Isolation of the Photoactive Reaction Center Complex that Contains Three Types of Fe-S Centers and a Cytochrome c Subunit from the Green Sulfur Bacterium Chlorobium limicola f. thiosulfatophilum, Strain Larsen

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The photoactive reaction center (RC) complex from the green sulfur bacterium Chlorobium limicola f. thiosulfatophilum, strain Larsen, was isolated after solubilization and ammonium sulfate fractionation followed by ion-exchange chromatography. The spectrum of the complex was almost identical with that of the similar RC complex isolated by Feiler et al. [(1992) Biochemistry 31 : 2608-2614] except for the presence of cytochrome c_<551> instead of c_<553> in the latter study. A molecular ratio of BChl a to P840 of the isolated RC complex was assayed to be 25-35. SDS-PAGE analysis revealed that the isolated complex contained three major polypeptides with apparent molecular masses of 68, 41 and 21 kDa, respectively. The 21-kDa polypeptide was identified to be a heme-binding protein by staining the gel for peroxidase activity. The cytochrome c_<551> was oxidized by flash light in a biphasic manner with half times of 90 and 390 μs, respectively, that coincided with the reduction half times of P840^+. Three distinct iron-sulfur centers assigned to F_A, F_B and F_X, respectively, from their g-values were detected by EPR spectroscopy at cryogenic temperature. These results suggest that the present preparation contains a minimal functional unit of the RC of this bacterium, and that this complex appears to lie on a evolutionary line between RC's of purple bacteria and photosystem I.
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