Bifurcated Electron Donations from Quinol Oxidoreductase and Soluble CycA to Cytochrome c_z of the Photosynthetic Reaction Center Complex in the Green Sulfur Bacterium Chlorobium tepidum

Abstract

In The Photosynthetic Green Sulfur Bacterium Chlorobium Tepidum, The Photo-Oxidized Primary Electron Donor P840⁺ Is Rapidly Rereduced By Cytochrome CZ, One Of Subunits Of The Reaction Center Complex. The Oxidized Cytochrome CZ Can Be Rereduced By Either Soluble Low-Molecular-Weight Cytochrome C, Cyca, Or Membrane-Bound Quinol Oxidoreductase Directly. By Setting Up The Reconstitution System Using Membranes Prepared From A Mutant Devoid Of Cyca, We Investigated The Photosynthetic Electron Transfer Pathways When Both Of Them Co-Exist As They Do In Vivo. It Was Found That Quinol Oxidoreductase Never Donates Electrons To Cyca When It Can Donate To Cytochrome CZ, Making A Clear Contrast With The Case In Purple Bacteria Where Cytochrome Bc ₁ Complex Donates Electrons To Cytochorme C₂. The Analysis Of Photosynthetic Growth Profiles Using Mutants Defective In Sulfur Oxidations Suggests That Quinol Oxidoreductase And Cyca Serve As Electron Carriers In The Two Different Pathways, That Is, The Sulfide And Thiosulfate Oxidation One, Respectively.