Role of the GlgX protein in glycogen metabolism of the cyanobacterium, Synechococcus elongatus PCC 7942

The putative glgX gene encoding isoamylase-type debranching enzyme was isolated from the cyanobacterium, Synechococcus elongatus PCC 7942. The deduced amino acid sequence indicated that the residues essential to the catalytic activity and substrate binding in bacterial and plant isoamylases and GlgX proteins were all conserved in the GlgX protein of S. elongatus PCC 7942. The role of GlgX in the cyanobacterium was examined by insertional inactivation of the gene. Disruption of the glgX gene resulted in the enhanced fluctuation of glycogen content in the cells during light-dark cycles of the culture, although the effect was marginal. The glycogen of the glgX mutant was enriched with very short chains with degree of polymerization 2 to 4. When the mutant was transformed with putative glgX genes of Synechocystis sp. PCC 6803, the short chains were decreased as compared to the parental mutant strain. The result indicated that GlgX protein contributes to form the branching pattern of polysaccharide in S. elongatus PCC 7942.