Herbicide binding at photosystem II. A new azido-triazinone photoaffinity label

Azido-triazinone (3-dimethylamino-4-methyl-6-(3′-azidophenyl)-1,2,4-triazin-5-one) was found to be an efficient inhibitor of Photosystem II electron transport. This compound has an I₅₀ value of 69 nM (extrapolated to zero chlorophyll concentration), a high-affinity binding constant of 12.6 nM, and a number of binding sites of 1.9 nmol/mg chlorophyll. This corresponds to 550–580 molecules of chlorophyll per bound inhibitor; i.e., one molecule inhibitor per electron transport chain. In isolated spinach thylakoids, ¹⁴C]azido-triazinone upon ultraviolet illumination covalently binds almost exclusively to a 34 kDa protein. Covalent binding is prevented in the presence of other Photosystem II inhibitors. The protein labeled by azido-triazinone is identical to the 34 kDa herbicide-binding protein which is tagged by another photoaffinity label azido-atrazine (2-azido-4-(ethylamino)-6-(isopropylamino)-s-triazine).