Nitrogen and Redox Metabolism in Cyanobacterium Anabaena sp. PCC 7120 Exposed to Different Sulfate Regimes.

Sulfur is an important key nutrient required for the growth and development of cyanobacteria. Several reports showed the effect of sulfate limitation in unicellular and filamentous cyanobacteria, but such studies have not yet been reported in heterocytous cyanobacteria to ascribe the mechanisms of nitrogen and thiol metabolisms. Thus, the present work was carried out to appraise the impacts of sulfate limitation on nitrogen and thiol metabolisms in Anabaena sp. PCC 7120 by analyzing the contents as well as enzymes of nitrogen and thiol metabolisms. Cells of Anabaena sp. PCC 7120 were exposed to different regimes of sulfate, i.e., 300, 30, 3, and 0 µM. Application of reduced concentration of sulfate showed negative impact on the cyanobacterium. Sulfate-limiting conditions reduces nitrogen-containing compounds in the cells of Anabaena. Additionally, reduced activities of nitrogen metabolic enzymes represented the role of sulfate in nitrogen metabolism. However, decreased activities of thiol metabolic enzymes indicated that sulfate-limited cyanobacterial cells have lower amount of glutathione and total thiol contents. Reduced accumulation of thiol components in the stressed cells indicated that sulfate-limited cells have lower ability to withstand stressful condition. Hence, Anabaena displays differential response to different concentrations of sulfate, and thus, stipulated that sulfur plays an important role in nitrogen and thiol metabolisms. To the best of our knowledge, this is the first report demonstrating the impact of sulfate stress on nitrogen and redox metabolisms in heterocytous cyanobacteria. This preliminary study provides a baseline idea that may help improve the production of paddy.

Bioinformatics analysis of enzymes involved in cysteine biosynthesis: first evidence for the formation of cysteine synthase complex in cyanobacteria.

The biosynthesis of cysteine is crucial and critically regulated by two enzymes. i.e., serine acetyl transferase (SAT) and O-acetyl serine (thiol) lyase (OAS-TL). A descriptive account on the activity and regulatory mechanism of the enzyme is available in bacteria and plants. But no such studies yet performed in cyanobacteria, to understand the evolutionary aspect of cysteine biosynthesis and its regulation. Therefore, in our study a detailed bioinformatic analysis has been performed to understand all the possible features of cyanobacterial SATs and OAS-TLs. The analysis of SAT and OAS-TL sequences from cyanobacteria depicted that the large genome and morphological complexities favoured acquisition of these genes. Besides, conserved function of these enzymes was presumed by their sequence similarity. Further, the phylogenetic tree consisted of distinct clusters for unicellular, filamentous, and heterocytous strains. Nevertheless, the specificity pocket, SVKDR for OAS-TL having K as catalytic residue was also identified. Additionally, in silico protein modelling of SAT (SrpG) and OAS-TL (SrpH) of Synechococcus elongatus PCC 7942 was performed to gain insight into the structural attributes of the proteins. Finally, here we showed the possibility of hetero-oligomeric bi-enzyme cysteine synthase complex formation upon interaction of SAT and OAS-TL through protein-protein docking analysis thus provides a way to understand the regulation of cysteine biosynthesis in cyanobacteria. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13205-021-02899-1.

Insights Into the Phylogenetic Distribution, Diversity, Structural Attributes, and Substrate Specificity of Putative Cyanobacterial Orthocaspases.

The functionality of caspase homologs in prokaryotic cell execution has been perceived, yet the dimensions of their metabolic pertinence are still cryptic. Here, a detailed in silico study on putative cyanobacterial caspase homologs, termed orthocaspases, in a sequenced genome of 132 strains was performed. We observed that 473 putative orthocaspases were distributed among 62% cyanobacterial strains subsumed within all the taxonomical orders. However, high diversity among these orthocaspases was also evident as the conventional histidine-cysteine (HC) dyad was present only in 72.03% of orthocaspases (wild-type), whereas the rest 28.18% were pseudo-variants having substituted the catalytic dyad. Besides, the presence of various accessory functional domains with Peptidase C14 probably suggested the multifunctionality of the orthocaspases. Moreover, the early origin and emergence of wild-type orthocaspases were conferred by their presence in Gloeobacter; however, the complex phylogeny displayed by these caspase-homologs perhaps suggested horizontal a gene transfer for their acquisition. However, morpho-physiological advancements and larger genome size favored the acquisition of orthocaspases. Moreover, the conserved caspase hemoglobinase fold not only in the wild-type but also in the pseudo-orthocaspases in Nostoc sp. PCC 7120 ascertained the least effect of catalytic motifs in the protein tertiary structure. Further, the 100-ns molecular dynamic simulation and molecular mechanics/generalized born surface area exhibited stable binding of arginylarginine dipeptide with wild-type orthocaspase of Nostoc sp. PCC 7120, displaying arginine-P1 specificity of wild-type orthocaspases. This study deciphered the distribution, diversity, domain architecture, structure, and basic substrate specificity of putative cyanobacterial orthocaspases, which may aid in functional investigations in the future.

Disentangling the Impact of Sulfur Limitation on Exopolysaccharide and Functionality of Alr2882 by In Silico Approaches in Anabaena sp. PCC 7120.

The wide applications, uniqueness, and high quality of cyanobacterial exopolysaccharides (EPSs) have attracted many biotechnologists. Despite it, the inducers and molecular determinants of EPS biosynthesis in cyanobacteria are lesser known. Although, studies revealed that environmental cues especially C/N ratio as the prime modulator, the factors like light, temperature, moisture, and nutrient availability, etc. have been overlooked. Due to this, the possibilities to modify cyanobacterial system for achieving higher quantity of EPS either by modifying growth medium or metabolic engineering are restricted to few optimisations. Therefore, the present work describes the impact of sulfate limitations on the EPS production and compositions in the cyanobacterium Anabaena sp. PCC 7120. Increased EPS production with enhanced expression of alr2882 was observed in lower sulfate supplementations; however, FTIR analysis depicted an altered composition of supramolecule. Furthermore, in silico analysis of Alr2882 depicted the presence of ExoD domain and three transmembrane regions, thereby indicating its membrane localisation and role in the EPS production. Additionally, the phylogeny and multiple sequence alignment showed vertical inheritance of exoD and conservation among cyanobacteria. The meta-threading template-based modelling and ab initio full atomic relaxation by LOMET and ModRefiner servers, respectively, also exhibited helical topology of Alr2882, with nine α-helices arranged antiparallel to the preceding one. Moreover, post-translational modifications predicted in Alr2882 indicated high order of molecular regulation underlining EPS production in Anabaena sp. PCC 7120. This study provides a foundation for understanding the EPS biosynthesis mechanism under sulfur limitation and the possible role of ExoD in cyanobacteria.