Modification of Histidine Residues of Photosystem II by Diethyl Pyrocarbonate Inhibits the Electron Transfer between the Primary (QA) and Secondary (QB) Quinone Acceptors

The effect o f diethyl pyrocarbonate (DEPC) on the photosynthetic electron transport was investigated in isolated spinach thylakoids by partial electron transport rate and thermolumi­ nescence measurements. Incubation o f thylakoids at pH 6.5 with 5 mM DEPC for 15 min re­ sulted in a considerable inhibition o f electron transport from water to dichlorophenolindophenol. The inhibition was only partially releaved by addition o f the donor, diphenylcarbazide indicating the effect o f DEPC both on the donor and acceptor sides o f PS II. In the thermolu­ minescence glow curve DEPC-treatment abolished the B band (S2QB~ radiative charge recom­ bination) at 30 °C with a concomitant appearance o f the Q band (S2QA~ charge recombina­ tion) at 10 °C. This suggests that in isolated thylakoids possessing an active water-splitting sys­ tem DEPC affects the electron transfer from QA to QB but does not inhibit the electron transport from manganese to QA during the St—>S2 transition o f the water-splitting system. At the acceptor side o f PS II the targets o f DEPC are probably the histidines which are coordinat­ ed to the non-heme iron. Illumination o f thylakoids at -8 0 °C following DEPC addition after two preflashes at 5 °C resulted in the replacement o f the A (A T) thermoluminescence band at -3 0 °C with a band appearing at -1 5 °C. This observation can be explained by the effect o f DEPC on a donor side histidine component participating in the generation o f the A(AT) band. Consequently, in the interpretation o f results obtained by DEPC treatment o f PS II, both the donor and acceptor side effects o f DEPC should be considered.