Tracking molecular evolution of photosynthesis by characterization of a major photosynthesis gene cluster from Heliobacillus mobilis

A DNA sequence has been obtained for a 35.6-kb genomic segment from Helioba cillus mobilis that contains a major cluster of photosynthesis genes. A tot al of 30 ORFs were identified, 20 of which encode enzymes for bacteriochlor ophyll and carotenoid biosynthesis, reaction-center (RC) apoprotein, and cy tochromes for cyclic electron transport. Donor side electron transfer compo nents to the RC include a putative RC-associated cytochrome C-553 and a uni que four-large-subunit cytochrome be complex consisting of Rieske Fe-S prot ein (encoded by petC), cytochrome bs (petB), subunit IV (petD), and a dihem e cytochrome c (petX), Phylogenetic analysis of various photosynthesis gene products indicates a consistent grouping of oxygenic lineages that are dis tinct 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 p hotosynthesis gene trees also indicates that purple bacteria are the earlie st emerging photosynthetic lineage. Our analysis also indicates that an anc ient gene-duplication event giving rise to the paralogous bchI and bchD gen es predates the divergence of all photosynthetic groups. In addition, our a nalysis of gene duplication of the photosystem I and photosystem II core po lypeptides supports a "heterologous fusion model" for the origin and evolut ion of oxygenic photosynthesis.