Chlamydiae are the major cause of preventable blindness and sexually transmitted disease. Genome analysis of a chlamydia-related symbiont of free-living amoebae revealed that it is twice as large as any of the pathogenic chlamydiae and had few signs of recent lateral gene acquisition. We showed that about 700 million years ago the last common ancestor of pathogenic and symbiotic chlamydiae was already adapted to intracellular survival in early eukaryotes and contained many virulence factors found in modern pathogenic chlamydiae, including a type III secretion system. Ancient chlamydiae appear to be the originators of mechanisms for the exploitation of eukaryotic cells.

ATP/ADP translocases catalyze the highly specific transport of ATP across a membrane in an exchange mode with ADP. Such unique transport proteins are employed by plant plastids and have among the prokaryotes so far only been identified in few obligate intracellular bacteria belonging to the Chlamydiales and the Rickettsiales. In this study, 12 phylogenetically diverse bacterial endosymbionts of free-living amoebae and paramecia were screened for the presence of genes encoding ATP/ADP transport proteins. The occurrence of ATP/ADP translocase genes was found to be restricted to endosymbionts related to rickettsiae and chlamydiae. We showed that the ATP/ADP transport protein of the Parachlamydia-related endosymbiont of Acanthamoeba sp. strain UWE25, a recently identified relative of the important human pathogens Chlamydia trachomatis and Chlamydophila pneumoniae, is functional when expressed in the heterologous host Escherichia coli and demonstrated the presence of transcripts during the chlamydial developmental cycle. These findings indicate that the interaction between Parachlamydia-related endosymbionts and their amoeba hosts concerns energy parasitism similar to the interaction between pathogenic chlamydiae and their human host cells. Phylogenetic analysis of all known ATP/ADP translocases indicated that the genes encoding ATP/ADP translocases originated from a chlamydial ancestor and were, after an ancient gene duplication, transferred horizontally to rickettsiae and plants.Obligate intracellular bacteria live within a highly specialized niche, the eukaryotic cell. This lifestyle gave rise to unique adaptations, for example, the reduction of bacterial metabolism (37) and the exploitation of host metabolites (36). Chlamydiae and rickettsiae (comprising major bacterial pathogens of humans and animals) have evolved a remarkable adaptation which enables them to utilize ATP generated by their eukaryotic hosts by making use of specific carrier proteins (49). These transmembrane proteins catalyze the import of host-derived ATP into the prokaryotic cell across the bacterial cell membrane, which is otherwise impermeable for the relatively large and charged nucleotides. In an exchange mode, these transport proteins export bacterial ADP back into the host cytosol.In total, four bacterial ATP/ADP transport proteins have been functionally characterized to date, including those of Chlamydia trachomatis (43), Rickettsia prowazekii (11, 48), and the Rickettsia-related paramecium parasites Holospora obtusa and Caedibacter caryophilus (28). Bacterial ATP/ADP transport proteins are highly specific for their substrates (ATP and ADP). They belong to the family of solute transporters exhibiting 12 predicted transmembrane helices and display several highly conserved motifs while sharing only moderate amino acid sequence similarity (34, 49). Interestingly, transport proteins exhibiting structural and functional features similar to bacterial ATP/ADP transport proteins have been identified and characterized from chloroplasts and hetero...

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