The amino terminus of PetD is essential for cytochrome b6 f function and the negative feedback control of STT7 kinase

Abstract The cytochrome b 6 f complex ( b 6 f ) links photosystem (PS) I and PSII in the photosynthetic electron transport chain and is distributed across appressed and non-appressed thylakoid membranes. The b 6 f also activates the state-transition 7 kinase (STT7), which phosphorylates light-harvesting complex proteins, triggering their migration to enable energy redistribution between PSII and PSI. STT7-dependent phosphorylation has also been observed at Thr4 in the amino-terminal domain of the b 6 f subunit-IV (PetD), though its functional significance has remained unclear. Here, to investigate its role, we generated several chloroplast mutants. The phosphomimic substitution PetD T4E—but not T4A—inhibited STT7 kinase activity, as indicated by the absence of STT7-dependent phosphorylation and a State 1-locked phenotype. This reveals a feedback mechanism regulating STT7-dependent phosphorylation. The deletion of the first five N-terminal amino acids similarly inhibited STT7 activity and additionally disrupted electron transfer, underscoring a crucial role of the PetD N terminus in b 6 f function.