Transposable Element - Classification

Classification

Transposable elements are only one of several types of mobile genetic elements. They are assigned to one of two classes according to their mechanism of transposition, which can be described as either "copy and paste" (for class I TEs) or "cut and paste" (for class II TEs).

Class I (retrotransposons): They copy themselves in two stages, first from DNA to RNA by transcription, then from RNA back to DNA by reverse transcription. The DNA copy is then inserted into the genome in a new position. Reverse transcription is catalyzed by a reverse transcriptase, which is often coded by the TE itself. Retrotransposons behave very similarly to retroviruses, such as HIV.

There are three main orders of retrotransposons (other orders are less abundant):

• Those with long terminal repeats (LTRs): encode reverse transcriptase, similar to retroviruses;
• LINEs: encode reverse transcriptase, lack LTRs, transcribed by RNA polymerase II;
• SINEs: do not code for reverse transcriptase, transcribed by RNA polymerase III.

Retroviruses can be considered as TEs. Indeed, after entering a host cell and converting their RNA into DNA, retroviruses integrate this DNA into the DNA of the host cell. The integrated DNA form (provirus) of the retrovirus is viewed as a particularly specialized form of eukaryotic retrotransposon, which is able to encode RNA intermediates that usually can leave the host cells and infect other cells. The transposition cycle of retroviruses also has similarities to that of prokaryotic TEs. The similarities suggest a distant familial relationship between these two TEs types.

Class II (DNA transposons): By contrast, the cut-and-paste transposition mechanism of class II TEs does not involve an RNA intermediate. The transpositions are catalyzed by various types of transposase enzymes. Some transposases can bind non-specifically to any target site, while others bind to specific sequence targets. The transposase makes a staggered cut at the target site producing sticky ends, cuts out the DNA transposon and ligates it into the target site. A DNA polymerase fills in the resulting gaps from the sticky ends and DNA ligase closes the sugar-phosphate backbone. This results in target site duplication and the insertion sites of DNA transposons may be identified by short direct repeats (a staggered cut in the target DNA filled by DNA polymerase) followed by inverted repeats (which are important for the TE excision by transposase). The duplications at the target site can result in gene duplication, which plays an important role in evolution.

Not all DNA transposons transpose through a cut-and-paste mechanism. In some cases a replicative transposition is observed in which transposon replicates itself to a new target site (e.g. Helitron (biology)).

Cut-and-paste TEs may be duplicated if transposition takes place during S phase of the cell cycle when the "donor" site has already been replicated, but the "target" site has not.

Both classes of TEs may lose their ability to synthesise reverse transcriptase or transposase through mutation, yet continue to jump through the genome because other TEs are still producing the necessary enzymes. Hence, they can be classified as either "autonomous" or "non-autonomous". For instance for the class II TEs, the autonomous ones have an intact gene that encodes an active transposase enzyme; the TE does not need another source of transposase for its transposition. In contrast, non-autonomous elements encode defective polypeptides and accordingly require transposase from another source. When a TE is used as a genetic tool, the transposase is supplied by the investigator, often from an expression cassette within a plasmid.