title: Plastid terminal oxidase 2 (PTOX2) is the major oxidase involved in chlororespiration in Chlamydomonas.
abstract: By homology with the unique plastid terminal oxidase (PTOX) found in plants, two genes encoding oxidases have been found in the Chlamydomonas genome, PTOX1 and PTOX2. Here we report the identification of a knockout mutant of PTOX2. Its molecular and functional characterization demonstrates that it encodes the oxidase most predominantly involved in chlororespiration in this algal species. In this mutant, the plastoquinone pool is constitutively reduced under dark-aerobic conditions, resulting in the mobile light-harvesting complexes being mainly, but reversibly, associated with photosystem I. Accordingly, the ptox2 mutant shows lower fitness than wild type when grown under phototrophic conditions. Single and double mutants devoid of the cytochrome b(6)f complex and PTOX2 were used to measure the oxidation rates of plastoquinols via PTOX1 and PTOX2. Those lacking both the cytochrome b(6)f complex and PTOX2 were more sensitive to light than the single mutants lacking either the cytochrome b(6)f complex or PTOX2, which discloses the role of PTOX2 under extreme conditions where the plastoquinone pool is overreduced. A model for chlororespiration is proposed to relate the electron flow rate through these alternative pathways and the redox state of plastoquinones in the dark. This model suggests that, in green algae and plants, the redox poise results from the balanced accumulation of PTOX and NADPH dehydrogenase.
authors.affiliation: Institut de Biologie Physico-Chimique, Centre National de la Recherche Scientifique Université Pierre et Marie Curie, Unité Mixte de Recherche 7141, 75005 Paris,  France.
authors.forename: Laura
authors.initials: L
authors.lastname: Houille-Vernes
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rappaport
authors.forename: Francis-André
authors.initials: FA
authors.lastname: Wollman
authors.forename: Jean
authors.initials: J
authors.lastname: Alric
authors.forename: Xenie
authors.initials: X
authors.lastname: Johnson
mesh_terms: D029681:Arabidopsis Proteins / Q000378:metabolism*; D002338:Carotenoids / Q000737:chemistry; D002696:Chlamydomonas / Q000201:enzymology*; D000460:Chlorophyta / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism; D002874:Chromosome Mapping; D015723:Gene Library; D005816:Genetic Complementation Test; D007700:Kinetics; D008027:Light; D009154:Mutation; D009252:NADPH Dehydrogenase / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D010641:Phenotype; D010971:Plastoquinone / Q000737:chemistry
chemical_list: D029681:Arabidopsis Proteins; D002338:Carotenoids; D009252:NADPH Dehydrogenase; D010088:Oxidoreductases; D010971:Plastoquinone; C405742:IM protein, Arabidopsis