Analysis of the Electron Transfer Pathway in the Green Sulfur Bacterial-Type Photochemical Reaction Center by a Conversion into the Pseudo Heterodimeric Structure

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We studied on the structural and functional relationship of the type 1 reaction center in green sulfur bacteria and heliobacteria in order to understand profoundly the principle for the molecular construction of photosystem I reaction center in higher plants and cyanobacteria. The research progress in these two years is summarized as follows. 1. Searching for the Secondary Electron Acceptor A_1 (Quinone): The ESR signal derived from reduced quinone was successfully detected in the reaction center core complex in Heliobacterium modesticaldum. At present, we are now trying to identify its chemical species with HPLC and mass spectroscopic analyses. Meanwhile, we tried to obtain a mutant deleted of menG which encode a SAM-dependent methyl transferase in the quinone biosynthetic pathway. The back reaction of the oxidized P800^+ in the mutant had been expected to show some kind of different kinetics after a flash excitation. But we failed to obtain it. 2: Analyses of Sulfur Oxidation Pathway: Green sulfur bacteria use reduced sulfur compounds as an electron source. However, information concerning the linkage between sulfur oxidation and photosynthetic electron transfer pathways has long been scarce. Three mutants, that is, ΔcycA (a deletion mutant of CycA, cytochrome c-554), ΔsoxB (the SoxB-deleted one, which is one of subunits forming a multi-enzyme system involved in S_2O_3^<2-> oxidation) and a double mutant of ΔcycAΔsoxB, were constructed and we studied on the relationship between sulfur oxidation and photosynthetic electron transfer pathways. Although CycA surely serves as a terminal electron acceptor in the S_2O_3^<2-> oxidation pathway, another component should be involved in it as a probable bypath. Furthermore, each electron liberated from the S^<2-> and S_2O_3^<2-> oxidations, respectively, would be transferred to the P840 reaction center by a discrete electron transfer pathway.