Selective nitration of PsbO1 inhibits oxygen evolution from isolated Arabidopsis thylakoid membranes.

Treatment of isolated Arabidopsis thaliana thylakoid membranes with nitrogen dioxide (NO2 ) induces selective nitration of the tyrosine residue at the ninth amino acid (9 Tyr) of PsbO1. This selective nitration is triggered by light and is inhibited by photosynthetic electron transport inhibitors. Therefore, we postulated that, similar to 161 Tyr of D1 (YZ ), 9 Tyr of PsbO1 is redox active and is selectively oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical that is highly susceptible to nitration. This tyrosyl radical may combine rapidly at diffusion-controlled rates with NO2 to form 3-nitrotyrosine. If this postulation is correct, the nitration of 9 Tyr of PsbO1 should decrease oxygen evolution activity. We investigated the effects of PsbO1 nitration on oxygen evolution from isolated thylakoid membranes, and found that nitration decreased oxygen evolution to ≥ 0% of the control. Oxygen evolution and nitration were significantly negatively correlated. This finding is consistent with redox active properties of the 9 Tyr gene of PsbO1, and suggests that PsbO1 9 Tyr acts as an electron relay, such as YZ in the photosystem II oxygenic electron transport chain.