A cytochrome b origin of photosynthetic reaction centers: an evolutionary link between respiration and photosynthesis |
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Authors: | Xiong Jin Bauer Carl E |
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Institution: | Department of Biology, Texas A&M University, College Station, TX 77843, USA. |
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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. |
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Keywords: | distant homology ancestral sequence reconstruction Bayesian phylogeny structural comparison threading |
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