A partial reaction in photosystem II: reduction of silicomolybdate prior to the site of dichlorophenyldimethylurea inhibition.

Silicomolybdate functions as an electron acceptor in a Photosystem II water oxidation (measured as O2 evolution) partial reaction that is 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) insensitive, that is, reduction os silicomolybdate occurs at or before the level of Q, the primary electron acceptor for Photosystem II. This report characterizes the partial reaction with the principal findings being as follows: 1. Electron transport to silicomolybdate significantly decreased room temperature Photosystem I side of the DCMU had no effect on the fluorescence level, consistent with silicomolybdate accepting electrons at or before Q. In the absence of DCMU, silicomolybdate is also reduced at a site on the Photosystem I side of the DCMU block, prior to or at plastoquinone, since the plastoquinone antagonist dibromothymoquinone (DBMIB) did not affect the electron transport rate. 3. Electron transport from water to silicomolybdate (+ DCMU) is not coupled to ATP formation, nor is there a measurable accumulation of protons within the membrane (measured by amine uptake). Silicomolybdate is not inhibitory to phosphorylation per se since neither cyclic nor post-illumination (XE) phosphorylation were inhibited. 4. Uncouplers stimulated electron transport from water to silicomolybdate in the pH range of 6 to 7, but inhibited at pH values near 8. These data are consistent with the view that when electron flow is through the abbreviated sequence of water to Photosystem II to silicomolybdate (+ DCMU), conditions are not established for the water protons to be deposited within the membrane. Experiments reported elsewhere (Fiaquinta, R.T., Dilley, R.A. and Horton, P.(19741 J. Bioenerg. 6, 167-177) and these data, are consistent with the hypothesis that electron transport between Q and plastoquinone energizes a membrane conformational change that is required to interact with the water oxication system so as to result in the deposition of water protons either within the membrane itself or within the inner oxmotic space.