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BIOENERGETICS AND PHOTOSYNTHESIS

Impact of PsbTc on Forward and Back Electron Flow, Assembly, and Phosphorylation Patterns of Photosystem II in Tobacco^1,[W],[OA]

Department of Biology I, Botany, Ludwig-Maximilians-University Munich, 80638 Munich, Germany (P.U., C.F., S.S., M.Z., L.A.E., R.G.H., J.M.); Department of Botany, Kakatiya University, Warangal 506 009, India (A.S.); and Department of Biological Chemistry and Minerva Center of Photosynthesis Research, Hebrew University of Jerusalem, Jerusalem 91904, Israel (I.O.)

Photosystem II (PSII) of oxygen-evolving cyanobacteria, algae, and land plants mediates electron transfer from the Mn₄Ca cluster to the plastoquinone pool. It is a dimeric supramolecular complex comprising more than 30 subunits per monomer, of which 16 are bitopic or peripheral, low-molecular-weight components. Directed inactivation of the plastid gene encoding the low-molecular-weight peptide PsbTc in tobacco (Nicotiana tabacum) does not prevent photoautotrophic growth. Mutant plants appear normal green, and levels of PSII proteins are not affected. Yet, PSII-dependent electron transport, stability of PSII dimers, and assembly of PSII light-harvesting complexes (LHCII) are significantly impaired. PSII light sensitivity is moderately increased and recovery from photoinhibition is delayed, leading to faster D1 degradation in psbTc under high light. Thermoluminescence emission measurements revealed alterations of midpoint potentials of primary/secondary electron-accepting plastoquinone of PSII interaction. Only traces of CP43 and no D1/D2 proteins are phosphorylated, presumably due to structural changes of PSII in psbTc. In striking contrast to the wild type, LHCII in the mutant is phosphorylated in darkness, consistent with its association with PSI, indicating an increased pool of reduced plastoquinone in the dark. Finally, our data suggest that the secondary electron-accepting plastoquinone of PSII site, the properties of which are altered in psbTc, is required for oxidation of reduced plastoquinone in darkness in an oxygen-dependent manner. These data present novel aspects of plastoquinone redox regulation, chlororespiration, and redox control of LHCII phosphorylation.

² These authors contributed equally to the article.

³ Present address: Plant Molecular Biology, International Centre for Genetic Engineering and Biotechnology, Aruna Asaf Ali Marg, New Delhi 110 067, India.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jörg Meurer (joerg.meurer{at}lrz.uni-muenchen.de).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.126060

^* Corresponding author; e-mail joerg.meurer{at}lrz.uni-muenchen.de.

Received July 8, 2008; accepted September 12, 2008; published September 19, 2008.