PREAMYLOPECTIN PROCESSING - A MANDATORY STEP FOR STARCH BIOSYNTHESIS IN PLANTS

It has been generally assumed that the alpha-(1-->4)-linked and alpha- (1-->6)-branched glucans of starch are generated by the coordinated ac tion of elongation (starch synthases) and branching enzymes, We have i dentified a novel Chlamydomonas locus (STA7) that when defective leads to a wipeout of starch and its replacement by a small amount of glyco gen-like material, Our efforts to understand the enzymological basis o f this phenotype have led us to determine the selective disappearance of an 88-kD starch hydrolytic activity. We further demonstrate that th is enzyme is a debranching enzyme, Cleavage of the alpha-(1-->6) linka ge in a branched precursor of amylopectin (preamylopectin) has provide d us with the ground rules for understanding starch biosynthesis in pl ants. Therefore, we propose that amylopectin clusters are synthesized by a discontinuous mechanism involving a highly specific glucan trimmi ng mechanism.