Effect of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone on the secondary electron acceptor B of photosystem II

Measurements of chlorophyll fluorescence have been used to monitor electron transport from the primary electron acceptor of photosystem II, Q, to the secondary acceptor, B, in chloroplasts in either the presence or the absence of the plastoquinone analog 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Electron transport is markedly slower from Q^? to either B or B^? in the presence of DBMIB. Binary oscillations in the rate of reoxidation of Q^? (equivalent to the reactions Q^?B → QB^? and Q^?B^? → QB^2?) after each of a series of flashes were of a phase opposite to those observed in the absence of DBMIB (J. M. Bowes, and A. R. Crofts, (1980) Biochim. Biophys. Acta590, 573–584). The results confirm that inhibition of electron transport by DBMIB in chloroplasts is not restricted to an inhibition of electron transfer from the plastoquinone pool, but that there is also a specific interaction between the reduced form of the inhibitor and the secondary electron acceptor B. Models are discussed to account for the mechanism of this interaction.