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 studiedwith Triton-solubilized PS II membranes depleted of functionalMn. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) non-competitivelyinhibited the diphenylcarbazide-supported (DPC-supported) photoreductionof silicomolybdate (SiMo) at concentrations more than ten timeshigher than that required for inhibition of the DPC-supportedphotoreduction of 2,6-dichlorophenolindophenol (DCIP). The maximumfluorescence intensity was also reduced by DCMU at a similarconcentration to that required for the inhibition of the SiMophotoreduction. These findings suggest two inhibitory sitesof action of DCMU in PS II: one on the reducing side and oneon the oxidizing side of PS II. The inhibition constant forDCMU in the DPC-supported SiMo-photoreduction was 10 µMin every examination. The extent of inhibition was attenuatedby modifications of the PS II oxidizing side by the presenceof functional Mn, by photoinhibition and by chemical modificationsof histidine residues and acidic amino acid residues. Our resultssuggest that DCMU binds to the PS II oxidizing side near Z,D and the high-affinity Mn-binding sites. ¹ Present Address and address for all communications: NoriakiTamura (Dr.), Plant Physiology Laboratory Fukuoka Women's University,Kasumigaoka 1-1, Higashi-ku, Fukuoka, 813 Japan. FAX 092-661-2415.