title: A cytochrome b origin of photosynthetic reaction centers: an evolutionary link between respiration and photosynthesis.
abstract: The evolutionary origin of photosynthetic reaction centers has long remained elusive. Here, we use sequence and structural analysis to demonstrate an evolutionary link between the cytochrome b subunit of the cytochrome bc(1) complex and the core polypeptides of the photosynthetic bacterial reaction center. In particular, we have identified an area of significant sequence similarity between a three contiguous membrane-spanning domain of cytochrome b, which contains binding sites for two hemes, and a three contiguous membrane-spanning domain in the photosynthetic reaction center core subunits, which contains binding sites for cofactors such as (bacterio)chlorophylls, (bacterio)pheophytin and a non-heme iron. Three of the four heme ligands in cytochrome b are found to be conserved with the cofactor ligands in the reaction center polypeptides. Since cytochrome b and reaction center polypeptides both bind tetrapyrroles and quinones for electron transfer, the observed sequence, functional and structural similarities can best be explained with the assumption of a common evolutionary origin. Statistical analysis further supports a distant but significant homologous relationship. On the basis of previous evolutionary analyses that established a scenario that respiration evolved prior to photosynthesis, we consider it likely that cytochrome b is the evolutionary precursor for type II reaction center apoproteins. With a structural analysis confirming a common evolutionary origin of both type I and type II reaction centers, we further propose a novel "reaction center apoprotein early" hypothesis to account for the development of photosynthetic reaction center holoproteins.
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D005075:Biological Evolution; D019069:Cell Respiration / Q000502:physiology*; D003573:Cytochrome b Group / Q000737:chemistry / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010788:Photosynthesis / Q000502:physiology*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment
chemical_list: D001426:Bacterial Proteins; D003573:Cytochrome b Group; D045322:Photosynthetic Reaction Center Complex Proteins