Evolutionary Relationships Among Antenna Proteins of Purple Phototrophic Bacteria

Abstract

A core, or LH1, antenna complex is present in all purple phototrophicbacteria, whereas peripheral, or LH2, antenna complexes are found in themembranes of many, but not all purple phototrophic bacteria. Thepresence of an LH2 complex is thought to provide a phototrophic growthcompetitive advantage under low-intensity illumination. Some specieshave only one type of LH2 complex, whereas other species have two ormore types of LH2 complex, giving rise to a variety of absorptionspectrum profiles. The great diversity of LH2 genes, sometimes within asingle bacterial strain, illustrates a gene family consisting of manyindividuals that have arisen over long evolutionary time scales. We usesequence alignments to infer evolutionary relationships of LH2 proteins,and compare phylogenetic trees to a 16S DNA-based tree to evaluate therelative extent of vertical and horizontal transmission of LH2 genes. Aphylogenetic tree of LH1 and LH2 proteins indicates that they arederived from a common ancestor. The LH2 genes of Phaeospirillummolischianum appear to have evolved independently of the LH2 genes ofother purple phototrophic bacteria.