CONICET | Buscador de Institutos y Recursos Humanos

Sucrose is one of the most common nonreducing disaccharides found in nature. To date its biosynthesis was reported only in oxygenic photosynthetic organisms. On the other hand, sucrose cleavage through the action of sucrose synthase (SuS, A/UDP-Glucose: D-fructose 2- α-D-glucosyl transferase, EC 2.4.1.13) yielding a sugar nucleotide and fructose, was described in plants and in filamentous nitrogen-fixing cyanobacterial strains like Anabaena (Nostoc) sp. PCC 7119 and 7120, Anabaena variabilis, Nostoc punctiforme, and Nostoc commune. In Anabaena strains Sus was suggested to control sucrose cell level through the cleavage of the disaccharide and to be involved in the control of carbon flux in the N2-fixing filament. Recently, when new genome sequences became available, a homolog to Anabaena sp. PCC 7120 SuS gene (susA) was uncovered for Gloeobacter violaceus PCC 7421, a unicellular early branching strain of the cyanobacterial lineage. The deduced protein sequence is 71% identical to that of SusA. The functional identification of the orf was carried out by heterologous expression in Escherichia coli. The recombinant His-tagged fusion protein exhibited SuS activity with higher substrate affinity towards ADP than to UDP and shared immunological properties with PCC 7119 SusA. Sequence alignments and modular analysis of G. violaceus SuS showed that its predicted protein structure was similar to those present in filamentous strains. These results suggest that both the phylogenetic origin and the role of SuS in the control of the carbon flux in N2-fixing filaments might be revised to account for these new findings. Supported by PICT Nº 38144, PIP 6105, UNMdP and FIBA.