N-terminal region of PetD is essential for cytochrome b6f function and controls STT7 kinase activity via STT7-dependent feedback loop phosphorylation

The cytochrome b ₆ f complex ( b ₆ f ) links photosystem I (PSI) and photosystem II (PSII) in the photosynthetic electron transfer 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 T4 in the N-terminal domain of the b ₆ f subunit-IV (PetD), though its functional significance has remained unclear. To investigate its role, we generated several chloroplast mutants. The phosphomimic mutation PetD T4E inhibited STT7 kinase activity, as indicated by the absence of STT7-dependent phosphorylation and the strain being locked in State 1. This reveals a novel feedback mechanism regulating phosphorylation facilitated by STT7. The deletion of five N-terminal amino acids resulted in a comparable inhibition of STT7 activity and, additionally, in the disruption of electron transfer which underscores a crucial role of the PetD N-terminus in b ₆ f function.