Comparison of Photosynthesis Gene Clusters Retrieved from Total Genome Sequences of Purple Bacteria

Studies on some purple photosynthetic bacteria have shown that the genes required for photosynthesis form a large cluster. This is often called a photosynthesis gene cluster (PGC) and has been described as a unique feature of purple photosynthetic bacteria. Here, photosynthesis genes were retrieved from the rapidly increasing genomic data of purple bacteria. Arrangements of these genes on the genomes were compared among 24 species and strains were selected from the wide distribution of alpha, beta, and gamma classes of purple bacteria. The presence of PGC was confirmed in the all of these, although it is divided into two or several islands in some species. Gene arrangements in several subclusters possibly forming an operon or a superoperon are well conserved while the directions and locations of these subclusters in the PGC vary according to the strains. Phylogenetic trees based on the sequences of the suspected products of the genes in the PGCs consistently place the species of the beta and gamma classes in a cluster within the branch of the alpha class, showing that a horizontal transfer of PGC had occurred in the process of evolution. Information obtained from the increasing genome projects on bacteria is very useful for the study on evolution of photosynthesis and even for its application in metabolic engineering.