Sucrose catabolism pathways in cyanobacteria

G.L. SALERNO; M.A. KOLMAN; M. PEREZ-CENCI; C.N. NISHI

Mar del Plata

Congreso; X Congreso Argentino de Microbiología General; 2014

Sociedad Argentina de Microbiología General

The increasing number of complete sequenced genomes of oxygenic photosynthetic microorganisms led us to continue our study on sucrose metabolism proteins. Sucrose has been studied extensively in plants and in a lesser extent in cyanobacteria. Sucrose metabolism occurs in cyanobacteria in a similar way as described previously in plants. On one hand, sucrose synthesis through a similar two-step pathway has been described in both unicellular and filamentous nitrogen-fixing strains. On the other hand, sucrose catabolism can take place through its irreversible hydrolysis to hexoses by alkaline/neutral invertases (A/N-Inv) or its cleavage by sucrose synthase (SuS, UDP?glucose:D-fructose 2-a-D-glucosyltransferase, EC 2.4.1.13), supplying sugar nucleotides, precursors in the formation of structural and storage polysaccharides. Recently, a new hydrolysis pathway has been described involving an amylosucrase (AMS) that belongs to glycoside hydrolase (GH) family 13 that not only to hydrolyze Suc to monosaccharides supplying carbon and energy, but also to synthesize amylose-like polymers using sucrose as sole substrate. First, based on the presence of sus homologs in the most recently radiated cyanobacterial species, whose genomes have been fully sequenced (198 in total), and after phylogenetic analysis, we concluded that SuS may play a key role in heterocyst-forming strains, while in unicellular strains is likely to be dispensable in some cases or related to environmental adverse conditions as has been demonstrated in M. aeruginosa. Second, the presence of inv homologs is found in the 45% of all cyanobacterial genomes available to date and its early origin is reflected by its presence at the base of the radiation. Additionally, in nearly 60% of the cases, the inv homologs are present in nitrogen-fixing cyanobacterial genomes and Inv-A/N are present in the majority of all heterocyst-forming cyanobacteria, as occurs with SuS, where they would play a relevant role in the heterotrophic metabolism within the nitrogen-fixing specialized cells named heterocysts. Third, there is little information about the AMS: homology searches showed its presence in 2 filamentous and in 8 unicellular cyanobacterial strains, where this was the only catabolic pathway of sucrose. Phylogenetic analysis of the three different pathways of degradation of sucrose in cyanobacteria showed that the origin of the A/N-Inv occurred early in the evolution, while the appearance of SuS happened later and was fundamental in the development of more evolved morphological forms. The phylogenetic analysis from AMS showed that this enzyme has a bacterial origin and would have been acquired in separate events. Taken together, the results presented here show that Sucrose catabolic pathways play different roles in cyanobacteria.