Molecular characterization of various microbial genomes has revealed that many pathogenic and mutualistic intracellular bacterial species have smaller genomes than their free-living relatives. The flux, streamlining and elimination of genes in these genomes constitute a selective and ongoing process. Obligatory intracellular parasites display marked similarities in patterns of protein length and frequency distribution, with substantial sharing of a ‘backbone genome’. The results highlight that gene loss is function-dependent, but is independent of protein length. It is suggested that obligate genomes have greater proportion of overlapping genes, which may be a result of evolutionary pressure to minimize genome size. The differential loss of genes in these organisms is also reflected in their metabolic plasticity. Here, we present a review on gene loss and gene conservation in obligatory intracellular parasites. These data are essential for understanding of genome evolution and microbial pathogenesis of these organisms.
Keywords: Obligate intracellular parasites, genome reduction, backbone genome, essential genes, comparative genomics
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