IS Overview

The idea that many prokaryotic genomes are mosaic, composed of a "central genome backbone" of essential and house-keeping genes (the core genome) interspersed with DNA segments constituting the "mobilome" (a variety of accessory genes that form part of the pan genome)( Fig 1.1.1), is now common currency (Medini, et al., 2005, Tettelin, et al., 2008). The mobilome embraces several types of genetic unit which, as their collective name indicates, can move from place to place in a particular genome or from cell to cell. These mobile genetic elements (MGE) can be divided into two major groups: those, such as plasmids and bacteriophages, that are transmissible from cell to cell (the intercellular MGE), and those that cannot themselves undergo transfer but which are transferred following integration into members of the first group (the intracellular MGE). Intracellular MGE or transposable elements (TE) include transposons (Tn) and insertion sequences (IS) but can embrace integrons (In) (Escudero, et al., 2015) and introns (Craig, et al., 2002, Lambowitz & Belfort, 2015, Zimmerly & Semper, 2015, Zimmerly & Wu, 2015). Originally Tn were distinguished from IS since they carry passenger (also called cargo) genes not involved in catalysing or regulating TE movement. Most eukaryotic DNA transposons have relatives among the prokaryotic IS (see (Hickman, et al., 2010)) and it is not surprising that a variety of these elements carrying passenger genes are now also being identified (Bao, et al., 2009, Bao & Jurka, 2013). Prokaryotes harbour a host of such elements as well as several types of structure possessing characteristics of both groups (e.g. Integrative Conjugative Elements, ICE, originally called conjugative transposons, as well as other types of non-conjugative genomic islands) (Burrus & Waldor, 2004, Dobrindt, et al., 2004, Guerillot, et al., 2013).

References :

• Bao W & Jurka J (2013) Homologues of bacterial TnpB_IS605 are widespread in diverse eukaryotic transposable elements. Mob DNA 4: 12.
• Bao W, Jurka MG, Kapitonov VV & Jurka J (2009) New superfamilies of eukaryotic DNA transposons and their internal divisions. Mol Biol Evol 26: 983-993.
• Burrus V & Waldor MK (2004) Shaping bacterial genomes with integrative and conjugative elements. Res Microbiol 155: 376-386.
• justify;line-height:normal'>Craig NL, Craigie R, Gellert M & Lambowitz A (2002) Mobile DNA II. American Society of Microbiology, Washington.
• Dobrindt U, Hochhut B, Hentschel U & Hacker J (2004) Genomic islands in pathogenic and environmental microorganisms. Nat Rev Microbiol 2: 414-424.
• Escudero JA, Loot C, Nivina A & Mazel D (2015) The Integron: Adaptation On Demand. Microbiol Spectr 3: MDNA3-0019-2014.
• Guerillot R, Da Cunha V, Sauvage E, Bouchier C & Glaser P (2013) Modular evolution of TnGBSs, a new family of integrative and conjugative elements associating insertion sequence transposition, plasmid replication, and conjugation for their spreading. J Bacteriol 195: 1979-1990.
• Hickman AB, Chandler M & Dyda F (2010) Integrating prokaryotes and eukaryotes: DNA transposases in light of structure. Critical Reviews in Biochemistry and Molecular Biology 45: 50-69.
• Lambowitz AM & Belfort M (2015) Mobile Bacterial Group II Introns at the Crux of Eukaryotic Evolution. Microbiol Spectr 3: MDNA3-0050-2014.
• Medini D, Donati C, Tettelin H, Masignani V & Rappuoli R (2005) The microbial pan-genome. Curr Opin Genet Dev 15: 589-594.
• Tettelin H, Riley D, Cattuto C & Medini D (2008) Comparative genomics: the bacterial pan-genome. Curr Opin Microbiol 11: 472-477.
• Zimmerly S & Semper C (2015) Evolution of group II introns. Mob DNA 6: 7.
• Zimmerly S & Wu L (2015) An Unexplored Diversity of Reverse Transcriptases in Bacteria. Microbiol Spectr 3: MDNA3-0058-2014.