Transposition is the movement of a particular fragment of DNA from one part of a genome to another. A transposon is a segment of DNA which is capable of moving from a specific location on a DNA molecule to another location on the same or different molecule. For this reason, it is known as a “jumping element”. The recombination that takes place involves two unrelated sequences. This is unlike other homologous recombination events such as crossovers in meiosis and in transposition; there is a completely new arrangement of genes along the chromosome. A transposon could contain antibiotic resistance genes so that when it inserts itself into its target, it could confer resistance to the host. Transposons have therefore aided the development of plasmids which give multiple drug resistance to certain bacteria. Transposition can be both beneficial and hazardous to the host. Over time, transposons have led to genetic variability and evolution. This is due to their ability to generate mutations by insertion within a host’s genome. However, their insertion can lead to alterations in DNA arrangement such as cause deletions, inversions and chromosome fusions. For this reason, transposition can be deleterious. It is important to understand how the activity of transposable elements is regulated. Transposition activity must be limited so that there is little capacity to damage host DNA but still maintain advantageous features. For this to be achieved, a balance must be struck between too much transposition occurring and too little. This is known as the frequency of transposition. This essay will review the different types of regulatory mechanisms employed.
A transposon element consists of three major regions. It contains a gene for transposase, insertion sequences (IS) and a coding region for proteins such as those which give antibiotic resistance. These multiple protein-coding regions lie in between the short, repeated sequences. Transposase employs the joining together of the transposon to the host’s genome through a cut and paste mechanism whereby the enzyme cleaves its transposon and splices its ends to the target sequence. This is known as conservative transposition. The insertion sequences can be direct or inverted repeats of DNA. Directionality is given for being on different ends of a transposon. Transposition occurs because of the insertion sequences in the terminals of the transposon and the transposase enzyme .
Composite transposons are similar to simple transposons. Tn5 is an example of a composite transposon because it is flanked by two separate IS elements. Its structure is shown below in figure 1:
Figure 1: Tn5 Transposon, adapted from Annual Review Microbiology, 47: 945-63, Reznikoff, 1993
The Tn5 encodes two proteins, the transposase enzyme with a related protein and the transposition inhibitor. Since transposons are defined by the specific sequence at its ends, changes in any base pair within these sequences can typically reduce the frequency of and in some cases, completely inhibit transposition. The abundance of the inhibitor is one means of determining the frequency of its transposition. The synthesis of the two proteins that Tn5 codes for is regulated by a set of genetic regulatory elements. The proteins that the host encodes also play a crucial role in the transposition process. The host DNA methylation function also plays a part in controlling transposition because the expression of transposase is sensitive to DNA methylation. The transposase enzyme is highly unstable and in any case, cannot accumulate to very high levels in the cell. This in itself is a self-regulatory mechanism. The expression of transposase is also controlled during translation. This is done by blocking ribosome...