Starchless mutants of Chlamydomonas reinhardtii lack the small subunit of a heterotetrameric ADP-glucose pyrophosphorylase

ADP-glucose synthesis through ADP-glucose pyrophosphorylase defines the maj or rate-controlling step of storage polysaccharide synthesis in both bacter ia and plants. We have isolated mutant strains defective in the STA6 locus of the monocellular green alga Chlamydomonas reinhardtii that fail to accum ulate starch and lack ADP-glucose pyrophosphorylase activity. We show that this locus encodes a 514-amino-acid polypeptide corresponding to a mature 5 0-kDa protein with homology to vascular plant ADP-glucose pyrophosphorylase small-subunit sequences. This gene segregates independently from the previ ously characterized STA1 locus that encodes the large 53-kDa subunit of the same heterotetramer enzyme. Because STA1 locus mutants have retained an AG Pase but exhibit lower sensitivity to 3-phosphoglyceric acid activation, we suggest that the small and large subunits of the enzyme define, respective ly the catalytic and regulatory subunits of AGPase in unicellular green alg ae. We provide preliminary evidence that both the small-subunit mRNA abunda nce and enzyme activity, and therefore also starch metabolism, may be contr olled by the circadian clock.