The stromal side of the cytochrome b6f complex regulates state transitions.

In oxygenic photosynthesis, state transitions distribute light energy between Photosystem I and Photosystem II. This regulation involves reduction of the plastoquinone pool, activation of the State Transitions 7 (STT7) protein kinase by the cytochrome b6f complex, and phosphorylation and migration of Light Harvesting Complex II (LHCII). Here, we show that in Chlamydomonas reinhardtii, the C-terminus of the cyt b6 subunit PetB 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 accumulated but the phosphorylation cascade was 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 accumulated transiently after reduction of the PQ pool and represent the active forms of the protein kinase. 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.