Abstract
Most cyanobacteria have the single branching enzyme (BE) responsible for glycogen synthesis. In the BE mutants of Synechococcus elongatus PCC 7942, the amount of glycogen accumulated in the cells was decreased to 55% of that in the wild type, and the length of the most abundant α-1,4-chains in the polysaccharide was shifted from 8 to 4. However, the synthesis of the branched polysaccharides was still observed even in the absence of the prominent BE band in the zymogram, suggesting the presence of unidentified BE isoform(s) in cyanobacteria. Recently, it has been reported that a product of a GH57 family gene in the archaeon Thermococcus kodakaraensis exhibited BE activity. Mutants defective in the GH57 gene in S. elongatus PCC 7942 were therefore constructed and characterized. The amount of glycogen in the GH57 mutants was decreased only slightly. In contrast, when both BE and GH57 genes were disrupted, the amount of glucans accumulated in the cells was substantially decreased and the growth rates of the mutants were significantly reduced as compared to the wild type. The role of both of the gene products in the polysaccharide metabolism is discussed.
