Transposable Elements

Transposable Elements

As more and more genomes have been sequenced and annotated, it has become increasingly apparent that the genomes of all organisms are rife with genetic elements that are able to move into and out of the genomes where they reside. Transposable elements are often referred to as ” jumping genes,” mobile genetic elements , and transposable elements. Transposition refers to the movement of a mobile genetic element.

  • Mobile genetic elements were first discovered in the 1940s by Barbara McClintock (1902-1992) during her studies on maize genetics (a discovery for which she was awarded the Nobel Prize in 1983).
  • As more and more types of mobile genetic elements were discovered, new nomenclature was developed to distinguish one type from another.
  • In 2008 a group of prominent scientists studying bacterial and archaeal mobile genetic elements proposed a new definition for transposable elements: Specific DNA segments that can repeatedly insert into one or more sites or into one or more genomes.
  • This new definition encompasses elements that differ in structure, mechanisms of integration and excision, target sites, and ability to be transferred from one cell to another by HGT.

Some Types of Transposable Elements

Transposable Element Description
Insertion sequences (IS) Small genetic elements consisting of a transposase gene bracketed by inverted repeats
Composite transposons Contain genes unrelated to transposition bounded at each end by IS elements, which supply the transposase activity needed for transposition.
Unit transposons Contain one or more genes encoding enzymes needed for transposition, as well as other genes unrelated to transposition (e.g., antibiotic-resistance genes). They are not associated with an IS element.
Integrative conjugative elements In addition to transposition functions, contain genes for conjugative transfer to a new host cell and other genes (e.g., antibiotic-resistance genes).
  • The enzymes that function in transposition are collectively termed recombinases.
  • The recombinase used by a specific mobile genetic element may be called an integrase, resolvase, or transposase.
  • There is tremendous interest in these enzymes because it has been discovered that their mechanisms of action are similar to those used to rearrange the gene segments that encode important immune system proteins such as antibodies and T-cell receptors.
  • The simplest mobile genetic elements in bacteria are insertion sequences, or IS elements for short.
  • An IS element is just a DNA sequence (around 750 to 1,600 base pairs (bp) in length). It contains only the gene for the enzyme transposase, and it is surrounded at each ends by inverted repeats-identical or extremely same sequences of nucleotides in reversed orientation.
Insertion sequence
Insertion sequences (IS) consist only of IRs on either side of the transposase gene.
  • Inverted repeats are normally about 15 to 25 base pairs long and vary among IS elements so that each type of IS has its own characteristic inverted repeats.
  • Transposase is necessary for transposition and correctly identifies the ends of the IS.
  • Each IS element is named by giving it the prefix IS followed by a number. IS elements have been observed in a variety of bacteria and some archaea.
  • Transposons are more complex in structure than IS elements.
  • Some transposons (composite transposons; are composed of a central region containing genes unrelated to transposition (e.g., antibiotic-resistance genes) flanked on each sides by IS elements that are similar or very similar in sequence.

Composite transposon

  • The flanking IS elements encode the transposase used by the transposon to move. Other transposons lack IS elements and encode their own transposition enzymes.
  • Most transposon names start with the prefix Tn.
  • Some transposons bear transfer genes and can easily move between bacteria during the process of conjugation, They are called conjugative transposons or integrative conjugative elements (ICEs).
Unit transposon
Unit transposons contain additional genes (e.g., antibiotic-resistance genes) in addition to the recombinases that enable them to transpose. In composite transposons, the additional genes are flanked by insertion sequences, which supply the transposase. Unit transposons are not associated with insertion sequences. DRs, direct repeats in host DNA, flank a transposable element.
  • A well-studied example of an ICE is Tn916 from Enterococcus faecalis.
  • Although Tn916 cannot replicate autonomously, it can transfer itself from E. faecalis to a various of recipients and combine into their chromosomes. ICEs are primarily observed in Gram-positive bacteria.

Two major transposition methods have been identified:

  1. Simple transposition
  2. Replicative transposition

Comparison of Simple and Replicative Transposition

  • Simple transposition is also called cut and paste transposition. In this method, transposase catalyzes excision of the transposable element, followed by cleavage of a new target site and ligation of the element into this site.
  • Target sites are specific sequences about five to nine base pairs long. When a mobile genetic element inserts at a target site, the target sequence is copied so that short, direct-sequence repeats flank the element’s terminal inverted repeats.
Simple Transposition
Transposable element; IR, Inverted repeat
  • Replicative transposition, the original transposon remains at the parental site on the chromosome and a copy is inserted at the target DNA site.
  • Transposable elements are of interest for many reasons. Their movement into a chromosome can change gene function either by resulting mutations in the genes into which they move or by providing promoters that are regulated another way than the normal promoter.
  • They contribute to the evolution of an organism’s chromosome, plasmids, and other mobile genetic elements of particular note is the role of mobile genetic elements that are genetic the spread of antibiotic resistance.

Reference and Sources


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