Characterization of reaction center/antenna complexes from bacteriochlorophyll a containing Ectothiorhodospiraceae

Abstract

Pigment-protein complexes have been isolated from three bacteriochlorophyll- a -containing Ectothiorhodospira species: E. mobilis, E. shaposhnikovii and E. halophila , using detergent solubilisation of photosynthetic membranes and sucrose-density gradient centrifugation. This isolation procedure yields pure LHII complexes and reaction centers with either one or both antenna complexes attached (i.e., RC/LHI and RC/LHI/LHII complexes). The RC/LHI/LHII complexes consist of three reaction center polypeptides, H (35 kDa), M (34 kDa), and L (26.5 kDa), plus two LHI and two LHII antenna polypeptides. In photosynthetic membranes of the three Ectothiorhodospira species two major types of heme c are present, of low (25 mV) and high (297 mV) midpoint potential, respectively. These heme groups belong to separate cytochromes. During solubilisation and purification, no loss of the H subunit from the reaction centers is observed. However, the reaction centers do lose their c -type cytochromes. The high-potential cytochrome c is completely lost during solubilisation of RC complexes from photosynthetic membranes, while the low-potential cytochrome is largely lost during isolation of RC/LHI complexes. The residual amount of this low-potential cytochrome, associated with the RC/LHI complexes, varies with the salt concentration used during the isolation, but remains below 0.2 mol heme c per mol reaction center. This is less than 20% of the amount originally present in photosynthetic membranes.