An
insertion sequence (also known as an
IS, an
insertion sequence element, or an
IS element) is a short
DNA sequence that acts as a simple
transposable element. Insertion sequences have two major characteristics: they are small relative to other transposable elements (generally around 700 to 2500
bp in length) and only code for proteins implicated in the
transposition activity (they are thus different from other
transposons, which also carry accessory genes such as antibiotic resistance genes). These proteins are usually the
transposase which catalyses the enzymatic reaction allowing the IS to move, and also one regulatory protein which either stimulates or inhibits the transposition activity. The coding region in an insertion sequence is usually flanked by
inverted repeats. For example, the well-known IS
911 (1250 bp) is flanked by two 36bp inverted repeat extremities and the coding region has two genes partially overlapping
orfA and
orfAB, coding the transposase (OrfAB) and a regulatory protein (OrfA).
[1]A particular insertion sequence may be named according to the form IS
n, where
n is a
number (e.g. IS
1, IS
2, IS
3, IS
10, IS
50, IS
911, IS
26 etc.); this is not the only naming scheme used, however. Although insertion sequences are usually discussed in the context of
prokaryotic genomes, certain
eukaryotic DNA sequences belonging to the family of Tc1/
mariner transposable elements may be considered to be, insertion sequences.
[2] In addition to occurring autonomously, insertion sequences may also occur as parts of
composite transposons; in a composite transposon, two insertion sequences flank one or more accessory genes, such as an antibiotic resistance gene (e.g.
Tn10, Tn
5). Nevertheless, there exist another sort of transposons, called unit transposons, that do not carry insertion sequences at their extremities (e.g. Tn
7).