Mechanism of Transposition

It has been shown that transposons have genes coding for an enzyme called transposase which cleaves off DNA segment of transposon by recognizing the reverse repeats. In a transposon Tn3, a gene transposase has been recognized along with a regulator gene synthesizing a repressor. The repressor regulates the synthesis of transposase and its own synthesis (Fig. 15.21).

It is also speculated now that the multiple drug resistance of plasmids has evolved due to collection of transposons, each carrying a gene for resistance against one or more antibiotics (Fig. 15.22). Since transposon like Tn3 can move among quite unrelated plasmids or bacteria, identical transposons can be found in different bacterial species.

In addition to their ability to join unrelated DNA segments, the transposable elements can also promote rearrangement and deletion of genetic material. In this category, we include a phage called Mu or mutator, which has a property of inserting at multiple sites in host bacterium thus causing different kinds of mutations. But Mu can also exist an an infectious virus. In the virus particle, Mu DNA is sandwiched between two short segments of bacterial DNA, which Mu picked up from bacterial chromosome. When Mu infects new cell, it sheds old bacterial DNA and is transposed to a site in the new host chromosome. It has also been shown that Mu can catalyse a remarkable series of chromosome rearrangements like: (i) fusion of two DNA molecules e.g. bacterial DNA and phage DNA, (ii) deletion of an adjoining DNA segment, (iii) inversion in adjoining region, and (iv) transposition of a segment of bacterial DNA to plasmid.

Through a detailed study of nucleotide sequence of insertion sequence element IS1, it could also be shown that insertion of the element is accompanied with duplication of a sequence 5, 7, 9 or 11 nucleotides long, on either side of insertion sequences. The origin of these repeats is explained to be due to staggered cleavage, which means that the two strands of DNA are cleaved off at different positions. Cleavage sites are separated by 5, 7, 9 or 11 nucleotides, generating single stranded regions which synthesize short single strands of complementary sequence, thus resulting in duplication (Fig. 15.22 and Fig. 15.23). It is through this process that in many chromosomal rearrangements, two copies of Mu are found on either side of a sequence. This method of illegitimate recombination is different from illegitimate recombination involving integration of phage lambda (λ)DNA into bacterial chromosome. It suggests that several systems of illegitimate recombination may actually exist. There is also some evidence that movement of controlling elements in maize from one chromosomal site to another may be brought about by mechanism different from that of transposition in bacteria.

There are several questions about the mechanism of transposition, which need to be answered in future. These questions include the following:

1. what is the mechanism of recognition of the inverted repeat ends of transposable genetic elements?
2. what proteins other than those coded by the transposon are needed for transposition?
3. what are the additional genetic aspects of the regulation of transposition?

The insertion of a transposable genetic element in one orientation will switch off a specific gene and switch on another gene. A reverse effect may be - observed due to insertion in a different orientation. It is in this respect that transposons and other transposable genetic elements in bacteria and higher organisms control gene regulation.