Differential Effects of Urea/Triazine-type and Phenol-type Photosystem II Inhibitors on Inactivation of the Electron Transport and Degradation of the D1 Protein during Photoinhibition

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The loss of PSII activity (water → silicomolybdate) and the degradation of the D1 protein during photoin-hibitory illumination were studied in the presence of the various PSII inhibitors that bind to the Q_B-site to interrupt the electron transfer from Q^-_A to Q_B, and the following results were obtained. (i) The inactivation of PSII was suppressed in the presence of urea/triazine-type inhibitors but was enhanced in the presence of phenol-type inhibitors. (ii) The urea/triazine-type inhibitors retarded not only the degradation of the D1 protein but also the formation of the 41 kDa adduct of the D1 protein and the a-subunit of Cyt b_<559>. (iii) The phenol-type inhibitors did not affect the degradation of the D1 protein but enhanced the formation of the 41 kDa adduct. (iv) The 23 and 9 kDa fragments of the D1 protein, induced by dark incubation with PNO8, were further degraded by photoinhibitory illumination. (v) Photoin-activation of PSII was not suppressed by DCMU, a urea/ triazine-type inhibitor, after the Q_B-site had been modified by trypsin. These and other results are discussed with respect to the possibility of a conformational change at the Q_B_-site of the D1 protein, induced by the binding of a PSII inhibitor, and the effects of such a change on the processes of photoinhibition.
日本植物生理学会の論文