Increase in the level of thylakoid protein phosphorylation in maize mesophyll chloroplasts by decrease in the transthylakoid pH gradient

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Abstract
Phosphorylation of the light‐harvesting chlorophyll protein (LHCP) has been measured in intact chloroplasts prepared from maize mesophyll protoplasts. Maximum levels of phosphorylation were obtained in the absence of added reducible photosynthetic substrate and the presence of low concentrations of the ΔpH dissipating ionophore, nigericin. Assays of chlorophyll fluorescence indicated a high reduction level of plastoquinone under these conditions. It is suggested that the size of the transthylakoid pH gradient exerts control over the redox activation of the protein kinase and that protein phosphorylation is involved in the regulation of cyclic and non‐cyclic electron flow. Further evidence for this view was obtained from stimulation of protein phosphorylation by pyruvate which lowers ΔpH by ATP consumption and by inhibition of phosphorylation in strong light and by low levels of DCMU.