Current knowledge and recent advances in understanding metabolism of the model cyanobacterium Synechocystis sp. PCC 6803

Abstract

Cyanobacteria are key organisms in the global ecosystem, useful models for studying metabolic and physiological processes conserved in photosynthetic organisms, and potential renewable platforms for production of chemicals. Characterizing cyanobacterial metabolism and physiology is key to understanding their role in the environment and unlocking their potential for biotechnology applications. Many aspects of cyanobacterial biology differ from heterotrophic bacteria. For example, most cyanobacteria incorporate a series of internal thylakoid membranes where both oxygenic photosynthesis and respiration occur, while CO2 fixation takes place in specialized compartments termed carboxysomes. In this review, we provide a comprehensive summary of our knowledge on cyanobacterial physiology and the pathways in Synechocystis sp. PCC 6803 (Synechocystis) involved in biosynthesis of sugar-based metabolites, amino acids, nucleotides, lipids, cofactors, vitamins, isoprenoids, pigments and cell wall components, in addition to the proteins involved in metabolite transport. While some pathways are conserved between model cyanobacteria, such as Synechocystis, and model heterotrophic bacteria like Escherichia coli, many enzymes and/or pathways involved in the biosynthesis of key metabolites in cyanobacteria have not been completely characterized. These include pathways required for biosynthesis of chorismate and membrane lipids, nucleotides, several amino acids, vitamins and cofactors, and isoprenoids such as plastoquinone, carotenoids, and tocopherols. Moreover, our understanding of photorespiration, lipopolysaccharide assembly and transport, and degradation of lipids, sucrose, most vitamins and amino acids, and haem, is incomplete. We discuss tools that may aid our understanding of cyanobacterial metabolism, notably CyanoSource, a barcoded library of targeted Synechocystis mutants, which will significantly accelerate characterization of individual proteins.

Modifying the Cyanobacterial Metabolism as a Key to Efficient Biopolymer Production in Photosynthetic Microorganisms

“…Due to the impact of the TCA cycle on the production of energy intermediates in aerobic respiratory organisms, researchers have attempted to determine how the lack of this enzyme is compensated for in photosynthetic cyanobacteria. Several metabolic pathways exist to make up for this enzyme, and they rely on the shunt pathways involved in the metabolism of γ-aminobutyric acid (GABA), succinic semialdehyde, or glyoxylate ( Figure 2 ) [ 27 , 28 ]. S. sp.…”

“…Whereas in S. sp. PCC 6803 it has been shown that the stream of metabolites from 2-oxoglutarate to succinate is weaker than in a typical TCA as it is bypassed by a GABA shunt ( Figure 2 b) [ 27 , 28 , 36 ]. The pathway referred here as typical TCA is presented in Figure 2 a and relies on the presence of citrate synthase, aconitase, isocitrate dehydrogenase, 2-oxoglutarate dehydrogenase complex, succinyl CoA ligase, succinate dehydrogenase, fumarase, and malate dehydrogenase [ 36 ].…”

“…In S. sp. PCC 6803 mutants incapable of producing one isoform, the compensation effect from the other isoform was observed [ 27 , 40 ].…”

“…In S. sp. PCC 6803, this catabolism is controlled by two GlgX isoforms (GlgX1 and GlgX2) and two GlgP isoforms (GlgP1 and GlgP2) [ 27 ]. The gene encoding the GlgP protein was overexpressed in Escherichia coli , resulting in a decrease in glycogen content [ 66 ].…”

Characterization of Light-Enhanced Respiration in Cyanobacteria

“…Due to the lack of organelles, both respiration and photosynthesis proceed in one cell, and they both depend on the cytosolic metabolism and on the electron transport chain in the thylakoid membrane ( Figure 1 ). The redox states of the photosynthetic electron transport components and the amounts of photosynthetic metabolites are affected in respiratory mutants [ 20 , 21 , 22 , 23 , 24 , 25 , 26 ], the interplay between respiration and photosynthesis in cyanobacterial cells. In the cyanobacterium Synechocystis sp.…”