The role of the protective shield against UV-C radiation and its molecular interactions in Nostoc species (Cyanobacteria).

Cyanobacteria possess special defense mechanisms to protect themselves against ultraviolet (UV) radiation. This study combines experimental and computational methods to identify the role of protective strategies in Nostoc species against UV-C radiation. To achieve this goal, various species of the genus Nostoc from diverse natural habitats in Iran were exposed to artificial UV-C radiation. The results indicated that UV-C treatment significantly reduced the photosynthetic pigments while simultaneously increasing the activity of antioxidant enzymes. Notably, N. sphaericum ISB97 and Nostoc sp. ISB99, the brown Nostoc species isolated from habitats with high solar radiations, exhibited greater resistance compared to the green-colored species. Additionally, an increase in scytonemin content occurred with a high expression of key genes associated with its synthesis (scyF and scyD) during the later stages of UV-C exposure in these species. The molecular docking of scytonemin with lipopolysaccharides of the cyanobacteria that mainly cover the extracellular matrix revealed the top/side positioning of scytonemin on the glycans of these lipopolysaccharides to form a UV-protective shield. These findings pave the way for exploring the molecular effects of scytonemin in forming the UV protection shield in cyanobacteria, an aspect that has been ambiguous until now.

Nitric oxide mediates positive regulation of Nostoc flagelliforme polysaccharide yield via potential S-nitrosylation of G6PDH and UGDH.

Based on our previous findings that salicylic acid and jasmonic acid increased Nostoc flagelliforme polysaccharide yield by regulating intracellular nitric oxide (NO) levels, the mechanism through which NO affects polysaccharide biosynthesis in Nostoc flagelliforme was explored from the perspective of S-nitrosylation (SNO). The addition of NO donor and scavenger showed that intracellular NO had a significant positive effect on the polysaccharide yield of N. flagelliforme. To explore the mechanism, we investigated the relationship between NO levels and the activity of several key enzymes involved in polysaccharide biosynthesis, including fructose 1,6-bisphosphate aldolase (FBA), glucokinase (GK), glucose 6-phosphate dehydrogenase (G6PDH), mitochondrial isocitrate dehydrogenase (ICDH), and UDP-glucose dehydrogenase (UGDH). The enzymatic activities of G6PDH, ICDH, and UGDH were shown to be significantly correlated with the shifts in intracellular NO levels. For further validation, G6PDH, ICDH, and UGDH were heterologously expressed in Escherichia coli and purified via Ni+-NAT affinity chromatography, and subjected to a biotin switch assay and western blot analysis, which revealed that UGDH and G6PDH were susceptible to SNO. Furthermore, mass spectrometry analysis of proteins treated with S-nitrosoglutathione (GSNO) identified the SNO modification sites for UGDH and G6PDH as cysteine 423 and cysteine 249, respectively. These findings suggest that NO modulates polysaccharide biosynthesis in N. flagelliforme through SNO of UGDH and G6PDH. This reveals a potential mechanism through which NO promotes polysaccharide synthesis in N. flagelliforme, while also providing a new strategy for improving the industrial production of polysaccharides.

Study of alginate-encapsulated phycoerythrin in promoting the biological activity of synbiotic ice cream with Lactobacillus casei.

This study examines the effect of phycoerythrin (PE) from a cyanobacterial Nostoc strain encapsulated with alginate as a potential prebiotic to produce synbiotic ice cream products with Lactobacillus casei. It was found that the addition of the encapsulated PE affected, mostly favourably, the physicochemical properties, antioxidant activity, probiotic survival, volatile compound contents, and sensory acceptability of the synbiotic ice cream samples before and after aging at the freezing periods of one day to eight weeks. Thus, it confirms the prebiotic potential of PE for synbiotic ice creams with L. casei.

Determination of nutrient concentration in liquid culture of cyanobacteria Nostoc sp. by capillary electrophoresis and inductively coupled plasma mass spectrometry.

In this work, we demonstrate the use of capillary electrophoresis and inductively coupled plasma mass spectrometry, as competitive methods primarily for ion chromatography, to determine changes in the concentration of small inorganic ions in the Nostoc sp. culture medium. Although macronutrients were analyzed by capillary electrophoresis with conductivity detection, micronutrients were analyzed by inductively coupled plasma mass spectrometry. The different light settings (light intensity and spectral composition) had a visible effect on the culture growth and depletion of calcium, magnesium, and phosphate ions, and iron and manganese elements when comparing the behavior under red or violet light with that under blue light.

Assembly and comparative analyses of the Geosiphon pyriformis metagenome.

Geosiphon pyriformis, a representative of the fungal sub-phylum Glomeromycotina, is unique in its endosymbiosis with cyanobacteria within a fungal cell. This symbiotic relationship occurs in bladders containing nuclei of G. pyriformis, Mollicutes-like bacterial endosymbionts (MRE), and photosynthetically active and dividing cells of Nostoc punctiforme. Recent genome analyses have shed light on the biology of G. pyriformis, but the genome content and biology of its endosymbionts remain unexplored. To fill this gap, we gathered and examined metagenomic data from the bladders of G. pyriformis, where N. punctiforme and MRE are located. This ensures that our analyses are focused on the organs directly involved in the symbiosis. By comparing this data with the genetic information of related cyanobacteria and MREs from other species of Arbuscular Mycorrhizal Fungi, we aimed to reveal the genetic content of these organisms and understand how they interact at a genetic level to establish a symbiotic relationship. Our analyses uncovered significant gene expansions in the Nostoc endosymbiont, particularly in mobile elements and genes potentially involved in xenobiotic degradation. We also confirmed that the MRE of Glomeromycotina are monophyletic and possess a highly streamlined genome. These genomes show dramatic differences in both structure and content, including the presence of enzymes involved in environmental sensing and stress response.

Reversing the directionality of reactions between non-oxidative pentose phosphate pathway and glycolytic pathway boosts mycosporine-like amino acid production in Saccharomyces cerevisiae.

BACKGROUND: Mycosporine-like amino acids (MAAs) are a class of strongly UV-absorbing compounds produced by cyanobacteria, algae and corals and are promising candidates for natural sunscreen components. Low MAA yields from natural sources, coupled with difficulties in culturing its native producers, have catalyzed synthetic biology-guided approaches to produce MAAs in tractable microbial hosts like Escherichia coli, Saccharomyces cerevisiae and Corynebacterium glutamicum. However, the MAA titres obtained in these hosts are still low, necessitating a thorough understanding of cellular factors regulating MAA production. RESULTS: To delineate factors that regulate MAA production, we constructed a shinorine (mycosporine-glycine-serine) producing yeast strain by expressing the four MAA biosynthetic enzymes from Nostoc punctiforme in Saccharomyces cerevisiae. We show that shinorine is produced from the pentose phosphate pathway intermediate sedoheptulose 7-phosphate (S7P), and not from the shikimate pathway intermediate 3-dehydroquinate (3DHQ) as previously suggested. Deletions of transaldolase (TAL1) and phosphofructokinase (PFK1/PFK2) genes boosted S7P/shinorine production via independent mechanisms. Unexpectedly, the enhanced S7P/shinorine production in the PFK mutants was not entirely due to increased flux towards the pentose phosphate pathway. We provide multiple lines of evidence in support of a reversed pathway between glycolysis and the non-oxidative pentose phosphate pathway (NOPPP) that boosts S7P/shinorine production in the phosphofructokinase mutant cells. CONCLUSION: Reversing the direction of flux between glycolysis and the NOPPP offers a novel metabolic engineering strategy in Saccharomyces cerevisiae.

Photoreversible Aggregation of the Biliprotein Containing the First and Second GAF Domains of a Cyanobacteriochrome All2699 in Nostoc sp. PCC7120.

As plant photoreceptors, phytochromes are capable of detecting red light and far-red light, thereby governing plant growth. All2699 is a photoreceptor found in Nostoc sp. PCC7120 that specifically responds to red light and far-red light. All2699g1g2 is a truncated protein carrying the first and second GAF (cGMP phosphodiesterase/adenylyl cyclase/FhlA) domains of All2699. In this study, we found that, upon exposure to red light, the protein underwent aggregation, resulting in the formation of protein aggregates. Conversely, under far-red light irradiation, these protein aggregates dissociated. We delved into the factors that impact the aggregation of All2699g1g2, focusing on the protein structure. Our findings showed that the GAF2 domain contains a low-complexity (LC) loop region, which plays a crucial role in mediating protein aggregation. Specifically, phenylalanine at position 239 within the LC loop region was identified as a key site for the aggregation process. Furthermore, our research revealed that various factors, including irradiation time, temperature, concentration, NaCl concentration, and pH value, can impact the aggregation of All2699g1g2. The aggregation led to variations in Pfr concentration depending on temperature, NaCl concentration, and pH value. In contrast, ΔLC did not aggregate and therefore lacked responses to these factors. Consequently, the LC loop region of All2699g1g2 extended and enhanced sensory properties.

The crane fly glycosylated triketide δ-lactone cornicinine elicits akinete differentiation of the cyanobiont in aquatic Azolla fern symbioses.

The restriction of plant-symbiont dinitrogen fixation by an insect semiochemical had not been previously described. Here we report on a glycosylated triketide δ-lactone from Nephrotoma cornicina crane flies, cornicinine, that causes chlorosis in the floating-fern symbioses from the genus Azolla. Only the glycosylated trans-A form of chemically synthesized cornicinine was active: 500 nM cornicinine in the growth medium turned all cyanobacterial filaments from Nostoc azollae inside the host leaf-cavities into akinetes typically secreting CTB-bacteriocins. Cornicinine further inhibited akinete germination in Azolla sporelings, precluding re-establishment of the symbiosis during sexual reproduction. It did not impact development of the plant Arabidopsis thaliana or several free-living cyanobacteria from the genera Anabaena or Nostoc but affected the fern host without cyanobiont. Fern-host mRNA sequencing from isolated leaf cavities confirmed high NH4-assimilation and proanthocyanidin biosynthesis in this trichome-rich tissue. After cornicinine treatment, it revealed activation of Cullin-RING ubiquitin-ligase-pathways, known to mediate metabolite signaling and plant elicitation consistent with the chlorosis phenotype, and increased JA-oxidase, sulfate transport and exosome formation. The work begins to uncover molecular mechanisms of cyanobiont differentiation in a seed-free plant symbiosis important for wetland ecology or circular crop-production today, that once caused massive CO2 draw-down during the Eocene geological past.

UV-A radiation increases biomass yield by enhancing energy flow and carbon assimilation in the edible cyanobacterium Nostoc sphaeroides.

Ultraviolet (UV) A radiation (315-400 nm) is the predominant component of solar UV radiation that reaches the Earth's surface. However, the underlying mechanisms of the positive effects of UV-A on photosynthetic organisms have not yet been elucidated. In this study, we investigated the effects of UV-A radiation on the growth, photosynthetic ability, and metabolome of the edible cyanobacterium Nostoc sphaeroides. Exposures to 5-15 W m-2 (15-46 µmol photons m-2 s-1) UV-A and 4.35 W m-2 (20 µmol photons m-2 s-1) visible light for 16 days significantly increased the growth rate and biomass production of N. sphaeroides cells by 18%-30% and 15%-56%, respectively, compared to the non-UV-A-acclimated cells. Additionally, the UV-A-acclimated cells exhibited a 1.8-fold increase in the cellular nicotinamide adenine dinucleotide phosphate (NADP) pool with an increase in photosynthetic capacity (58%), photosynthetic efficiency (24%), QA re-oxidation, photosystem I abundance, and cyclic electron flow (87%), which further led to an increase in light-induced NADPH generation (31%) and ATP content (83%). Moreover, the UV-A-acclimated cells showed a 2.3-fold increase in ribulose-1,5-bisphosphate carboxylase/oxygenase activity, indicating an increase in their carbon-fixing capacity. Gas chromatography-mass spectrometry-based metabolomics further revealed that UV-A radiation upregulated the energy-storing carbon metabolism, as evidenced by the enhanced accumulation of sugars, fatty acids, and citrate in the UV-A-acclimated cells. Therefore, our results demonstrate that UV-A radiation enhances energy flow and carbon assimilation in the cyanobacterium N. sphaeroides.IMPORTANCEUltraviolet (UV) radiation exerts harmful effects on photo-autotrophs; however, several studies demonstrated the positive effects of UV radiation, especially UV-A radiation (315-400 nm), on primary productivity. Therefore, understanding the underlying mechanisms associated with the promotive effects of UV-A radiation on primary productivity can facilitate the application of UV-A for CO2 sequestration and lead to the advancement of photobiological sciences. In this study, we used the cyanobacterium Nostoc sphaeroides, which has an over 1,700-year history of human use as food and medicine, to explore its photosynthetic acclimation response to UV-A radiation. As per our knowledge, this is the first study to demonstrate that UV-A radiation increases the biomass yield of N. sphaeroides by enhancing energy flow and carbon assimilation. Our findings provide novel insights into UV-A-mediated photosynthetic acclimation and provide a scientific basis for the application of UV-A radiation for optimizing light absorption capacity and enhancing CO2 sequestration in the frame of a future CO2 neutral, circular, and sustainable bioeconomy.

Unravelling HetC as a peptidase-based ABC exporter driving functional cell differentiation in the cyanobacterium Nostoc PCC 7120.

The export of peptides or proteins is essential for a variety of important functions in bacteria. Among the diverse protein-translocation systems, peptidase-containing ABC transporters (PCAT) are involved in the maturation and export of quorum-sensing or antimicrobial peptides in Gram-positive bacteria and of toxins in Gram-negative organisms. In the multicellular and diazotrophic cyanobacterium Nostoc PCC 7120, the protein HetC is essential for the differentiation of functional heterocysts, which are micro-oxic and non-dividing cells specialized in atmospheric nitrogen fixation. HetC shows similarities to PCAT systems, but whether it actually acts as a peptidase-based exporter remains to be established. In this study, we show that the N-terminal part of HetC, encompassing the peptidase domain, displays a cysteine-type protease activity. The conserved catalytic residues conserved in this family of proteases are essential for the proteolytic activity of HetC and the differentiation of heterocysts. Furthermore, we show that the catalytic residue of the ATPase domain of HetC is also essential for cell differentiation. Interestingly, HetC has a cyclic nucleotide-binding domain at its N-terminus which can bind ppGpp in vitro and which is required for its function in vivo. Our results indicate that HetC is a peculiar PCAT that might be regulated by ppGpp to potentially facilitate the export of a signaling peptide essential for cell differentiation, thereby broadening the scope of PCAT role in Gram-negative bacteria.IMPORTANCEBacteria have a great capacity to adapt to various environmental and physiological conditions; it is widely accepted that their ability to produce extracellular molecules contributes greatly to their fitness. Exported molecules are used for a variety of purposes ranging from communication to adjust cellular physiology, to the production of toxins that bacteria secrete to fight for their ecological niche. They use export machineries for this purpose, the most common of which energize transport by hydrolysis of adenosine triphosphate. Here, we demonstrate that such a mechanism is involved in cell differentiation in the filamentous cyanobacterium Nostoc PCC 7120. The HetC protein belongs to the ATP-binding cassette transporter superfamily and presumably ensures the maturation of a yet unknown substrate during export. These results open interesting perspectives on cellular signaling pathways involving the export of regulatory peptides, which will broaden our knowledge of how these bacteria use two cell types to conciliate photosynthesis and nitrogen fixation.

Effects of ultraviolet and photosynthetically active radiation on morphogenesis, antioxidants and photoprotective defense mechanism in a hot-spring cyanobacterium Nostoc sp. strain VKB02.

The continuous increase in global temperature and ultraviolet radiation (UVR) causes profound impacts on the growth and physiology of photosynthetic microorganisms. The hot-spring cyanobacteria have a wide range of mitigation mechanisms to cope up against current unsustainable environmental conditions. In the present investigation, we have explored the indispensable mitigation strategies of an isolated hot-spring cyanobacterium Nostoc sp. strain VKB02 under simulated ultraviolet (UV-A, UV-B) and photosynthetically active radiation (PAR). The adaptive morphological changes were more significantly observed under PAB (PAR, UV-A, and UV-B) exposure as compared to P and PA (PAR and UV-A) irradiations. PAB exposure also exhibited a marked decline in pigment composition and photosynthetic efficiency by multi-fold increment of free radicals. To counteract the oxidative stress, enzymatic and non-enzymatic antioxidants defense were significantly enhanced many folds under PAB exposure as compared to the control. In addition, the cyanobacterium has also produced shinorine as a strong free radicals scavenger and excellent UV absorber for effective photoprotection against UV radiation. Therefore, the hot-spring cyanobacterium Nostoc sp. strain VKB02 has unique defense strategies for survival under prolonged lethal UVR conditions. This study will help in the understanding of environment-induced defense strategies and production of highly value-added green photo-protectants for commercial applications.

Arsenite S-Adenosylmethionine Methyltransferase Is Responsible for Antimony Biomethylation in Nostoc sp. PCC7120.

Antimony (Sb) biomethylation is an important but uninformed process in Sb biogeochemical cycling. Methylated Sb species have been widely detected in the environment, but the gene and enzyme for Sb methylation remain unknown. Here, we found that arsenite S-adenosylmethionine methyltransferase (ArsM) is able to catalyze Sb(III) methylation. The stepwise methylation by ArsM forms mono-, di-, and trimethylated Sb species. Sb(III) is readily coordinated with glutathione, forming the preferred ArsM substrate which is anchored on three conserved cysteines. Overexpressing arsM in Escherichia coli AW3110 conferred resistance to Sb(III) by converting intracellular Sb(III) into gaseous methylated species, serving as a detoxification process. Methylated Sb species were detected in paddy soil cultures, and phylogenetic analysis of ArsM showed its great diversity in ecosystems, suggesting a high metabolic potential for Sb(III) methylation in the environment. This study shows an undiscovered microbial process methylating aqueous Sb(III) into the gaseous phase, mobilizing Sb on a regional and even global scale as a re-emerging contaminant.

Identification of a morphogene required for tapered filament termini in filamentous cyanobacteria.

Although the photosynthetic cyanobacteria are monophyletic, they exhibit substantial morphological diversity across species, and even within an individual species due to phenotypic plasticity in response to life cycles and environmental signals. This is particularly prominent among the multicellular filamentous cyanobacteria. One example of this is the appearance of tapering at the filament termini. However, the morphogenes controlling this phenotype and the adaptive function of this morphology are not well defined. Here, using the model filamentous cyanobacterium Nostoc punctiforme ATCC29133 (PCC73102), we identify tftA, a morphogene required for the development of tapered filament termini. The tftA gene is specifically expressed in developing hormogonia, motile trichomes where the tapered filament morphology is observed, and encodes a protein containing putative amidase_3 and glucosaminidase domains, implying a function in peptidoglycan hydrolysis. Deletion of tftA abolished filament tapering inidcating that TftA plays a role in remodelling the cell wall to produce tapered filaments. Genomic conservation of tftA specifically in filamentous cyanobacteria indicates this is likely to be a conserved mechanism among these organisms. Finally, motility assays indicate that filaments with tapered termini migrate more efficiently through dense substratum, providing a plausible biological role for this morphology.

Structural Diversity and Biological Activity of Cyanopeptolins Produced by Nostoc edaphicum CCNP1411.

Cyanopeptolins (CPs) are one of the most commonly occurring class of cyanobacterial nonribosomal peptides. For the majority of these compounds, protease inhibition has been reported. In the current work, the structural diversity of cyanopeptolins produced by Nostoc edaphicum CCNP1411 was explored. As a result, 93 CPs, including 79 new variants, were detected and structurally characterized based on their mass fragmentation spectra. CPs isolated in higher amounts were additionally characterized by NMR. To the best of our knowledge, this is the highest number of cyanopeptides found in one strain. The biological assays performed with the 34 isolated CPs confirmed the significance of the amino acid located between Thr and the unique 3-amino-6-hydroxy-2-piperidone (Ahp) on the activity of the compounds against serine protease and HeLa cancer cells.

Zinc-Induced Fluorescence Turn-On in Native and Mutant Phycoerythrobilin-Binding Orange Fluorescent Proteins.

Cyanobacteriochrome (CBCR)-derived fluorescent proteins are a class of reporters that can bind bilin cofactors and fluoresce across the ultraviolet to the near-infrared spectrum. Derived from phytochrome-related photoreceptor proteins in cyanobacteria, many of these proteins use a single small GAF domain to autocatalytically bind a bilin and fluoresce. The second GAF domain of All1280 (All1280g2) from Nostoc sp. PCC7120 is a DXCF motif-containing protein that exhibits blue-light-responsive photochemistry when bound to its native cofactor, phycocyanobilin. All1280g2 can also bind non-photoswitching phycoerythrobilin (PEB), resulting in a highly fluorescent protein. Given the small size, high quantum yield, and that unlike green fluorescent proteins, bilin-binding proteins can be used in anaerobic organisms, the orange fluorescent All1280g2-PEB protein is a promising platform for designing new genetically encoded metal ion sensors. Here, we show that All1280g2-PEB undergoes a ∼5-fold reversible zinc-induced fluorescence enhancement with a blue-shifted emission maximum (572 to 517 nm), which is not observed for a related PEB-bound GAF from Synechocystis sp. PCC6803 (Slr1393g3). Zn2+ significantly enhances All1280g2-PEB fluorescence across a biologically relevant pH range from 6.0 to 9.0, with pH-dependent dissociation constants from 1 µM to ∼20-80 nM. Site-directed mutants aiming to sterically decrease and increase access to PEB show a decreased and similar amount of zinc-induced fluorescence enhancement. Mutation of the cysteine residue within the DXCF motif to alanine abolishes the zinc-induced fluorescence enhancement. Collectively, these results support the presence of a unique fluorescence-enhancing Zn2+ binding site in All1280g2-PEB likely involving coordination to the bilin cofactor and requiring a nearby cysteine residue.

The role of SepF in cell division and diazotrophic growth in the multicellular cyanobacterium Anabaena sp. strain PCC 7120.

The cyanobacterium Anabaena forms filaments of cells that grow by intercalary cell division producing adjoined daughter cells connected by septal junction protein complexes that provide filament cohesion and intercellular communication, representing a genuine case of bacterial multicellularity. In spite of their diderm character, cyanobacterial genomes encode homologs of SepF, a protein normally found in Gram-positive bacteria. In Anabaena, SepF is an essential protein that localized to the cell division ring and the intercellular septa. Overexpression of sepF had detrimental effects on growth, provoking conspicuous alterations in cell morphology that resemble the phenotype of mutants impaired in cell division, and altered the localization of the division-ring. SepF interacted with FtsZ and with the essential FtsZ tether ZipN. Whereas SepF from unicellular bacteria generally induces the bundling of FtsZ filaments, Anabaena SepF inhibited FtsZ bundling, reducing the thickness of the toroidal aggregates formed by FtsZ alone and eventually preventing FtsZ polymerization. Thus, in Anabaena SepF appears to have an essential role in cell division by limiting the polymerization of FtsZ to allow the correct formation and localization of the Z-ring. Expression of sepF is downregulated during heterocyst differentiation, likely contributing to the inhibition of Z-ring formation in heterocysts. Finally, the localization of SepF in intercellular septa and its interaction with the septal-junction related proteins SepJ and SepI suggest a role of SepF in the formation or stability of the septal complexes that mediate cell-cell adhesion and communication, processes that are key for the multicellular behavior of Anabaena.

Expanding the FurC (PerR) regulon in Anabaena (Nostoc) sp. PCC 7120: Genome-wide identification of novel direct targets uncovers FurC participation in central carbon metabolism regulation.

FurC (PerR, Peroxide Response Regulator) from Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120) is a master regulator engaged in the modulation of relevant processes including the response to oxidative stress, photosynthesis and nitrogen fixation. Previous differential gene expression analysis of a furC-overexpressing strain (EB2770FurC) allowed the inference of a putative FurC DNA-binding consensus sequence. In the present work, more data concerning the regulon of the FurC protein were obtained through the searching of the putative FurC-box in the whole Anabaena sp. PCC 7120 genome. The total amount of novel FurC-DNA binding sites found in the promoter regions of genes with known function was validated by electrophoretic mobility shift assays (EMSA) identifying 22 new FurC targets. Some of these identified targets display relevant roles in nitrogen fixation (hetR and hgdC) and carbon assimilation processes (cmpR, glgP1 and opcA), suggesting that FurC could be an additional player for the harmonization of carbon and nitrogen metabolisms. Moreover, differential gene expression of a selection of newly identified FurC targets was measured by Real Time RT-PCR in the furC-overexpressing strain (EB2770FurC) comparing to Anabaena sp. PCC 7120 revealing that in most of these cases FurC could act as a transcriptional activator.

PatU3 plays a central role in coordinating cell division and differentiation in pattern formation of filamentous cyanobacterium Nostoc sp. PCC 7120.

Spatial periodic signal for cell differentiation in some multicellular organisms is generated according to Turing's principle for pattern formation. How a dividing cell responds to the signal of differentiation is addressed with the filamentous cyanobacterium Nostoc sp. PCC 7120, which forms the patterned distribution of heterocysts. We show that differentiation of a dividing cell was delayed until its division was completed and only one daughter cell became heterocyst. A mutant of patU3, which encodes an inhibitor of heterocyst formation, showed no such delay and formed heterocyst pairs from the daughter cells of cell division or dumbbell-shaped heterocysts from the cells undergoing cytokinesis. The patA mutant, which forms heterocysts only at the filament ends, restored intercalary heterocysts by a single nucleotide mutation of patU3, and double mutants of patU3/patA and patU3/hetF had the phenotypes of the patU3 mutant. We provide evidence that HetF, which can degrade PatU3, is recruited to cell divisome through its C-terminal domain. A HetF mutant with its N-terminal peptidase domain but lacking the C-terminal domain could not prevent the formation of heterocyst pairs, suggesting that the divisome recruitment of HetF is needed to sequester HetF for the delay of differentiation in dividing cells. Our study demonstrates that PatU3 plays a key role in cell-division coupled control of differentiation.

Differential effects of nitrate and ammonium on the growth of algae and microcystin production by nitrogen-fixing Nostoc sp. and non-nitrogen-fixing Microcystis aeruginosa.

Cyanotoxins produced by cyanobacteria are a significant threat to human health. However, their responses to nitrogen (N) supplies could differ between N-fixing and non-N-fixing species, which has been poorly understood. This study aimed to compare the responses of the non-N-fixing Microcystis aeruginosa and N-fixing Nostoc sp. to varying concentrations of nitrate and ammonium. This comparison had been conducted by analyzing chlorophyll-a contents, maximum quantum efficiencies of photosystem II, microcystin production, and related gene expressions. Our findings revealed that nitrate substantially stimulated the growth of both M. aeruginosa and Nostoc sp. with biomass increase by 366.2 ± 56.5 and 93.0 ± 14.0%, respectively, at 16 mg-N/L. In contrast, high ammonium concentrations suppressed their growth. Furthermore, the intracellular concentration of microcystins produced by M. aeruginosa was higher under high nitrate. Extracellular microcystins showed an opposite trend to increases in nitrate and ammonium. Ammonium increases the production and releases microcystin from Nostoc sp. N metabolism genes showed a similar trend with toxin formation genes, which were up-regulated under the high N treatments. This study provides valuable insights into the impacts of N supplies on growths of N- and non-N-fixing cyanobacteria, as well as microcystin production, which helps to develop effective strategies for managing cyanobacterial blooms.

Interlink between ExoD (Alr2882), exopolysaccharide synthesis and metal tolerance in Nostoc sp. strain PCC 7120: Insight into its role, paralogs and evolution.

Exopolysaccharides (EPS) produced by bacterial species are an important component of bacteria's survival strategy. Synthesis of EPS, principal component of extracellular polymeric substance, occurs through multiple pathways involving multitude of genes. While stress-induced concomitant increase in exoD transcript levels and EPS content have been shown earlier, experimental evidence for direct correlation is lacking. In the present study, role of ExoD in Nostoc sp. strain PCC 7120 was evaluated by generating a recombinant Nostoc strain AnexoD+, wherein the ExoD (Alr2882) protein was constitutively overexpressed. AnexoD+ exhibited higher EPS production, propensity for formation of biofilms and tolerance to Cd stress compared to vector control AnpAM cells. Both Alr2882 and its paralog All1787 exhibited 5 transmembrane domains, with only All1787 predicted to interact with several proteins in polysaccharide synthesis. Phylogenetic analysis of orthologs of these proteins across cyanobacteria indicated that the two paralogs Alr2882 and All1787 and their corresponding orthologs arose divergently during evolution, and could have distinct roles to perform in the biosynthesis of EPS. This study has thrown open the possibility of engineering overproduction of EPS and inducing biofilm formation through genetic manipulation of EPS biosynthesis genes in cyanobacteria, thus building a cost-effective green platform for large scale production of EPS.