title: Restriction-Modification Systems as Mobile Genetic Elements in the Evolution of an Intracellular Symbiont.
abstract: Long-term vertical transmission of intracellular bacteria causes massive genomic erosion and results in extremely small genomes, particularly in ancient symbionts. Genome reduction is typically preceded by the accumulation of pseudogenes and proliferation of mobile genetic elements, which are responsible for chromosome rearrangements during the initial stage of endosymbiosis. We compared the genomes of an endosymbiont of termite gut flagellates, "Candidatus Endomicrobium trichonymphae," and its free-living relative Endomicrobium proavitum and discovered many remnants of restriction-modification (R-M) systems that are consistently associated with genome rearrangements in the endosymbiont genome. The rearrangements include apparent insertions, transpositions, and the duplication of a genomic region; there was no evidence of transposon structures or other mobile elements. Our study reveals a so far unrecognized mechanism for genome rearrangements in intracellular symbionts and sheds new light on the general role of R-M systems in genome evolution.
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
authors.forename: Hao
authors.initials: H
authors.lastname: Zheng
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Dietrich
authors.affiliation: Department of Biological Sciences, Tokyo Institute of Technology, Tokyo, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Hongoh
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Terrestrial Microbiology, Marburg, Germany brune@mpi-marburg.mpg.de.
authors.forename: Andreas
authors.initials: A
authors.lastname: Brune
mesh_terms: D005075:Biological Evolution; D064113:CRISPR-Cas Systems; D015280:DNA Restriction-Modification Enzymes; D015321:Gene Rearrangement; D016678:Genome; D023281:Genomics; D020071:Interspersed Repetitive Sequences; D013559:Symbiosis / Q000235:genetics
chemical_list: D015280:DNA Restriction-Modification Enzymes
keywords: CRISPR-Cas system; endosymbiont; genome reduction; mobile genetic elements; restriction-modification system