Transposition
Transposition, also sometimes referred to as translocation, is a process by which segments of a chromosome are relocated through the genome. Commonly, this process involves "transposable elements" [TEs] or "transposons", but can also occur with genes. Gene transposition may involve the machinery of transposons, including target DNA sequences used by the transposon translocation machinery. Transposition can be either "replicative" or "conservative" in nature. In replicative transposition, the transposable element is copied and and the copy is relocated, leaving behind the original. This results in duplicate copies of the TE existing within the genome. In conservative transposition, the transposable element is excised from its original location then reinserted at a new location.[1] Because the excision of TEs is not always perfect, adjacent sequences can be removed and relocated as well. This process is know as "exon shuffling", and is one important method by which organisms develop new gene products.[2]
Two classes of Transposable Elements[2]
1. Class 1 Transposons - "Retrotransposons"
- Retrotransposons move via RNA intermediaries. The TE is copied to an RNA transcript, which is then converted back to DNA via reverse transcriptase. The resulting DNA is then inserted back into the genome.
2. Class 2 Transposons - "DNA Transposons"
- DNA Transposons encode the protein "transposase", which is responsible for removing and inserting the TE from chromosomes. Thus, Class 2 TEs do not require an RNA intermediary for the transposition process. To help transposase identify the ends of the TE, each end is marked with a "terminal inverted repeat".
Autonomous vs Non-autonomous Transposons[2]
- Both Class 1 and Class 2 TEs can be classified as either "autonomous" or "non-atonomous"
- Autonomous TEs contain coding for all necessary proteins for moving, and are thus capable of moving independently.
- Non-atonomous TEs lack either transposase or reverse transcriptase, and thus to move must borrow the necessary cellular machinery from another TE in order to relocate.
Silencing of Transposons[2]
Because transposons can damage genomes by interfering with essential gene expression, silencing of transposons is extremely common. In fact, the vast majority of transposons are effectively silenced and no longer multiply or move throughout the genome. Multiple methods for silencing exist, with the most well known being:
- Silencing due to mutation - mutations have rendered the TE incapable of transposing
- Epigenetic silencing - Epigenetic defense mechanisms prevent intact TEs from transposing.
- siRNA silencing - Some TEs silence their own activity through small interfering RNAs (siRNAs)
Alternate Terminology
Transposition can also refer to the process in which genes are transferred between organisms, but not through reproduction. This process is also known as "Horizontal Gene Transfer" or "Lateral Gene Transfer", and is most notable among prokaryotes but also exists in eukaryotes.[3]
[1] Griffiths AJF, Gelbart WM, Miller JH, et al. Modern Genetic Analysis. New York: W. H. Freeman; 1999. Mechanism of Transposition.
[2] Pray, L. (2008). Transposons: The jumping genes. Nature Education 1(1): 204.
[3] Andersson, JO. (2005). Lateral gene transfer in eukaryotes. Cell Mol Life Sci 62(11): 1182-97
