title: Photosynthetic growth despite a broken Q-cycle.
abstract: Central in respiration or photosynthesis, the cytochrome bc(1) and b(6)f complexes are regarded as functionally similar quinol oxidoreductases. They both catalyse a redox loop, the Q-cycle, which couples electron and proton transfer. This loop involves a bifurcated electron transfer step considered as being mechanistically mandatory, making the Q-cycle indispensable for growth. Attempts to falsify this paradigm in the case of cytochrome bc(1) have failed. The rapid proteolytic degradation of b(6)f complexes bearing mutations aimed at hindering the Q-cycle has precluded so far the experimental assessment of this model in the photosynthetic chain. Here we combine mutations in Chlamydomonas that inactivate the redox loop but preserve high accumulation levels of b(6)f complexes. The oxidoreductase activity of these crippled complexes is sufficient to sustain photosynthetic growth, which demonstrates that the Q-cycle is dispensable for oxygenic photosynthesis.
authors.affiliation: Institut de Biologie Physico-Chimique, Unité Mixte de Recherche 7141, Centre National de la Recherche Scientifique-Université Paris 6, 13 Rue Pierre et Marie Curie, 75005 Paris, France.
authors.forename: Alizée
authors.initials: A
authors.lastname: Malnoë
authors.forename: Francis-André
authors.initials: FA
authors.lastname: Wollman
authors.forename: Catherine
authors.initials: C
authors.lastname: de Vitry
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rappaport
mesh_terms: D016227:Benzoquinones; D016825:Chlamydomonas reinhardtii / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D045346:Cytochrome b6f Complex / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D045348:Cytochromes f / Q000378:metabolism; D004579:Electron Transport; D006418:Heme / Q000172:deficiency; D006873:Hydroquinones; D015151:Immunoblotting; D009154:Mutation; D010088:Oxidoreductases / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*
chemical_list: D016227:Benzoquinones; D006873:Hydroquinones; C025232:semiquinone radicals; D006418:Heme; D045346:Cytochrome b6f Complex; D045348:Cytochromes f; D010088:Oxidoreductases