Insertion element (also known as an
IS, an
insertion sequence element, or an
IS element) is a short
DNA sequence that acts as a simple
transposon. Insertion sequences have two major characteristics: they are small relative to other transposable elements (generally around 700 to 2500
base pair 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).
A particular insertion sequence may be named according to the form IS n, where n is a natural 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.
Composition
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
. 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).
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).
A complex transposon does not rely on flanking insertion sequences for resolvase. The resolvase is part of the tns genome and cuts at flanking inverted repeats.
Evolutionary role
Although insertion sequences are usually discussed in the context of
prokaryote , certain
eukaryote DNA sequences belonging to the family of Tc1/
mariner transposable elements may be considered to be insertion sequences.
Transposition frequency of IS elements is dependent of multiple parameters, including culture growth phase, medium composition, oxygen tension, growth scale, and structural conformation of target sites (e.g.: curvature, presence of certain motifs, DNA composition).
See also
-
Campbell, Neil A. and Reece, Jane B. (2002). Biology (6th ed.), pp. 345–346. San Francisco: Benjamin Cummings. .
-
Prescott, Lansing M.; Harley, John P.; and Klein, Donald A. (2002). Microbiology (5th ed.), pp. 298–299. New York: McGraw-Hill. .
-
Shuler, Michael L. and Kargi, Fikret (2002). Bioprocess Engineering: Basic Concepts (2nd ed.), p. 220. Upper Saddle River, NJ: Prentice Hall PTR. .
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