IS91 and ISCR
A final example of the subtle line dividing IS
and transposons is found in the IS91/ISCR group (Fig 1.17.1). IS91 was identified some
time ago (Diaz-Aroca, et al., 1984) and carries a single Tpase orf. More recently, a group of related
elements, ISCR (IS with a "Common Region") was
described [reviewed in (Toleman, et al., 2006, Toleman & Walsh,
2010)]. Although there has been no formal demonstration that these
actually transpose, the CR is an orf which resembles the IS91 family Tpases (Chandler, et al., 2013). The major feature of ISCR elements
is that they are associated with a diverse variety of antibiotic resistance
genes and, particularly in the case of Pseudomonas ISCR, aromatic degradation pathways, both upstream and downstream of
the Tpase orf.
It is thought that these genes are transmitted
during the rolling circle type of transposition mechanism postulated to occur
in IS91 transposition (Fig 1.17.2). This involves an initiation event
at one IS end, polarized transfer of the IS strand into a target molecule and
termination at the second end (Garcillan-Barcia, et al., 2002). However, the de
la Cruz lab has identified circular IS91 forms and it seems possible that these are transposition intermediates. An
alternative model involving transposon circle formation is shown in Fig 1.17.3. This model is attractive since it
would liberate the transposon circle intermediate to locating a target site
following circle formation rather than requiring target engagement during the
replicative transposition process (Fig. 1.17.2)
Flanking gene acquisition is thought to occur
when the termination mechanism fails and rolling circle transposition extends
into neighboring DNA where it may encounter a second surrogate end (Garcillan-Barcia,
et al., 2002). This type of mobile element may prove to play an
important role in the assembly and transmission of multiple antibiotic
resistance (Toleman, et al., 2006, Toleman & Walsh, 2011).
M, de la Cruz F, Dyda F, Hickman AB, Moncalian G & Ton-Hoang B (2013)
Breaking and joining single-stranded DNA: the HUH endonuclease superfamily. Nat Rev Microbiol 11: 525-538.
E, de la Cruz F, Zabala JC & Ortiz JM (1984) Characterization of the new
insertion sequence IS91 from an alpha- hemolysin plasmid of Escherichia coli. Mol.Gen.Genet. 193: 493-499.
- Garcillan-Barcia MP, Bernales
I, Mendiola MV & De la Cruz F (2002) IS91 rolling circle transposition. Mobile DNA, Vol. II (Craig NL, Craigie R, Gellert M & Lambowitz A, eds.), pp.
891-904. ASM press, Washington DC.
MA & Walsh TR (2010) ISCR elements are key players in IncA/C plasmid
evolution. Antimicrob Agents Chemother 54: 3534; author reply 3534.
MA & Walsh TR (2011) Combinatorial events of insertion sequences and ICE in
Gram-negative bacteria. FEMS Microbiol
Rev 35: 912-935.
- Toleman MA, Bennett PM & Walsh TR
(2006) ISCR Elements: Novel Gene-Capturing Systems of the 21st Century? Microbiol Mol Biol Rev 70: 296-316.