Significance Sunlight is a powerful but fluctuating energy source used by photosynthetic organisms as a substrate for biochemistry. To optimize photosynthetic efficiency, the distribution of energy input between the two photosystems is regulated through state transitions. This mechanism is common to all organisms of the green lineage and depends on the concerted response of the cytochrome b ₆ f complex and the Stt7 kinase to changing redox conditions along the electron transport chain. The identification of a cytochrome b ₆ f stromal motif involved in state transitions gives insight into the mechanism of Stt7 kinase activation.