The Antenna-Reaction Center Complex of Heliobacteria

Abstract

SummaryThe heliobacteria are a recently discovered group of photosynthetic bacteria that are characterized by the presence of bacteriochlorophyll (BChl) g as major pigment. All of the pigments are contained in a single pigment-protein complex, which contains about 35 BChls g per reaction center. The primary electron donor, P798, is presumably a dimer of BChl g or of its epimer, BChl g′; the primary electron acceptor has been identified as hydroxy chlorophyll a. There is evidence that iron-sulfur centers act as secondary electron acceptors, while a c-type cytochrome, cytochrome c-553, acts as secondary electron donor to P798. Energy transfer from the antenna pigments to the reaction center appears to be distinctly faster than in purple bacteria. The major subunit of the antenna reaction center complex is a peptide of 68 kDa, containing 609 amino acid residues, which shows an about 20% homology to the PS I-A and PS I-B reaction center-core peptides of Photosystem I.The above observations all indicate that the reaction center of heliobacteria, like that of green sulfur bacteria is related to that of Photosystem I of plants and, in fact, the heliobacteria are in many ways excellent material to study the basic processes of Photosystem I-type reaction centers. However, the reaction center is homodimeric, i.e., it contains two identical subunits that bind the antenna pigments and the reaction center components. This indicates that the ancestral lines of Photosystem I and heliobacteria separated before gene duplication occurred in the first one.