XIX International Botanical Congress
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Phylogenetic analyses of other photosynthesis gene products (BchI, BchD, BchB, BchN, and BchM) are in agreement with the three representative trees shown in Fig. A–C (data not shown). Thus, one can conclude that photosynthesis genes from anoxygenic photosynthetic organisms are ancestral to those found in oxygenic photosynthetic organisms. Comparing phylogenetic trees of photosynthesis genes vs. nonphotosynthesis genes, however, has been complicated by the various versions of trees available in the literature (e.g., 16S rRNA; refs. , ), which further justifies the use of photosynthesis genes for tracking the molecular evolution of photosynthesis.
With the exception of the sequence analysis of PSI and PSII structural polypeptides, there is little information on the origin and evolution of photosynthesis. Complete sequence information on photosynthesis genes is only available for the purple nonsulfur bacterium, Rhodobacter capsulatus, which has a major clustering of photosynthesis genes (), and for the cyanobacterium Synechocystis sp. PCC 6803, for which sequence analysis has been completed for the entire chromosome. Information on photosynthesis genes from the three other eubacterial photosynthetic phyla is limited mainly to the reaction-center (RC) core polypeptides and a few cytochromes.
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H. mobilis photosynthesis genes were compared with homologous sequences in the GenBank database to study evolutionary relationships. Phylogenetic trees were generated from multiple aligned sequences by using the maximum-parsimony, neighbor-joining, and maximum-likelihood methods. As the results from the three methods are highly congruent, we shall mainly discuss results obtained from the neighbor-joining tree analysis. A BchL/ChlL phylogenetic tree was constructed by using the nitrogenase iron protein NifH from an archaeon species Methanococcus jannaschii as an outgroup (Fig. A). Fujita et al. () and Burke et al. () found remarkable similarity between NifH and BchL. NifH was also used previously as an outgroup for studying phylogenetic relationships among BchL homologs from bacteria and plants (). Its choice as an outgroup is based on the premise that nitrogen fixation occurs in both archaea and bacteria, whereas tetrapyrrole-based photosynthesis occurs only in bacteria and bacterially derived chloroplasts. Thus, unlike nitrogen fixation, photosynthesis may not predate the divergence of archaea and bacteria. A notable feature of the highly supported BchL tree is that oxygenic phototrophs together form a monophyletic group distinct from the anoxygenic phototrophs R. capsulatus and H. mobilis (Fig. A). The paraphyletic R. capsulatus and H. mobilis lineages are rooted closer to the base of the tree and form an outgroup to the oxygenic clade, suggesting that anoxygenic phototrophs are ancestral to the oxygenic phototrophs. This result is in agreement with the previous phylogenetic analysis by Burke et al. (), who concluded that bacteriochlorophyll biosynthesis evolved earlier than chlorophyllous biosynthesis. However, this conclusion was later challenged by Lockhart et al. () who argued for an alternative hypothesis. Another notable feature of the tree is that R. capsulatus is a deeper branching lineage than H. mobilis, suggesting its more primitive evolutionary status.
A DNA sequence has been obtained for a 35.6-kb genomic segment from Heliobacillus mobilis that contains a major cluster of photosynthesis genes. A total of 30 ORFs were identified, 20 of which encode enzymes for bacteriochlorophyll and carotenoid biosynthesis, reaction-center (RC) apoprotein, and cytochromes for cyclic electron transport. Donor side electron-transfer components to the RC include a putative RC-associated cytochrome c553 and a unique four-large-subunit cytochrome bc complex consisting of Rieske Fe-S protein (encoded by petC), cytochrome b6 (petB), subunit IV (petD), and a diheme cytochrome c (petX). Phylogenetic analysis of various photosynthesis gene products indicates a consistent grouping of oxygenic lineages that are distinct and descendent from anoxygenic lineages. In addition, H. mobilis was placed as the closest relative to cyanobacteria, which form a monophyletic origin to chloroplast-based photosynthetic lineages. The consensus of the photosynthesis gene trees also indicates that purple bacteria are the earliest emerging photosynthetic lineage. Our analysis also indicates that an ancient gene-duplication event giving rise to the paralogous bchI and bchD genes predates the divergence of all photosynthetic groups. In addition, our analysis of gene duplication of the photosystem I and photosystem II core polypeptides supports a “heterologous fusion model” for the origin and evolution of oxygenic photosynthesis.
General & Introductory Chemistry
Xiong J., W.M. Fischer, K. Inoue, M. Nakahara, C.E. Bauer, 2000. Molecular evidence for the early evolution of photosynthesis. Science 289:1724-1730.
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Kubiak Research Group
Statistical Techniques | Statistical Mechanics
Kubiak Research Group Department of Chemistry and Biochemistry Graduate Program in Materials Science & Engineering
A Cytochrome b Origin of Photosynthetic Reaction …
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