Non-autonomous IS derivatives.
Many prokaryotic genomes are littered with IS
fragments and small, non-autonomous IS derivatives whose transposition can, in
principal, be catalyzed in trans by the Tpase of a related complete IS (Fig 1.13.1). Structures called MITEs (Miniature Inverted
repeat Transposable Elements) (Feschotte, et al., 2002) are related to IS
with DDE Tpases (Brugger, et al., 2002, Filee, et al.,
2007). Among those derived from IS with DDE transposases, representatives
of the IS1, IS4, IS5, IS6, and even Tn3 family members such as ISRf1 from Sinorhizobium fredii have been identified. They are
generally less than 300 bp long, include appropriately oriented left and right terminal
IR but no Tpase and generally generate flanking DR. MITEs are probably derived
from IS by internal deletion. Some carry short non-coding sequences between
these IRs which may or may not be IS-derived. MITEs are considered to be
non-autonomous transposable elements mobilisable in trans by Tpases of
full-length parental genomic copies. They were first identified in plants (Feschotte, et
al., 2002, Zhang, et al., 2004,
Feschotte, et al., 2005, Yang, et al., 2007) and are related
to Tc/mariner elements (distantly related to bacterial IS630 family). IS630-related
MITEs were also the first described bacterial examples (Correia, et al., 1988, Oggioni
& Claverys, 1999, Buisine, et al.,
2002). MITEs showing similarities to other IS families are observed in
bacteria and archaea.
MITE-like structures related to elements with
other types of transposase have also been identified. Among these are IS200/IS605 family derivatives (Filee, et al., 2007);(Siguier unpublished) (Fig 1.18.1) now called PATEs (Palindrome-associated Transposable Elements) (Dyall-Smith, et
al., 2011) reflecting the sub-terminal secondary structures which constitute the ends of
these IS. Although originally observed in the Archaea as derivatives of known
IS200/IS605 family members, they have also been observed in
certain cyanobacteria and Salmonella (Siguier unpublished). They presumably
represent decay products which appear quite frequently for this IS family.
Full-length copies of the parental IS may not
be available or may be so divergent as to escape detection in standard BLAST
analysis. This is the case for certain MITEs from the archaea (Brugger, et al.,
2002) and is probably also true for the bacteria. Their detection and
analysis is therefore arduous.
Another group of IS derivatives related to MITEs
are called MICs (Mobile Insertion Cassette) (Chen, et al., 1999) (Fig 1.13.1).
These, like MITEs, are flanked by IR, do not include a Tpase gene and generate
flanking DR. They carry various coding sequences, and are present in relatively
low copy number. The IS231 sub-group of the large IS4 family includes examples of many of these
IS-derivatives (canonical ISs, MITEs, MICs and tIS) (De Palmenaer, et al., 2004).
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