title: A Chlamydomonas reinhardtii low-starch mutant is defective for 3-phosphoglycerate activation and orthophosphate inhibition of ADP-glucose pyrophosphorylase.
abstract: 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.
authors.affiliation: Laboratoire de Chimie Biologique, Université des Sciences et Techniques de Lille Flandres-Artois, Bâtiment C9, F-59655, Villeneuve d'Ascq cedex, France.
authors.forename: S
authors.initials: S
authors.lastname: Ball
authors.forename: T
authors.initials: T
authors.lastname: Marianne
authors.forename: L
authors.initials: L
authors.lastname: Dirick
authors.forename: M
authors.initials: M
authors.lastname: Fresnoy
authors.forename: B
authors.initials: B
authors.lastname: Delrue
authors.forename: A
authors.initials: A
authors.lastname: Decq