Abstract
In the photosynthetic electron transfer chain, PSII and PSI are connected in series through the plastoquinone (PQ) pool, the cytochrome b6f complex and plastocyanin. Optimal electron flow requires a balanced redox poise of these intermediate carriers. State transitions reflect a regulatory feedback loop that adjusts within minutes the redox balance of the PQ pool in response to changes in external conditions and cellular demands for ATP and NADPH. Thus, state transitions are tightly integrated into the regulation of cellular energy metabolism. State transitions involve the dynamic allocation of components of the light-harvesting complex II (LHCII) antenna to PSI or PSII. They are regulated by a protein kinase (STT7) and two counteracting phosphatases (CrPPH1 and CrPBCP) that control LHCII phosphorylation. Specific phosphorylated subunits of LHCII direct its docking to PSI, forming PSI-LHCI-LHCII supercomplexes. The segregation of PSI and PSII in different domains of the thylakoid membranes imposes the need to laterally redistribute LHCII during state transitions.
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