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Published on June 7, 2007; 10.1104/pp.107.100131

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Received March 23, 2007
Accepted June 2, 2007

Role of the Low-Molecular-Weight Subunits PetL, PetG, and PetN in Assembly, Stability, and Dimerisation of the Cytochrome b6f Complex in Tobacco

Serena Schwenkert , Julia Legen , Tsuneaki Takami , Toshiharu Shikanai , Reinhold G. Herrmann , and Jörg Meurer *

Ludwig-Maximilians-University Munich, Department Biology I, Botany, Menzingerstr. 67, 80638 Munich, Germany

* Corresponding author; email: joerg.meurer{at}lrz.uni-muenchen.de.

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 Nicotiana tabacum cv. Petit Havanna to analyze and compare their roles in assembly and stability of the complex. Deletion of petG or petN caused a bleached phenotype, loss of photosynthetic electron transport and of 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, {Delta}petL plants accumulate about 50% of other Cyt b6f subunits, appear green and grow photoautotrophically. However, {Delta}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 (PS) II are significantly decreased under state II conditions although the plastoquinone pool is largely reduced in {Delta}petL, as revealed by measurements of PSI and PSII redox states. This confirms the sensory role of Cyt b6f complex in activation of the corresponding kinase. The reduced LHCII phosphorylation did not affect state transition and association of LHCII to PSI under state II conditions. Ferredoxin-dependent plastoquinone reduction, which functions in cyclic electron transport around PSI in vivo, was not impaired in {Delta}petL.




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