Iron-sulfur centers in the photosynthetic reaction center complex from Chlorobium vibrioforme. Differences from and similarities to the iron-sulfur centers in Photosystem I

The photosynthetic reaction center complex from the green sulfur bacterium Chlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochrome c₅₅₁ encoded by the pscA and pscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophyll a-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded by pscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of F_A and F_B of Photosystem I, but also show significantly different g-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculated g-values of 2.085, 1.898, 1.841), and Center II (with calculated g-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to the pscB polypeptide.