STORAGE, PHOTOSYNTHESIS, AND GROWTH - THE CONDITIONAL NATURE OF MUTATIONS AFFECTING STARCH SYNTHESIS AND STRUCTURE IN CHLAMYDOMONAS

Growth-arrested Chlamydomonas cells accumulate a storage polysaccharid e that bears strong structural and functional resemblance to higher pl ant storage starch. It is synthesized by similar enzymes and responds in an identical fashion to the presence of mutations affecting these a ctivities. We found that log-phase photosynthetically active algae acc umulate granular a(1-->4)-linked, a(1-->6)-branched glucans whose shap e, cellular location, and structure differ markedly from those of stor age starch. That synthesis of these two types of polysaccharides is co ntrolled by both a common and a specific set of genes was evidenced by the identification of a new Chlamydomonas (STA4) locus specifically i nvolved in the biosynthesis of storage starch. Mutants defective in ST A4 accumulated a new type of high-amylose storage starch displaying an altered amylopectin chain size distribution. It is expected that the dual nature and functions of starch synthesis in unicellular green alg ae will yield new insights into the biological reasons for the emergen ce of starch in the eukaryotic plant cell.