Functional Genomics of Anoxygenic Green Bacteria Chloroflexi Species and Evolution of Photosynthesis

In addition to the most recently reported aerobic anoxygenic phototrophic bacterium Chloroacidobacterium thermophilium [1], five phyla of phototrophic bacteria have been reported, including four phyla anoxygenic phototrophic bacteria (anaerobic and aerobic anoxygenic phototrophic Proteobacteria, filamentous anoxygenic phototrophs (FAPs), green sulfur bacteria and heliobacteria) and oxygenic phototrophic bacteria (cyanobacteria). According to 16S rRNA analysis, Chloroflexi species in FAPs are the earliest branching bacteria capable of photosynthesis [2,3] (Fig. 1), and the thermophilic bacterium Chloroflexus [Cfl.] aurantiacus among the Chloroflexi species has been long regarded as a key organism to resolve the obscurity of the origin and early evolution of photosynthesis. Cfl. aurantiacus can grow phototrophically under anaerobic conditions or chemotrophically under aerobic and dark conditions [4]. During phototrophic growth of Cfl. aurantiacus, the light energy is first absorbed by the peripheral light-harvesting complex chlorosomes, then transferred to the integral membrane B808-866 core antenna complex and finally to the reaction center (RC). Cfl. aurantiacus contains a chimeric photosystem that comprises some characters of green sulfur bacteria (chlorosomes) and anoxygenic phototrophic Proteobacteria (the B808-866 core antenna complex), and also has some unique electron transport proteins compared to other photosynthetic bacteria. The complete genomic sequence of Cfl. aurantiacus has been recently determined, analyzed and compared to the genomes of other photosynthetic bacteria [5].