Study on mycosporine-like amino acids (MAAs) in cyanobacteria: A biochemical, bioinformatics and molecular biology approach
Year: 2009
Type: dissertation
Abstract: The cyanobacterium Anabaena variabilis was found to synthesize a single MAA, shinorine when isolated and purified by HPLC. Although there was significant induction of MAA synthesis from its initial value under 395 or 320 nm cut-off filters, MAA induction was significantly more pronounced in samples covered with 295 nm cut-off filters after 72 h of exposure. Heat as a stress factor had no effect on MAA induction with or without UVR. In contrast, salt and ammonium treatment had synergistic effects with UVR stress. MAA synthesis was also induced by salt and ammonium in a concentration-dependent manner without UV stress in samples covered with 395 nm cut-off filters. The study of induction of shinorine in A. variabilis in various culture media was performed to select the most appropriate medium that can support the highest synthesis of MAA, and can be used for industrial production of these multifunctional compounds. Also, in vivo photosynthetic activity was measured under shinorine inducing conditions in all media. The shinorine content and photosynthetic activity were highest in BG11 medium in comparison to others after 72 h of exposure to UV radiation. Thus, growth media with a nitrogen source supports higher synthesis of MAA and can be used for mass production of MAA from cyanobacteria. The MAA profile of Anabaena doliolum was studied under PAR and PAR + UVR conditions. The HPLC analysis of water-soluble compounds reveals the biosynthesis of three MAAs. The results indicate that mycosporine-glycine (mono-substituted) acts as a precursor for the biosynthesis of the bi-substituted MAAs shinorine and porphyra-334. Mycosporine-glycine was under constitutive control while porphyra-334 and shinorine were induced by UV-B radiation. The cyanobacterium Anabaena flos-aquae was found to synthesize a single MAA shinorine as purified by HPLC, similar to other Anabaena strains, except A. doliolum. There was significant induction of MAA synthesis from its initial value in all samples receiving PAR, PAR + UV-A or PAR + UV-A + UV-B radiation; however, MAA induction was highest in samples receiving PAR + UV-A + UV-B radiation after 72 h of irradiation. MAA biosynthesis in this organism is dependent on photosynthesis only for the carbon source since the inhibitory effect of DCMU on MAA biosynthesis was overcome by externally added fructose. Results also suggest that there is no direct involvement of photosystem II in MAA biosynthesis. However, utilization of energy for MAA biosynthesis derived from photosystem I can not be ruled out. Bioconversion of primary MAAs into secondary MAAs was found to be regulated by sulfur deficiency in the cyanobacterium A. variabilis PCC 7937. This cyanobacterium synthesizes the primary MAA shinorine under normal conditions, however, under sulfur deficiency a secondary MAA palythine-serine appears. Addition of methionine to sulfur-deficient cultures resulted in the disappearance of palythine-serine, suggesting the role of primary MAA under sulfur deficiency. This is the first time that palythine-serine was found to be synthesised by cyanobacteria which has been so far reported only in corals. Addition of methionine also affected the steady state between mycosporine-glycine biosynthesis and its conversion into shinorine, consequently, resulted in the appearance of mycosporine-glycine. Results also support that palythine-serine is synthesized from shinorine after decarboxylation and demethylation of the glycine subunit as proposed earlier. Out of the four investigated species, only A. variabilis was able to synthesize MAA. Genome mining identified a combination of genes, YP_324358 (predicted DHQ synthase) and YP_324357 (O-methyltransferase), which were present only in A. variabilis and were missing in other studied cyanobacteria. Phylogenetic analysis revealed that these two genes are transferred from a cyanobacterial donor to dinoflagellates and finally to metazoan by lateral gene transfer. All other cyanobacteria, which have these two genes, also had another copy of the DHQ synthase gene. The predicted protein structure for YP_324358 also suggested that this product is different from the chemically characterized DHQ synthase of Aspergillus nidulans contrary to the YP_324879, which was predicted to be similar to the DHQ synthase of A. nidulans. Based on the results it was proposed that the YP_324358 and YP_324357 gene products are involved in the biosynthesis of the common core of all MAAs. The knockout development for DHQS (YP_324358) in A. variabilis was performed using a tri-parental conjugation as a mode of gene transfer. An attempt to generate YP_324358 mutant by insertional mutagenesis to prove the role of this gene product in MAA biosynthesis in A. variabilis was unsuccessful due to incomplete segregation of the mutant from the wild-type forms.
Author: Shailendra P. Singh
Language: English
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W3148012548