Differential pattern expression of sucrose-related genes in heterocysts and vegetative cells of Nostoc sp. PCC7120

C. MARCOZZI, A.C. CUMINO AND G.L. SALERNO

Pau, France (27 August-1 September

Simposio; 12th International Symposium on Phototrophic Prokaryotes; 2006

International Society of Photosynthesis Research

Sucrose (Suc) occurrence is mainly limited to oxygenic photosynthetic organisms, including vascular plants, unicellular algae and cyanobacteria. In these prokaryotic organisms, Suc function has not been fully elucidated although it is associated with environmental stress responses. In filamentous heterocyst-forming cyanobacteria Suc has been also suggested as a carbon carrier molecule during N2 fixation. Recently, it was shown that in Nostoc sp. PCC 7120 Suc is synthesised in a two-step pathway involving sucrose-phosphate synthase (SPSA and SPSB) and sucrose-phosphate phosphatase (SPP), and it is cleaved by sucrose synthase (SuS), enzymes encoded by spsA and spsB, sppA, and susA, respectively. The crucial role of Suc in N2 fixation was shown in a susA overexpressing mutant strain that did not accumulate Suc, in which diazotrophic growth was impaired. We demonstrated that during N2 fixation not only Suc synthesis is increased but also the Suc turnover is enhaced. In an attempt to investigate the cell localisation of the expression of Suc-biosynthesis related genes in nitrogen-fixing filaments, we performed transcriptional fusions of the promoter regions of those genes with the gfp-mut2 (coding for an optimised version of the green fluorescent protein). While spsB and sppA expression were localised in both vegetative and heterocyst cells, spsA expression was only detected in the vegetative cells. These results were supported with biochemical and RT-PCR experiments. We conclude that Suc biosynthesis may take place either in vegetative or heterocyst cells. In addition, spsB promoter analysis led us to suggest that spsB expression may be connected with the global nitrogen metabolism regulation by NtcA, and, therefore, Suc metabolism may be coordinated with nitrogen fixation.