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 1998).

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.

References :

• Mahillon, J. and M. Chandler (1998). "Insertion sequences." Microbiol Mol Biol Rev 62(3): 725-774.