Glycogen synthases in cyanobacteria: relationship with sucrose synthesis in Synechocystis salt-stressed cells

GIARROCCO, L; KOLMAN, MA; CURATTI, L; SALERNO, G

Tubingen

Simposio; 15th International Symposium on Phototrophic Prokaryotes; 2015

University of Freiburg

In cyanobacteria, glycogen, the major storage polysaccharide, is synthesized by the action of ADP-glucose pyrophosphorylase, glycogen synthase and branching enzyme. Its accumulation has been extensively described. Particularly, glycogen role in salt stress responses has been shown in Synechocystis sp. PCC 6803. The presence of two glycogen synthases in this strain led us to study the physiological function of the isozymes in relation to salinity, which has not been investigated yet, and the origin and distribution of the two encoding genes (glgA1 and glgA2) in the available cyanobacterial genomes. Phylogenetic analyses show an ancestral origin for glgA1. A later gene duplication originated glgA2 (phylogenetically related to the origin of plant starch synthases) and the coexistence of both putative genes in most Oscillatoriales, Stignonematales and Pleurocapsales strains, and in some Nostocales and Chroococcales. While several strains lack glgA sequences (mostly belonging to Chroococcales), a few ones have three sequences. To investigate the role of each glgA in Synechocystis, we generated null mutants in each glgA and in spsA (involved in sucrose synthesis). Our data revealed that an opposite effect of each GlgA protein at the early step of the salt shock altering several times sucrose accumulation, a transient signal to cope the stress. While mutant strains lacking spsA accumulate glycogen at the onset of salt stress, double mutant strains lacking spsA and glgA do not. Differential expression profiles of genes involved in sucrose and glycogen metabolism were obtained in salt stress treatment and in different growth phase cells.