Origin and Evolution of Photosynthesis: Clues from Genome Comparison

Abstract

Fifteen complete cyanobacterial genome sequences were characterized revealing 1,054 protein families (Cyanobacterial clusters of Orthologous Groups of proteins, or CyOGs), encoded in at least 14 of them. While the majority of the core CyOGs were shared with other bacteria, 84 were exclusively shared by cyanobacteria and plants and/or other plastid-carrying eukaryotes that indicate their relation to the photosynthetic machinery (hereafter denoted as photosynthetic CyOGs). Only a few photosynthetic CyOGs find counterparts in the genomes of anoxygenic phototrophic bacteria Chlorobium tepidum, Rhodopseudomonas palustris, Chloroflexus aurantiacus or Heliobacillus mobilis. These observations, coupled with the recent geological data on the properties of the ancient phototrophs, suggest that photosynthesis has originated in the cyanobacterial lineage and spread to other phyla via horizontal gene transfer under the selective pressure of the UV light, availability of electron donors and, eventually, oxygen. We propose that the first phototrophs were anaerobic ancestors of cyanobacteria that conducted anoxygenic photosynthesis using photosystem I-type reaction center, similarly to the heterocysts of modern-day filamentous cyanobacteria.