CONICET | Buscador de Institutos y Recursos Humanos

Sucrose (Suc) metabolism has been reported in oxygenic photosynthetic organism. Suc is synthesized through a two-step pathway involving Sucrose-Phosphate Synthase (SPS) and Sucrose-Phosphate Phosphatase (SPP), either in unicellular or filamentous cyanobacterial strains. However, to date Suc cleavage by Sucrose Synthase (SuS) has only been reported in filamentous heterocyst-forming cyanobacteria. Microcystis aeruginosa is a unicellular non-N2 fixing strain, well known as one of the most common bloom-forming cyanobacteria in fresh water environments. By genome sequence analyses we retrieved three contiguous nucleotide sequences (IPF_1564, IPF_1566 and IPF_1565) corresponding to open reading frames (orfs) homologous to Suc metabolism genes. Their deduced amino-acid sequences are 53%, 55% and 72% identical to the protein sequences of Synechocystis sp. PCC 6803 SPP, and SPS-A and SuS-A from Anabaena sp. PCC 7120, respectively. To carry out their functional characterization, the sequences were cloned and expressed in Escherichia coli cells. The recombinant proteins exhibited SPP, SPS and SuS activity, respectively. In addition, cell free extracts from M. aeruginosa were chromatographed through an ion exchange column and the enzyme activities were assayed in the eluted fractions. Expression analyses by RT-PCR showed that the three genes are transcribed during standard culture conditions (BG11 medium, 20±1 ºC) and are induced after a salt stress. This is the first report on the presence of Suc enzymes activities in a potential toxic and bloom-forming cyanobacterium. It is worth noting the presence of a SuS protein in a unicellular strain. Suc metabolism role and its relationship with bloom formation remain to be elucidated.