title: Recent Mobility of Casposons, Self-Synthesizing Transposons at the Origin of the CRISPR-Cas Immunity.
abstract: Casposons are a superfamily of putative self-synthesizing transposable elements that are predicted to employ a homolog of Cas1 protein as a recombinase and could have contributed to the origin of the CRISPR-Cas adaptive immunity systems in archaea and bacteria. Casposons remain uncharacterized experimentally, except for the recent demonstration of the integrase activity of the Cas1 homolog, and given their relative rarity in archaea and bacteria, original comparative genomic analysis has not provided direct indications of their mobility. Here, we report evidence of casposon mobility obtained by comparison of the genomes of 62 strains of the archaeon Methanosarcina mazei. In these genomes, casposons are variably inserted in three distinct sites indicative of multiple, recent gains, and losses. Some casposons are inserted into other mobile genetic elements that might provide vehicles for horizontal transfer of the casposons. Additionally, many M. mazei genomes contain previously undetected solo terminal inverted repeats that apparently are derived from casposons and could resemble intermediates in CRISPR evolution. We further demonstrate the sequence specificity of casposon insertion and note clear parallels with the adaptation mechanism of CRISPR-Cas. Finally, besides identifying additional representatives in each of the three originally defined families, we describe a new, fourth, family of casposons.
authors.affiliation: Unité Biologie Moléculaire Du Gène Chez Les Extrêmophiles, Department of Microbiology, Institut Pasteur, Paris, France krupovic@pasteur.fr.
authors.forename: Mart
authors.initials: M
authors.lastname: Krupovic
authors.affiliation: National Library of Medicine, National Center for Biotechnology Information, National Institutes of Health, Bethesda, Maryland Skolkovo Institute of Science and Technology, Skolkovo, Russia.
authors.forename: Sergey
authors.initials: S
authors.lastname: Shmakov
authors.affiliation: National Library of Medicine, National Center for Biotechnology Information, National Institutes of Health, Bethesda, Maryland.
authors.forename: Kira S
authors.initials: KS
authors.lastname: Makarova
authors.affiliation: Unité Biologie Moléculaire Du Gène Chez Les Extrêmophiles, Department of Microbiology, Institut Pasteur, Paris, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Forterre
authors.affiliation: National Library of Medicine, National Center for Biotechnology Information, National Institutes of Health, Bethesda, Maryland.
authors.forename: Eugene V
authors.initials: EV
authors.lastname: Koonin
mesh_terms: D019843:Archaeal Proteins / Q000235:genetics*; D001483:Base Sequence; D064112:Clustered Regularly Interspaced Short Palindromic Repeats; D004251:DNA Transposable Elements / Q000235:genetics*; D004706:Endodeoxyribonucleases / Q000235:genetics*; D022761:Gene Transfer, Horizontal; D020745:Genome, Archaeal; D017021:Methanosarcina barkeri / Q000145:classification / Q000235:genetics; D008969:Molecular Sequence Data; D010802:Phylogeny
chemical_list: D019843:Archaeal Proteins; D004251:DNA Transposable Elements; D004706:Endodeoxyribonucleases
keywords: CRISPR-Cas; casposons; mobile genetic elements; self-synthesizing transposons; transposition