Interaction of the Quinone Analogue, DBMIB, with the Photosynthetic Rieske Iron‐Sulfur Center

Addition of the plastoquinone analogue, DBMIB, to chloroplasts results in the appearance of an EPR signal atg= 1.94 which is observed at cryogenic temperatures. Studies with subchloroplast fragments which either contain or lack the Rieske iron‐sulfur center indicate the presence of this electron carrier is required for the DBMIB‐induced EPR signal. A correlation between the disappearance of the Rieske center signal atg= 1.89 and the appearance of the DBMIB‐induced signal atg= 1.94 suggests DBMIB alters the Rieske center to produce the new signal. Similar effects were observed in photosynthetic membrane fragments from a blue‐green alga and a photosynthetic bacterium. It has been possible to use the DBMIB‐induced signal to test for interaction of other inhibitors in the region of the Rieske center. These results are considered in terms of models in which quinone and the Rieske center function as a donor‐acceptor pair during physiological electron transport.