Sucrose metabolism and carbon partitioning in Nostoc sp. PCC 7120 during diazotrophic growth

W.A. VARGAS; G.L. SALERNO

Sant Feliu de Guixols

Simposio; Molecular Bioenergetics of Cyanobacteria: From Cell to Community; 2011

European Science Foundation - EMBO

The occurrence of sucrose (Suc) metabolism in cyanobacteria and its importance for nitrogen metabolism led us to investigate Suc-metabolizing enzymes and their physiological functions in cells growing under different nitrogen sources. A comprehensive analysis including studies on gene expression regulation, enzyme activities, cell localization and mutagenesis, led us to demonstrate an important interplay between Suc and glycogen to support diazotrophic growth of Nostoc sp. PCC 7120 (also known as Anabaena sp. PCC 7120). Suc has been proposed as the carbon carrier molecule from the vegetative cell to the heterocyst in nitrogen-fixing filaments. Despite not being fully demonstrated, the physical transport of Suc towards the heterocyst, we show that Suc hydrolysis inside the heterocysts is essential for Nostoc growth during nitrogen fixation. In this analysis we complete the study of the whole sucrose metabolism in the PCC 7120 strain and its relationship with nitrogen fixation. Our results highlight a direct correlation between a Suc cycling inside the heterocyst by specific isosymes for Suc synthesis and hydrolysis, and the synthesis of glycogen, suggesting a net carbon flux from Suc towards glycogen. Also our findings undoubtedly support the role of the disaccharide as an intermediate in the reduced carbon flux in heterocyst-forming cyanobacteria, under the control of NtcA, a global nitrogen regulator.