Abstract
Abstract Numbers of the Photosystem I reaction center complexes and the cytochrome b 6 - f complexes with which a cytochrome c -553 molecule can interact within the limiting time of photosynthetic electron transport were examined by measuring flash-induced absorption changes of P-700, cytochrome c -553 and cytochrome f in the thermophilic cyanobacterium Synechococcus sp. The addition of 2,5-dibromo-3-methyl-6-isopropyl- p -benzoquinone (DBMIB) did not affect the common 2 ms half-time of P-700, cytochrome c -553 and cytochrome f reduction, which is ascribed to electron transfer from the plastoquinone pool. The inhibitor decreased, however, amounts of the three electron carriers which underwent the 2 ms reduction in the order of cytochrome f , cytochrome c -553 and P-700. On excitation with weak flashes which oxidized only a small fraction of cytochrome c -553 molecules present in cells, P-700 remained in the oxidized state after the flashes was reduced with electrons from the Rieske center or plastoquinone but not from cytochrome c -553. The ratios of cytochrome c -553 to cytochrome f oxidized at various flash intensities were constant and similar to the ratio of the two cytochromes present in cells. It is concluded that cytochrome c -553 cannot exchange electrons with large numbers of the Photosystem I reaction center complexes and the cytochrome b 6 - f complexes in the limiting time, but has a mobility sufficient to mediate electron transfer between the two complexes, which are present at an unbalanced ratio in Synechococcus cells.
Published Version
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