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Photosynthetic unit size and the electron-transport chain in a photoreaction center-depleted mutant of Rs. rubrum.

AutorMar, Ted; Picorel Castaño, Rafael ; Gingras, Gabriel
Palabras claveReaction center
Bacterial photosynthesis
Electron transport
Fecha de publicación1982
CitaciónBiochimica et Biophysica Acta (BBA) - Bioenergetics 682: 354-363 (1982)
ResumenThe aim of this work was to explain the relatively fast growth of a mutant of Rhodospirillum rubrum (F24.1) which contains 7–8% of an apparently normal photoreaction center. We explored the double hypothesis that the size of its photosynthetic unit is larger than that of the wild type and that its electron-transport chain is organized in a network rather than in isolated loops. The first feature would allow faster growth under less than saturating light intensities and the second would allow faster maximal electron fluxes than would be predicted from the photoreaction center content. With respect to the first possibility, measurements of absorbance changes at 793 nm induced by short flashes of increasing intensity indicate that the photosynthetic unit of strain F24.1 is 5.6-fold larger than that of strain S1. The second possibility was verified by measuring relative electron fluxes at the photoreaction center in the two strains. This was established in the steady state from the amount of primary donor oxidized by a continuous light beam of increasing intensity. This electron flux was found to be about 70% as high in strain F24.1 as in strain S1. A more detailed study of the electron-transport chain indicated that cytochrome c2 is by far the main secondary electron donor in strain F24.1. No evidence could be obtained for the existence of another secondary donor in that strain. The mole ratio of cytochrome c2 to photoreaction center is about 6 in strain F24.1 as conpared to about 0.5 in strain S1. In strain 24.1, the pool of secondary donor appears to be collectively involved in the reduction of the oxidized primary donor. The replacement time at the photoreaction center of a first equivalent of oxidized cytochrome c2 by a second equivalent of reduced cytochrome c2 is less than or equal to 0.2 ms. The effect of the photoreaction center content on the size of the photosynthetic unit is discussed in terms of the different models proposed for the organisation of the photosynthetic unit. We propose that the electron-transport chain is organized in a network, perhaps by virtue of the lateral mobility of some of the electron carriers such as ubiquinone and cytochrome c2.
Versión del editorhttp://dx.doi.org/10.1016/0005-2728(82)90049-4
Identificadoresdoi: 10.1016/0005-2728(82)90049-4
issn: 0005-2728
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