Role of the Low-Molecular-Weight Subunits PetL, PetG, and PetN in Assembly, Stability, and Dimerization of the Cytochrome b6 f Complex in Tobacco 1[C]

The cytochrome b6 f (Cyt b6 f) 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 b6 f complex were faintly detectable, indicating that both proteins are essential for the stability of the membrane complex. In contrast, DpetL plants accumulate about 50% of other Cyt b6 f subunits, appear green, and grow photoautotrophically. However, DpetL 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 b6 f 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 DpetL, as revealed by measurements of PSI and PSII redox states. This confirms the sensory role of the Cyt b6 f 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 DpetL.