Differential inhibition by plastoquinone analogues of photoreduction of cytochromeb‐559 in chloroplasts

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Abstract
The high‐potential form of cytochromeb‐559 (b‐559 HP) is closely linked to the oxygenic photosystem (photosystem II) but its relation to other redox components of the photosynthetic apparatus, including plastoquinone, is still obscure. We investigated the photoreduction of cytochromeb‐559 HP by isolated chloroplasts in the presence of 3 antagonists of plastoquinone, of which, DBMIB (dibromothymoquinone) and DNP‐INT (dinitrophenyl ether of iodonitrothymol) are known to inhibit the oxidation of the plastoquinone pool (PQ) by the FeS‐cytochrome ƒ/b6complex and one, UHDBT (5‐n‐undecyl‐6‐hydroxy‐4,7‐dioxobenzothiazole) is known to inhibit the reduction of PQ by QB.QBis a protein‐bound plastoquinone that serves as a two‐electron gate for the reduction of PQ. We found that DBMIB and DNP‐INT did not inhibit but low concentrations of UHDBT severely inhibited the photoreduction of cytochromeb‐559 HP. These results suggest that the electron donor for the reduction of cytochromeb‐559 HP was either QBor a portion of the PQ pool that was oxidized by a new pathway free of binding sites for DBMIB and DNP‐INT.