Transpositional Recombination

Transpositional recombination is a process in which a mobile element is inserted into a target DNA.  It may occur by one of two mechanisms: (1) directly as DNA, (2) through RNA.  The mobile elements that transpose through DNA are called transposons and those via RNA are referred to as  retrotransposons.  Some examples are given in the following figure.

Figure 8-D-9.  Classification of mobile elements.

Mechanisms of transpositional recombination

Figure 8-D-8.  Mechanisms of transpositional recombination.  
(a) For transposons.  The transposon in donor DNA is cut by a special enzyme and then inserted into a target DNA.  In bacteria, this enzyme is called transposase which has both nuclease and ligase activities.
(b) For retrotransposons.  The retrotransposon in donor DNA is first transcribed into RNA and then reverse-transcribed into DNA, which is inserted into a target DNA by the same recombination mechanism as the DNA intermediates of transposons.

The transpositional recombination as shown above involves DNA recombination whose mechanism is explained in the following figure.

Figure 8-D-10.  Mechanism of DNA recombination in transposition.

In bacteria, the target DNA is cut by transposase, producing sticky ends (single strands, easy to pair with complementary sequences).  The transposase also has ligase activity which may ligate the intermediate DNA of transposons.  The gaps at sticky ends are filled by DNA polymerase, generating direct repeats.

Other organisms are expected to use the same mechanism to insert the DNA intermediates of either transposons or retrotransposons into target DNA.  The direct repeats generated by this mechanism is found in ALL mobile elements.

In recombinant DNA technology, a DNA fragment is inserted into a target DNA by a slightly different mechanism, which is explained in Chapter 9.

Review Article:

Mammalian Retroelements - Genome Research, 2002.