming cyanobacteria.

FurA influences heterocyst differentiation in Anabaena sp. PCC 7120

In Anabaena sp. PCC 7120, FurA is a global transcriptional regulator whose expression is strongly induced by NtcA in proheterocysts and remains stably expressed in mature heterocysts. In the present study, overexpression of furA partially suppressed heterocyst differentiation by impairing morphogenesis at an early stage. Recombinant purified FurA specifically bound in vitro to the promoter regions of ntcA, while quantitative RT-PCR analyses indicated that furA overexpression strongly affected the transient increase of ntcA expression that occurs shortly after nitrogen step-down. Overall, the results suggest a connection between iron homeostasis and heterocyst differentiation via FurA, by modulating the expression of ntcA.

Alr5068, a Low-Molecular-Weight protein tyrosine phosphatase, is involved in formation of the heterocysts polysaccharide layer in the cyanobacterium Anabaena sp. PCC 7120

The filamentous cyanobacterium Anabaena sp. PCC 7120 forms nitrogen-fixing heterocysts after deprivation of combined nitrogen. Under such conditions, vegetative cells provide heterocysts with photosynthate and receive fixed nitrogen from the latter. Heterocyst envelope contains a glycolipid layer and a polysaccharide layer to restrict the diffusion of oxygen into heterocysts. Low-Molecular-Weight protein tyrosine phosphatases (LMW-PTPs) are involved in the biosynthesis of exopolysaccharides in bacteria. Alr5068, a protein from Anabaena sp. PCC 7120, shows significant sequence similarity with LMW-PTPs. In this study we characterized the enzymatic properties of Alr5068 and showed that it can dephosphorylate several autophosphorylated tyrosine kinases (Alr2856, Alr3059 and All4432) of Anabaena sp. PCC 7120 in vitro. Several conserved residues among LMW-PTPs are shown to be essential for the phosphatase activity of Alr5068. Overexpression of alr5068 results in a strain unable to survive under diazotrophic conditions, with the formation of morphologically mature heterocysts detached from the filaments. Overexpression of an alr5068 allele that lost phosphatase activity led to the formation of heterocyst with an impaired polysaccharide layer. The alr5068 gene was upregulated after nitrogen step-down and its mutation affected the expression of hepA and hepC, two genes necessary for the formation of the heterocyst envelope polysaccharide (HEP) layer. Our results suggest that Alr5068 is associated with the production of HEP in Anabaena sp. PCC 7120.