Evidence that plant-like genes in Chlamydia species reflect an ancestral relationship between Chlamydiaceae, cyanobacteria, and the chloroplast.

An unusually high proportion of proteins encoded in Chlamydia genomes are most similar to plant proteins, leading to proposals that a Chlamydia ancestor obtained genes from a plant or plant-like host organism by horizontal gene transfer. However, during an analysis of bacterial-eukaryotic protein similarities, we found that the vast majority of plant-like sequences in Chlamydia are most similar to plant proteins that are targeted to the chloroplast, an organelle derived from a cyanobacterium. We present further evidence suggesting that plant-like genes in Chlamydia, and other Chlamydiaceae, are likely a reflection of an unappreciated evolutionary relationship between the Chlamydiaceae and the cyanobacteria-chloroplast lineage. Further analyses of bacterial and eukaryotic genomes indicates the importance of evaluating organellar ancestry of eukaryotic proteins when identifying bacteria-eukaryote homologs or horizontal gene transfer and supports the proposal that Chlamydiaceae, which are obligate intracellular bacterial pathogens of animals, are not likely exchanging DNA with their hosts.