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Planta. 1991 Aug;185(1):17-26. doi: 10.1007/BF00194509.

A Chlamydomonas reinhardtii low-starch mutant is defective for 3-phosphoglycerate activation and orthophosphate inhibition of ADP-glucose pyrophosphorylase.

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Laboratoire de Chimie Biologique, Université des Sciences et Techniques de Lille Flandres-Artois, Bâtiment C9, F-59655, Villeneuve d'Ascq cedex, France.


A low-starch mutant accumulating less than 5% of wild-type amounts was isolated after X-ray mutagenesis of Chlamydomonas reinhardtii cells. The recessive st-1-1 defect segregated as a single mendelian mutation through meiosis, and led to a severe decrease in starch accumulation under all culture conditions tested, whether in the light or in darkness. Adenosine 5'-diphosphoglucose pyrophosphorylase (in the absence of 3-phosphoglycerate), starch synthase, phosphoglucomutase, phosphorylase and starch-branching enzyme were all characterized and shown to be unaffected by the mutation. However, ADP-glucose pyrophosphorylase in the mutant had its sensitivity to activation by 3-phosphoglycerate lowered dramatically and became less responsive to orthophosphate. Our results are consistent both with a mutation in a structural gene of a multisubunit enzyme or in a regulatory gene responsible for switching ADP-glucose pyrophosphorylase from a 3-phosphoglycerate-insensitive to a 3-phosphoglycerate-sensitive form. These results provide definite proof of the in-vivo requirement for 3-phosphoglycerate activation to obtain substantial starch synthesis in plants. The conclusions hold both for synthesis from CO2 in the light or from exogenous organic carbon sources in darkness. A model is presented in which the existence of a 3-phosphoglycerate gradient explains localized starch synthesis around the pyrenoid of lower plants.


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