Donation of Electrons from Cytosolic Components to the Intersystem Chain in the Cyanobacterium Synechococcus sp. PCC 7002 as Determined by the Reduction of P700^+ : ENVIRONMENTAL AND STRESS RESPONSES : MEMBRANES AND BIOENERGETICS

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Cells of Synechococcus sp. PCC 7002 showed a low oxidation level of P700 under a far-red light at 6 W m^<-2>, which induced nearly complete oxidation of P700 in spinach leaves, and a stronger far-red light was required to observe the oxidation of P700. DCMU did not affect the level of P700^+, but 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone induced the oxidation of P700 under far-red light, indicating that the low oxidation level of P700 was due to the donation of electrons to P700^+ from the cyiosolic respiratory donors through the intersystem chain at the plastoquinone pool. The electron transfer from the cyiosolic donors to the intersystem chain was inhibited by HgCl_2, but not by antimycin A. The reduction of P700^+ in Synechococcus cells, after illumination by strong far-red light was mostly accounted for by the electron flow to the inter system chain from the respiratory donors (t_<1/2>=0.5 s). P700^+ was reduced also after a 50-ms, saturating pulse light in the presence of DCMU (t_<1/2>=38 ms), which shows the electron flow to the intersystem chain from the photoreductant of PS I, possibly NAD(P)H.
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