The stromal side of cytochrome b6f complex regulates state transitions

In oxygenic photosynthesis, state transitions distribute light energy between Photosystem I and Photosystem II. This regulation involves the reduction of the plastoquinone pool, activation of the STT7 protein kinase by cytochrome b6f complex, phosphorylation and migration of Light Harvesting Complexes II (LHCII). Here we show the C-term of cyt b6 subunit acts on phosphorylation of STT7 and state transitions. We used site-directed mutagenesis of the chloroplast petB gene to truncate (remove L215b6) or elongate (add G216b6) the cyt b6 subunit. Modified complexes are devoid of heme ci and degraded by FTSH protease, revealing that salt bridge formation between cyt b6 (PetB) and subunit IV (PetD) is key to the assembly of the complex. In double mutants where FTSH is inactivated, modified cyt b6f are accumulated but the phosphorylation cascade is blocked. We also replaced the arginine interacting with heme ci propionate (R207Kb6). In this modified complex, heme ci is present but the kinetics of phosphorylation are slower. We show that highly phosphorylated forms of STT7 are accumulated transiently after reduction of the PQ pool, and represent the active forms of the protein kinase. 96% protein coverage using phosphoproteomics showed 4 new phosphorylated peptides in the kinase domain of STT7. Phosphorylation of the LHCII targets is favored at the expense of the protein kinase, and the migration of LHCII towards PSI is the limiting step for state transitions. Significance statementState transitions are regulatory mechanisms that optimize the quantum yield of photosynthesis. Chlamydomonas reinhardtii is the choice organism to study this regulation at the molecular level. Our study describes an unprecedented mechanism of stereochemical changes at the Qi site of the cytochrome b6f complex that trigger STT7 protein kinase activation through autophosphorylation.