The Reaction Center from Green Sulfur Bacteria

Green sulfur bacterial reaction centers have been usually cited as counterparts of Photosystem I. However, they have remained poorly characterized for a long time, since the main interest was directed towards understanding the unique antenna system of green bacteria, the so-called chlorosomes. Many exciting developments in green sulfur bacterial reaction center research have occurred during the years 1990 to 1995. Important similarities between the reaction centers of green sulfur bacteria, heliobacteria, and Photosystem I have been discovered, such as the presence of the same set of electron acceptors and a similar polypeptide composition. Furthermore, similarities to the purple bacterial-type reaction centers have been described. These include the presence of a bacteriochlorophyll dimer as the primary donor and a reaction-center-associated cytochrome subunit as electron donor to P840. Most surprising, however, has been the discovery of only one gene coding for the dimeric, large reaction center subunits. This feature is in contrast, not only to Photosystem I, but also, to purple bacterial-type reaction centers, and it has raised interesting evolutionary questions which are discussed in this chapter. It has been shown that both types of reaction centers, i.e. purple bacterial- and Photosystem I -type reaction centers share common features, such as the charge separation processes, those pigments involved and a similar organization of the main polypeptides. All this led to the speculation that the different kinds of photosynthetic reaction centers have a common evolutionary origin. In this review we summarize the current state of knowledge about the green sulfur bacterial reaction center structure and function and compare these with those of other Photosystem I-type and purple bacterial reaction centers.