title: The role of the high potential form of the cytochrome b559: Study of Thermosynechococcus elongatus mutants.
abstract: Cytochrome b559 is an essential component of the photosystem II reaction center in photosynthetic oxygen-evolving organisms, but its function still remains unclear. The use of photosystem II preparations from Thermosynechococcus elongatus of high integrity and activity allowed us to measure for the first time the influence of cytochrome b559 mutations on its midpoint redox potential and on the reduction of the cytochrome b559 by the plastoquinone pool (or QB). In this work, five mutants having a mutation in the α-subunit (I14A, I14S, R18S, I27A and I27T) and one in the β-subunit (F32Y) of cytochrome b559 have been investigated. All the mutations led to a destabilization of the high potential form of the cytochrome b559. The midpoint redox potential of the high potential form was significantly altered in the αR18S and αI27T mutant strains. The αR18S strain also showed a high sensitivity to photoinhibitory illumination and an altered oxidase activity. This was suggested by measurements of light induced oxidation and dark re-reduction of the cytochrome b559 showing that under conditions of a non-functional water oxidation system, once the cytochrome is oxidized by P680(+), the yield of its reduction by QB or the PQ pool was smaller and the kinetic slower in the αR18S mutant than in the wild-type strain. Thus, the extremely positive redox potential of the high potential form of cytochrome b559 could be necessary to ensure efficient oxidation of the PQ pool and to function as an electron reservoir replacing the water oxidation system when it is not operating.
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Seville, Spain; Laboratoire de Bioénergétique Moléculaire et Photosynthèse, Institut de Biologie et de Technologies de Saclay (iBiTec-S), CEA Saclay, 91191 Gif-sur-Yvette cedex, France. Electronic address: Fernando.guerrero@helsinki.fi.
authors.forename: Fernando
authors.initials: F
authors.lastname: Guerrero
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Seville, Spain; Laboratoire de Bioénergétique Moléculaire et Photosynthèse, Institut de Biologie et de Technologies de Saclay (iBiTec-S), CEA Saclay, 91191 Gif-sur-Yvette cedex, France. Electronic address: jorge.zurita@juntadeandalucia.es.
authors.forename: Jorge L
authors.initials: JL
authors.lastname: Zurita
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Seville, Spain. Electronic address: mroncel@us.es.
authors.forename: Mercedes
authors.initials: M
authors.lastname: Roncel
authors.affiliation: Laboratoire de Bioénergétique Moléculaire et Photosynthèse, Institut de Biologie et de Technologies de Saclay (iBiTec-S), CEA Saclay, 91191 Gif-sur-Yvette cedex, France. Electronic address: kirilovsky@cea.fr.
authors.forename: Diana
authors.initials: D
authors.lastname: Kirilovsky
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Seville, Spain. Electronic address: ortega@us.es.
authors.forename: José M
authors.initials: JM
authors.lastname: Ortega
mesh_terms: D002455:Cell Division; D003573:Cytochrome b Group / Q000737:chemistry*; D004579:Electron Transport; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D046940:Synechococcus / Q000737:chemistry* / Q000201:enzymology / Q000235:genetics
chemical_list: D003573:Cytochrome b Group; D045332:Photosystem II Protein Complex; C019313:cytochrome b559
keywords: Cytochrome b559; High potential form; Photosystem II; Redox potentiometry; Site-directed mutagenesis; Thermosynechococcus elongatus