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Plant and Cell Physiology, 1992, Vol. 33, No. 4 437-443
© 1992

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

Hiroyuki Kamachi, Noriaki Tamura1 and Hiroshi Inoué

Department of Biology, Faculty of Science, Toyama University 3190 Gofuku, Toyama, 930 Japan

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.

1 Present Address and address for all communications: Noriaki Tamura (Dr.), Plant Physiology Laboratory Fukuoka Women's University, Kasumigaoka 1-1, Higashi-ku, Fukuoka, 813 Japan. FAX 092-661-2415.

(Received March 19, 1991; Accepted March 14, 1992)
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