CONICET | Buscador de Institutos y Recursos Humanos

Sucrose (Suc) is synthesized by plants, unicellular algae and cyanobacteria as part of the carbon dioxide assimilation pathway. While Suc metabolism is an essential pathway in plants, its significance in oxygenic photosynthetic bacteria is still not fully understood. Particularly, Suc synthase (SuS, EC 2.4.1.13), which catalyzes a readily reversible reaction of Suc synthesis and cleavage, is widespread in the plant kingdom and in filamentous heterocyst-forming cyanobacteria [1]. In higher plants, SuS has critical functions, and particularly, in Suc to starch and other polysaccharides interconversion [2]. However, knowledge of the relationship between Suc and glycogen metabolism and the function and regulation of SuS in cyanobacteria, remains still fragmentary. The interconnection between glycogen and Suc metabolism in Anabaena sp. PCC 7119 N2-fixing filaments was recently studied. It has been shown that SuS activity is modulated by the nitrogen source at the transcriptional level [3,4]. Different lines of evidence point to a key role of Suc cleavage by SuS, located in the photosynthetic vegetative cells, in the control of carbon flux in the nitrogen-fixing filaments of heterocystous cyanobacteria [3,4]. In the present work we have studied the functional relationship between SuS and the metabolism of polysaccharides in Anabaena strains. We show that the nitrogen and carbon sources and light regulate the expresión of the SuS encoding gene (susA), in a similar way that they regulate the accumulation of polysaccharides. Furthermore, glycogen content in an Anabaena sp. mutant strain with an insertion inactivation of susA was lower than in the wild-type strain under diazotrophic conditions, while both glycogen and polysaccharides levels were higher in a mutant strain constitutively overexpressing susA. We also show that there are soluble and membrane-bound SuS forms in Anabaena. Taken together, these results strongly suggest that SuS is involved in the Suc to polysaccharides conversion according to nutritional and environmental signals in filamentous nitrogenfixing cyanobacteria. These findings support a role of SuS in the cycling of sugar-nucleotides and in the regulation of the flux of carbon between Suc and polysaccharides. Also, this study contributes to an emerging concept that shows that not only the enzymology of Suc and polysaccharides metabolism has been conserved during evolution from cyanobacteria to higher plants, but also that some basic regulatory aspects of carbon assimilation have been conserved as well. Supported by grants of ANPCyT (PICT 2004 Nº 21227-53), UNMdP and CONICET (PID 6105).