Transposase

Transposase Tn5 dimerisation domain
tn5 transposase: 20mer outside end 2 mn complex
Identifiers
Symbol	Dimer_Tnp_Tn5
Pfam	PF02281
InterPro	IPR003201
SCOP	1b7e
SUPERFAMILY	1b7e
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Transposase is an enzyme that binds to the end of a transposon and catalyzes the movement of the transposon to another part of the genome by a cut and paste mechanism or a replicative transposition mechanism. The word "transposase" was first coined by the individuals who cloned the enzyme required for transposition of the Tn3 transposon. The existence of transposons was postulated in the late 1940s by , who was studying the inheritance of maize, but the actual molecular basis for transposition was described by later groups. McClintock discovered that pieces of the chromosomes changed their position, jumping from one chromosome to another. The repositioning of these transposons (which coded for color) allowed other genes for pigment to be expressed. Transposition in maize causes changes in color; however, in other organisms, such as bacteria, it can cause antibiotic resistance. Transposition is also important in creating genetic diversity within species and adaptability to changing living conditions. During the course of human evolution, as much as 40% of the human genome has moved around via methods such as transposition of transposons.

Transposases are classified under EC number EC 2.7.7.

Genes encoding transposases are widespread in the genomes of most organisms and are the most abundant genes known.

Transposase (Tnp) Tn5 is a member of the RNase superfamily of proteins which includes retroviral integrases. Tn5 can be found in Shewanella and Escherichia bacteria. The transposon codes for antibiotic resistance to kanamycin and other aminoglycoside antibiotics.

Tn5 and other transposases are notably inactive. Because DNA transposition events are inherently mutagenic, the low activity of transposases is necessary to reduce the risk of causing a fatal mutation in the host, and thus eliminating the transposable element. One of the reasons Tn5 is so unreactive is because the N- and C-termini are located in relatively close proximity to one another and tend to inhibit each other. This was elucidated by the characterization of several mutations which resulted in hyperactive forms of transposases. One such mutation, L372P, is a mutation of amino acid 372 in the Tn5 transposase. This amino acid is generally a leucine residue in the middle of an alpha helix. When this leucine is replaced with a proline residue the alpha helix is broken, introducing a conformational change to the C-Terminal domain, separating it from the N-Terminal domain enough to promote higher activity of the protein. The transposition of a transposon often needs only three pieces: the transposon, the transposase enzyme, and the target DNA for the insertion of the transposon. This is the case with Tn5, which uses a cut-and-paste mechanism for moving around transposons.