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A schematic model for changes in PSII in unicellular nitrogen-fixing cyanobacteria during light-dark transitions. During the dark period, D1 protein in PSII is replaced by sD1. PSII complexes with sD1 are inactive. A transition into the light phase results in the replacement of sD1 by D1, followed by binding of other lumenal proteins to form a functional PSII complex.
