The reduction kinetics of EPR signal IIslow in the presence of iodide were measured with EPR spectroscopy and the amount of 125I incorporated into the D2-protein in the reaction center of photosystem II was quantitated in a PS II core complex preparation. The halftime of signal IIslow reduction in the dark was estimated at 31, 25 and 20 min in the presence of 0, 20 and 50 mM KI, respectively, reflecting the reduction of signal IIslow by iodide. The amount of 125I that was incorporated into the D2-protein in the dark was correlated to the reduction of signal IIslow by iodide. This correlation indicates that the site of iodide oxidation in the dark and the component giving rise to EPR signal IIslow are located on the D2-protein in the PS II reaction center.
