Donation of Electrons from Cytosolic Components to the Intersystem Chain in the Cyanobacterium Synechococcus sp. PCC 7002 as Determined by the Reduction of P700+

Cells, of Synechococcus sp. PCC 7002 showed a low oxidationlevel of P700 under a far-red light at 6 W m^–2 which inducednearly complete oxidation of P700 in spinach leaves, and a strongerfar-red light was required to observe the oxidation of P700.DCMU did not affect the level of P700⁺² but 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinoneinduced the oxidation of P700 under far-red light, indicatingthat the low oxidation level of P700 was due to the donationof electrons to P700⁺² from the cytosolic respiratory donorsthrough the intersystem chain at the plastoquinone pool. Theelectron transfer from the cytosolic donors to the intersystemchain was inhibited by HgCl₂ but not by antimycin A. The reductionof P700⁺ in Synechococcus cells, after illumination by strongfar-red light was mostly accounted for by the electron flowto the inter system chain from the respiratory donors (t