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.
Highlights
Cyanobacteria are the only prokaryotes capable of oxygenic photosynthesis
While activity of glyoxylate cycle enzymes has been detected in some cyanobacteria, it is unclear whether Synechocystis encodes active variants of isocitrate lyase (Icl) and malate synthase (Msy)
Enzymes involved in nucleotide biosynthesis (Figure 5) are highly conserved between E. coli and Synechocystis (Supplementary Table S1), and this pathway has not been investigated in great detail in cyanobacteria
Summary
Cyanobacteria are the only prokaryotes capable of oxygenic photosynthesis. Since their appearance >2.4 billion years ago [1], cyanobacteria have profoundly impacted Earth’s climate and ecosystem, most notably in generation of an oxygenic atmosphere [2]. PCC 6803 (Synechocystis), the most widely studied cyanobacterium, less than 1200 coding sequences (∼30%) have assigned function (469 in metabolism and 115 in transport: Highlighted in red in Supplementary Table S1; ∼558 in other cellular processes (including transposons and transposon related functions): Highlighted in red in Supplementary Table S3), which is less than half compared with Escherichia coli [20]. In Synechocystis, it has been demonstrated that the majority of characterized TM localized proteins are involved in photosynthetic and respiratory energy generation, suggesting that this is the primary function of this compartment [31,32]. These advantages must outweigh potential burdens arising from the additional complexity imposed on the cell. A dense material was observed between this junction that may play a role in ‘attachment’ of the thylakoids to the cell wall but the exact process and the proteins/compounds involved, has not been determined
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