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.,
- 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:
- Burrus V &
Waldor MK (2004) Shaping bacterial genomes with integrative and conjugative
elements. Res Microbiol 155: 376-386.
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& 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:
- Zimmerly S & Wu L (2015) An Unexplored
Diversity of Reverse Transcriptases in Bacteria. Microbiol Spectr 3: