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BIOENERGETICS AND PHOTOSYNTHESIS

Role of the Low-Molecular-Weight Subunits PetL, PetG, and PetN in Assembly, Stability, and Dimerization of the Cytochrome b₆f Complex in Tobacco^1,[C]

Department of Biology I, Botany, Ludwig-Maximilians-University, 80638 Munich, Germany

The cytochrome b₆f (Cyt b₆f) complex in flowering plants contains nine conserved subunits, of which three, PetG, PetL, and PetN, are bitopic plastid-encoded low-molecular-weight proteins of largely unknown function. Homoplastomic knockout lines of the three genes have been generated in tobacco (Nicotiana tabacum ‘Petit Havana’) to analyze and compare their roles in assembly and stability of the complex. Deletion of petG or petN caused a bleached phenotype and loss of photosynthetic electron transport and photoautotrophy. Levels of all subunits that constitute the Cyt b₆f complex were faintly detectable, indicating that both proteins are essential for the stability of the membrane complex. In contrast, petL plants accumulate about 50% of other Cyt b₆f subunits, appear green, and grow photoautotrophically. However, petL plants show increased light sensitivity as compared to wild type. Assembly studies revealed that PetL is primarily required for proper conformation of the Rieske protein, leading to stability and formation of dimeric Cyt b₆f complexes. Unlike wild type, phosphorylation levels of the outer antenna of photosystem II (PSII) are significantly decreased under state II conditions, although the plastoquinone pool is largely reduced in petL, as revealed by measurements of PSI and PSII redox states. This confirms the sensory role of the Cyt b₆f complex in activation of the corresponding kinase. The reduced light-harvesting complex II phosphorylation did not affect state transition and association of light-harvesting complex II to PSI under state II conditions. Ferredoxin-dependent plastoquinone reduction, which functions in cyclic electron transport around PSI in vivo, was not impaired in petL.

² Present address: Institute of Biology, Department of Biology, Chemistry and Pharmacy, Freie Universität Berlin, Koenigin-Luise-Str. 12-16, 14195 Berlin, Germany.

³ Present address: Graduate School of Agriculture, Kyushu University, Fukuoka 812–8581, Japan.

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).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.100131

^* Corresponding author; e-mail joerg.meurer{at}lrz.uni-muenchen.de; fax 49–89–1782274.

Received March 23, 2007; accepted June 2, 2007; published June 7, 2007.