Nature Reviews Microbiology - Reviews: "The small genomes of obligate intracellular bacteria are often presumed to be impervious to mobile DNA and the fluid genetic processes that drive diversification in free-living bacteria. Categorized by reductive evolution and streamlining, the genomes of some obligate intracellular bacteria manifest striking degrees of stability and gene synteny. However, recent findings from complete genome sequences of obligate intracellular species and their mobile genetic associates favour the abandonment of these wholesale terms for a more complex and tantalizing picture.

Mobile genetic elements are a common feature of most prokaryotes. Specific examples include plasmids, bacteriophages, DNA transposable elements and RNA transposable elements. Some prophages can occupy up to 20% of the host bacterial chromosome and comprise the majority of strain-specific DNA in genome comparisons.

Facultative intracellular bacteria have a four-fold higher genomic content of mobile DNA than obligate intracellular bacteria. This disparity is probably explained by accelerated deletion rates, intracellular confinement and the deleterious effects of mobile DNA in obligate intracellular species.

DNA transposable elements are the most abundant group of mobile elements in intracellular bacteria. They also preferentially suffer from inactivating mutations compared with other genetic parasites. Inserted plasmid genes are uncommon. Extrachromosomal plasmids might be more common in mutualistic endosymbionts and aid the primary functions of dietary endosymbionts in insects.

The 'intracellular arena' hypothesis posits that eukaryotic cells that are co-infected by multiple intracellular bacteria will elevate contact with novel gene pools and increase rates of mobile-DNA transfer between intracellular bacteria. Consistent with this hypothesis are the recent inferences of lateral phage transfer between co-infecting Wolbachia endosymbionts.

Increasing discoveries of mobile genetic elements in obligate intracellular species should motivate new insights and the development of tractable hypotheses, such as whether transmission differences among intracellular bacteria generally predict distinct outcomes for mobile-DNA content."

Seth R. Bordenstein & William S. Reznikoff MOBILE DNA IN OBLIGATE INTRACELLULAR BACTERIA Nature Reviews Microbiology 3, 688-699 (2005); doi:10.1038/nrmicro1233

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