Transposon is another word for a transposable element—that is, a DNA segment that can move between nonhomologous positions in a genome via a type of recombination called transposition. A transposable element generally encodes a recombinase, a transposase that executes the DNA breakage and joining reactions that underlie transposition, by acting on special recombination sequences, inverted terminal repeats, at the ends of the element. Transposable elements are present in Eubacteria, Archea, and Eucaryotes—indeed, in virtually every organism that has been examined. The term "transposon" is often, but certainly not exclusively, used in conjunction with bacterial transposable elements that encode, in addition to their transposition functions, other determinants such as antibiotic-resistance genes. The structure of a number of bacterial transposons comprises special recombination sequences up to perhaps 150 bp in length at the termini of the element, and the interior of the element encodes a transposase (and possibly other transposition proteins) and a gene (s) for another determinant, for example, an antibiotic-resistance gene. Small elements that lack determinants other than transposase are usually called insertion sequences.
Some transposons are actually composite elements in which two insertion sequence (IS) elements flank a determinant such as an antibiotic-resistance gene. One IS element may move independently of the other, and both IS elements can collaborate to move the entire IS-drug resistance-IS segment. For example, the transposon Tn10 is composed to two IS10 elements flanking a segment encoding a tetracycline-resistance determinant. The transposon Tn5 consists of 2 IS50 elements flanking a kanamycin-resistance determinant. Transposase supplied by both (or sometimes one) of the IS elements acts on the outside ends of the composite element to move them from place to place.
Despite the custom of using the terms insertion sequences and transposons to describe certain bacterial elements, it is important to remember that both types of DNA segments are actually transposable elements, highly related to many eukaryotic transposable elements in structure and transposition mechanism. However, some types of transposable elements have, to date, been identified in bacteria. For example, composite transposons with two internal individually transposable elements (IS elements) have not been identified to date in eukaryotes. Furthermore, elements that execute replicative transposition such as elements bacteriophage Mu and Tn3 have also not been identified in eukaryotes.
