Role of sucrose in different cyanobacteria

KOLMAN, MA; PEREZ-CENCI, M; NISHI, CN; SALERNO, G

Naantali

Congreso; 8th European Workshop on Molecular Biology of Cyanobacteria; 2011

Turku University Foundation

Sucrose (Suc) is one of the most common non-reducing disaccharides in nature, produced by oxygenic photosynthetic organisms. Suc metabolism has been extensively studied in plants and more recently in cyanobacteria. Suc is biosynthesized through a two-step pathway involving sucrose-phosphate synthase (an -D-glucosyltransferase) and sucrose-phosphate phosphatase (a phospho-hydrolase) in both unicellular and filamentous cyanobacterium strains. Suc is hydrolysed either by alkaline/neutral invertases (Inv) or sucrose synthase (SuS) (mainly present in filamentous heterocyst-forming cyanobacteria). To investigate the role of Suc in different cyanobacteria we analysed the presence and location of its metabolism genes in genomes sequenced to date, in addition to the functional characterization of key species genes and analyses of null mutants. While in most strains, genes are scattered in the genomes, in a few cases, genes of the biosynthesis pathway are grouped. Moreover, in Synechococcus sp. PCC 7002, the two genes were shown to be cotranscribed. A particular case is found in the unicellular bloom-forming strain Microcystis aeruginosa PCC 7806 where the three functionally characterized genes of Suc metabolism are grouped. Investigations carried out with null mutants in Suc biosynthesis or hydrolysis encoding genes led us to conclude that Suc is likely not to be essential in unicellular strains which it does not seem to be the case for filamentous diazotrophic strains, where nitrogen fixation depends on Suc hydrolysis. Suc seems to play an important role in early stages of a salt treatment in unicellular strains (like Synechocystis sp. PCC 6803) but it is dispensable for stress tolerance, which may be ascribed to the presence of other compatible solutes. Importantly, in filamenous strains, we showed a salt sensitive phenotype related to the lack of polymers derived from Suc. Taken together our results suggest that in cyanobacteria the acquisition of Suc metabolism gave some advantage to cyanobacteria to cope with salt/stress conditions, but it is not as an essential molecule, which in turn might have led to its lost in some unicellular marine strains. Later gene reacquisition events, probably by lateral gene transfer either from other cyanobacteria or from proteobacteria that might act as gene reservoirs, might give origin to novel roles in some bloom forming and in filament diazotrophic strains.