CONTROL OF STARCH COMPOSITION AND STRUCTURE THROUGH SUBSTRATE SUPPLY IN THE MONOCELLULAR ALGA CHLAMYDOMONAS-REINHARDTII

In Chlamydomonas, as in higher plants, synthesis of ADP glucose cataly zed by ADP-glucose pyrophosphorylase is rate-limiting for the building of starch in the chloroplast. We have isolated disruptions of the STA 1 ADP-glucose pyrophosphorylase structural gene that rendered the enzy me less responsive to the allosteric activator 3-phosphoglycerate, The structure and composition of the residual starch synthesized by all m utants of the STA1 locus is dramatically altered. The residual polysac charide is shown to be devoid of amylose despite the presence of granu le-bound starch synthase, the amylose biosynthetic enzyme. In addition , the fine structure of the mutant amylopectin revealed the presence o f an altered chain-length distribution. This distribution mimicks that which is observed during growth and photosynthesis and differs marked ly from that observed during storage, We therefore propose that low nu cleotide sugar concentrations are either directly or indirectly respon sible for the major differences observed in the composition or structu re of starch during storage and photosynthesis.