INVESTIGATION OF THE S3 ELECTRON-PARAMAGNETIC-RESONANCE SIGNAL FROM THE OXYGEN-EVOLVING COMPLEX OF PHOTOSYSTEM-2 - EFFECT OF INHIBITION OF OXYGEN EVOLUTION BY ACETATE

An S3 electron paramagnetic resonance (EPR) signal is observed in a va riety of photosystem 2 (PS2) samples in which the oxygen-evolving comp lex (OEC) has been inhibited. These signals have been proposed to be d ue to an interaction, S2X+, between the manganese cluster in an oxidat ion state equivalent to S2 and an organic radical, either oxidized his tidine [Boussac et al. (1990) Nature 347, 303-3061 or the tyrosine rad ical Y(z)+ [Hallahan et al. (1992) Biochemistry 31, 4562-45731. We rep ort that treatment of PS2 with acetate at pH 5.5 leads to a slowing of the reduction of Y(z)+ and allows the trapping of an S3-type state on freezing to 77 K following illumination at 277 K. The S3 EPR signal i n acetate-treated PS2 has a broader and more complex line shape but ot herwise has similar properties to other S3 signals. The addition to ac etate-treated samples in the S1 state of the herbicide 3-(3,4-dichloro phenyl)-1,1-dimethylurea (DCMU), which allows only a single turnover o f the reaction center, causes a large reduction in the yield of the S3 signal. Various anion and cation treatments change the S3 signal line shape and are used to show that acetate probably acts by binding and displacing chloride. We propose that a variety of treatments which aff ect calcium and chloride cofactor binding cause a modification of the S2 state of the manganese cluster, slow the reduction of Y(z)+ and all ow an S3 EPR signal to be observed following illumination. The origin of the S3 signal, whether a modified S3 or S2X+ where X is an organic radical, remains in doubt as the involvement and identity of the organ ic radical is still uncertain.