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

Treatment of isolated Arabidopsis thaliana thylakoid membranes with nitrogen dioxide (NO 2 ) 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 (Y Z ), 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 NO 2 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 Y Z in the photosystem II oxygenic electron transport chain.