Putative Second Binding Site of DCMU on the Oxidizing Side of Photosystem II in Photosystem II Membranes Depleted of Functional Mn

The effects of DCMU on the oxidizing side of PS II were studied with Triton-solubilized PS II membranes depleted of functional Mn. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) non-competitively inhibited the diphenylcarbazide-supported (DPC-supported) photoreduction of silicomolybdate (SiMo) at concentrations more than ten times higher than that required for inhibition of the DPC-supported photoreduction of 2,6-dichlorophenolindophenol (DCIP). The maximum fluorescence intensity was also reduced by DCMU at a similar concentration to that required for the inhibition of the SiMo photoreduction. These findings suggest two inhibitory sites of action of DCMU in PS II: one on the reducing side and one on the oxidizing side of PS II. The inhibition constant for DCMU in the DPC-supported SiMo-photoreduction was 10 μM in every examination. The extent of inhibition was attenuated by modifications of the PS II oxidizing side by the presence of functional Mn, by photoinhibition and by chemical modifications of histidine residues and acidic amino acid residues. Our results suggest that DCMU binds to the PS II oxidizing side near Z, D and the high-affinity Mn-binding sites.