Abstract
Some cyanobacterial species including Cyanobacterium sp. MBIC 10216 synthesize distinct α-polyglucans semi-amylopectin instead of glycogen. Semi-amylopectin is characterized as the content of long α-1,4-glucan chains being much more abundant as compared to glycogen, and its apparent molecular size close to that of rice amylopectin. To elucidate the genetic background of semi-amylopectin synthesis, the genomic sequence of the strain MBIC 10216 is being determined. Among the genes responsible for the biosynthesis of storage polysaccharides, we have found three distinct genes whose products possess all the essential amino acid residues of starch/glycogen branching enzymes (BE) belonging to GH13 family. The distinct BE activities were demonstrated with the recombinant proteins, although they remain undetectable in the crude extracts of the cyanobacterium. Catalytic specificities of the three BE isoforms were studied with synthetic α-1,4-glucans as the substrate. Chain length distribution of the branched glucans with α-1,6-linkage was markedly different between these isoforms.
