Reconstitution and expression of mcy gene cluster in the model cyanobacterium Synechococcus 7942 reveals a role of MC-LR in cell division

Cyanobacterial blooms pose a serious threat to public health due to the presence of cyanotoxins. Microcystin-LR (MC-LR) produced by Microcystis aeruginosa is the most common cyanotoxins. Due to the limitation of isolation, purification, and genetic manipulation techniques, it is difficult to study and verify in situ the biosynthetic pathways and molecular mechanisms of MC-LR. We reassembled the biosynthetic gene cluster (mcy cluster) of MC-LR in vitro by synthetic biology, designed and constructed the strong bidirectional promoter biPpsbA₂ , transformed it into Synechococcus 7942, and successfully expressed MC-LR at a level of 0.006-0.018 fg cell^-1  d^-1 . We found the expression of MC-LR led to abnormal cell division and cellular filamentation, further using various methods proved that by irreversibly competing its GTP-binding site, MC-LR inhibits assembly of the cell division protein FtsZ. The study represents the first reconstitution and expression of the mcy cluster and the autotrophic production of MC-LR in model cyanobacterium, which lays the foundation for resolving the microcystins biosynthesis pathway. The discovered role of MC-LR in cell division reveals a mechanism of how blooming cyanobacteria gain a competitive edge over their nonblooming counterparts.

Study of the growth and biochemical composition of 20 species of cyanobacteria cultured in cylindrical photobioreactors

BACKGROUND

Cyanobacteria are prokaryotic organisms with wide morphological and metabolic diversity. By means of photosynthesis, they convert inorganic compounds into biomolecules, which may have commercial interest. In this work, we evaluated 20 cyanobacterial strains regarding their physiological aspects such as growth, photosynthesis and biochemical composition, some of which are revealed here for the first time. The organisms were cultivated in cylindrical photobioreactors (CPBR) for 144 h and the biomass was obtained. The light inside cultures was constant throughout experimental time and maintained at the saturation irradiance (Ik) of each species. Culture pH was maintained within 7.8 and 8.4 by automatic CO₂ bubbling. Growth rate, dry biomass, chlorophyll a, carotenoids, phycocyanin, proteins, carbohydrates, lipids, polyhydroxyalkanoate (PHA) and antioxidant activity were determined.

RESULTS

The proportionality of the biochemical composition varied among species, as well as the growth rates. Leptolyngbya sp. and Nostoc sp. (CCIBt3249) showed growth rates in the range of 0.7-0.8 d^-1, followed by Rhabdorderma sp. (~ 0.6 d^-1), and Phormidium sp. (~ 0.5 d^-1). High carotenoid content was obtained in Rhabdoderma sp. (4.0 μg mL^-1) and phycocyanin in Leptolyngbya sp. (60 μg mL^-1). Higher total proteins were found in the genus Geitlerinema (75% DW), carbohydrates in Microcystis navacekii (30% DW) and lipids in Phormidium sp. (15% DW). Furthermore, Aphanocapsa holsatica showed the highest antioxidant activity (65%) and Sphaerocavum brasiliense, Microcystis aeruginosa, Nostoc sp. (CCIBt3249) and A. holsatica higher levels of PHA (~ 2% DW).

CONCLUSIONS

This study reports on the biochemical composition of cyanobacteria that can impact the biotechnology of their production, highlighting potential strains with high productivity of specific biomolecules.

Four New Genes of Cyanobacterium Synechococcus elongatus PCC 7942 Are Responsible for Sensitivity to 2-Nonanone

Microbial volatile organic compounds (VOCs) are cell metabolites that affect many physiological functions of prokaryotic and eukaryotic organisms. Earlier we have demonstrated the inhibitory effects of soil bacteria volatiles, including ketones, on cyanobacteria. Cyanobacteria are very sensitive to ketone action. To investigate the possible molecular mechanisms of the ketone 2-nonanone influence on cyanobacterium Synechococcus elongatus PCC 7942, we applied a genetic approach. After Tn5-692 transposon mutagenesis, several 2-nonanone resistant mutants have been selected. Four different mutant strains were used for identification of the impaired genes (Synpcc7942_1362, Synpcc7942_0351, Synpcc7942_0732, Synpcc7942_0726) that encode correspondingly: 1) a murein-peptide ligase Mpl that is involved in the biogenesis of cyanobacteria cell wall; 2) a putative ABC transport system substrate-binding proteins MlaD, which participates in ABC transport system that maintains lipid asymmetry in the gram-negative outer membrane by aberrantly localized phospholipids transport from outer to inner membranes of bacterial cells; 3) a conserved hypothetical protein that is encoding by gene belonging to phage gene cluster in Synechococcus elongatus PCC 7942 genome; 4) a protein containing the VRR-NUC (virus-type replication-repair nuclease) domain present in restriction-modification enzymes involved in replication and DNA repair. The obtained results demonstrated that 2-nonanone may have different targets in Synechococcus elongatus PCC 7942 cells. Among them are proteins involved in the biogenesis and functioning of the cyanobacteria cell wall (Synpcc7942_1362, Synpcc7942_0351, Synpcc7942_0732) and protein participating in stress response at DNA restriction-modification level (Synpcc7942_0726). This paper is the first report about the genes that encode protein products, which can be affected by 2-nonanone.

Systematic Identification of Target Genes for Cellular Morphology Engineering in Synechococcus elongatus PCC7942

Cyanobacteria are serving as promising microbial platforms for development of photosynthetic cell factories. For enhancing the economic competitiveness of the photosynthetic biomanufacturing technology, comprehensive improvements on industrial properties of the cyanobacteria chassis cells and engineered strains are required. Cellular morphology engineering is an up-and-coming strategy for development of microbial cell factories fitting the requirements of industrial application. In this work, we performed systematic evaluation of potential genes for cyanobacterial cellular morphology engineering. Twelve candidate genes participating in cell morphogenesis of an important model cyanobacteria strain, Synechococcus elongatus PCC7942, were knocked out/down and overexpressed, respectively, and the influences on cell sizes and cell shapes were imaged and calculated. Targeting the selected genes with potentials for cellular morphology engineering, the controllable cell lengthening machinery was also explored based on the application of sRNA approaches. The findings in this work not only provided many new targets for cellular morphology engineering in cyanobacteria, but also helped to further understand the cell division process and cell elongation process of Synechococcus elongatus PCC7942.