Genome reduction in prokaryotic obligatory intracellular parasites of humans: a comparative analysis

Obligatory intracellular parasites have undergone significant genome reduction by gene loss over time in the context of their obligate associations with the host. The flux, streamlining and elimination of genes in these genomes constitute a selective and ongoing process. Comparative analyses of five completely sequenced obligatory intracellular parasite genomes reveal that these genomes display marked similarities in patterns of protein length and frequency distribution, with substantial sharing of a 'backbone genome'. From category distribution based on the database of cluster of orthologous groups of proteins (COG), it is clear that habitat is a major factor contributing to genome reduction. It is also observed that, in all five obligatory intracellular parasites, the reduction in number of genes/proteins is greater for proteins with lengths of 200-600 amino acids. These comparative analyses highlight that gene loss is function-dependent, but is independent of protein length. These comparisons enhance our knowledge of the forces that drive the extreme specialization of the bacteria and their association with the host.