A study of the impact of mazie SBE I on the [alpha]-polyglucan produced in Synechocystis sp. strain PCC 6803
Starch is a biopolymer synthesized primarily in plants and is made of alpha-1,4 linked glucose chains with alpha-1,6 glycosidic bonds at branch points. Amylose and amylopectin, the component molecules of starch are essentially linear and 4-5% branched respectively. Amylopectin, thought to be responsible for the semi-crystalline nature of starch, is chemically similar but physically distinct from water-soluble glycogen. Amylopectin is distinct from glycogen in having longer chains, fewer branch points and a clustered branching pattern. The hypothesis that the presence of a branching enzyme that can transfer long branches will facilitate the formation and perpetuation of a cluster branch pattern was tested using the cyanobacterium, Synechocystis, by introducing maize starch branching enzyme I (MSBE I) into a recipient strain, M3, that has only residual levels of branching enzyme activity to produce the double mutant SM3. In order to determine the effects of MSBE I on the nature of the glucan produced, the M3 mutant (Yoo et al., 2002) was also further characterized. Results indicate that the transfer of long, branch chains did lead to a molecule that has characteristics that are similar to amylopectin in size, branch chain length distribution, average chain length, solubility, and density characteristics. In addition, the glucan produced in SM3 had the appearance of stacked, plate-like structures when viewed under the transmission electron microscope. The further characterization of the M3 mutant revealed some similar characteristics when compared to SM3 although the data gave indications that the processes that led to these characteristics were different in the two mutants. The effort to produce a background free of glycogen branching enzyme in Synechocystis also led to the tentative conclusion that there is an apparent need for glycogen synthesis and that MSBE I cannot fully compensate for glycogen branching enzyme activity.