Characterization of function of the GlgA2 glycogen/starch synthase in Cyanobacterium sp. Clg1 highlights convergent evolution of glycogen metabolism into starch granule aggregation.

At variance with the starch accumulating plants and most of the glycogen-accumulating cyanobacteria, Cyanobacterium sp. CLg1 synthesizes both glycogen and starch. We now report the selection of a starchless mutant of this cyanobacterium that retains wild-type amounts of glycogen. Unlike other mutants of this type found in plants and cyanobacteria, the mutant proved to be selectively defective for one of the two types of glycogen/starch synthase: GlgA2. This enzyme is phylogenetically related to the previously reported SSIII/SSIV starch synthase that is thought to be involved in starch granule-seeding in plants. This suggests that in addition to the selective polysaccharide debranching demonstrated to be responsible for starch rather than glycogen synthesis, the nature and properties of the elongation enzyme defines a novel determinant of starch versus glycogen accumulation. We show that the phylogenies of GlgA2 and of 16S rRNA display significant congruence. This suggests that this enzyme evolved together with cyanobacteria when they diversified over 2 billion years ago. . However cyanobacteria can be ruled out as direct progenitors of the SSIII-IV ancestral gene found in Archaeplastida. Hence both cyanobacteria and plants recruited similar enzymes independently to perform analogous tasks, further emphasizing the importance of convergent evolution in the appearance of starch from a pre-existing glycogen metabolism network.