| Photosynthesis in plants involves photosystem I (PSI) and photosystem II (PSII), both of which convert light energy to electrochemical energy. To keep optimal photosynthetic rate under variable light conditions, excitation levels of the two photosystems are balanced via redistribution of light-harvesting complexes II (LHCII) (state transitions). It has been known that LHCII phosphorylation is required for a transition from State 1, where mobile LHCIIs are associated with PSII, to State 2, where they are associated with PSI. However, it still remains unclear which particular step in the LHCII migration requires the phosphorylation. Here, we investigate the step of LHCII reassociation with PSI using a mutant deficient in the STT7 kinase. We show that under State 2-promoting conditions the excitation level of PSI increased, while that of PSII did not change, in the stt7 mutant. Moreover, a formation of PSI-LHCI/II supercomplex was observed in the stt7 mutant much as like in WT. Notably, the LHCII proteins in the PSI-LHCI/II supercomplex were barely phosphorylated. We concluded that LHCII phosphorylation is not a prerequisite for PSI binding mobile LHCII.
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