Major IS groups are defined by the type of the transposase they use
The principal factor in
IS classification is the similarity, at the primary sequence level, of the
enzymes which catalyse their movement, their transposases (Tpases) ( Table
1). In addition, a
variety of characteristics are also taken into account. These include: the
length and sequence of the short imperfect terminal inverted repeat sequences
(IRs) carried by many ISs at their ends (TIRs or ITRs in eukaryotes); the
length and sequence of the short flanking direct target DNA repeats (DRs) (TSD, Target Site Duplication, in eukaryotes) often generated on
insertion; the organisation of their open reading frames or the target sequences
into which they insert (Mahillon and Chandler
IS and some transposons
can also be divided into two major types based on the chemistry used in
breaking and re-joining DNA during TE displacement: the DDE (and DEDD) and HUH
enzymes. Additional types of transposase enzymes have been identified (Fig 1.7.1) but are generally associated with
other types of transposon rather than IS.
A relatively new type of
potential transposase, Cas1, is associated with so-called casposons, elements
that may resemble complex IS and are related to CRISPRs.
J. and M. Chandler (1998). "Insertion sequences." Microbiol Mol
Biol Rev 62(3): 725-774.