Demonstration of Two Sites of Inhibition of Electron Transport by Protein Phosphorylation in Wheat Thylakoids

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Abstract
The effect of protein phosphorylation on electron transport activities of thylakoids isolated from wheat leaves was investigated. Protein phosphorylation resulted in a reduction in the apparent quantum yield of whole chain and photosystem II (PSII) electron transport but had no effect on photosystem I (PSI) activity. The affinity of the D1 reaction centre polypeptide of PSII to bind atrazine was diminished upon phosphorylation, however, this did not reduce the light-saturated rate of PSII electron transport. Phosphorylation also produced an inhibition of the light-saturated rate of electron transport from water or durohydroquinone to methyl viologen with no similar effect being observed on the light-saturated rate of either PSII or PSI alone. This suggests that phosphorylation produces an inhibition of electron transport at a site, possibly the cytochrome b6/f complex, between PSII and PSI. This inhibition of whole-chain electron transport was also observed for thylakoids isolated from leaves grown under intermittent light which were deficient in polypeptides belonging to the light-harvesting chlorophyll-protein complex associated with photosystem II (LHCII). Consequently, this phenomenon is not associated with phosphorylation of LCHII polypeptides. A possible role for cytochrome b6/f complexes in the phosphorylation-induced inhibition of whole chain electron transport is discussed.

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