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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Mechanism of Transposition

Several different mechanisms of transposition are employed by prokaryotic transposable elements. And, as we shall see later, eukaryotic elements exhibit still additional mechanisms of transposition.

In E. coli, we can identify replicative and conservative (nonreplicative) modes of transposition. In the replicative pathway, a new copy of the transposable element is generated during the transposition event. The results of the transposition are that one copy appears at the new site and one copy remains at the old site. In the conservative pathway, there is no replication. Instead, the element is excised from the chromosome or plasmid and is integrated into the new site.

Replicative Transposition

When transposition is from one locus to a second locus for certain transposons, a copy of the transposable element is left behind at the first locus. An analysis of transposon mutants revealed an interesting fact about the mechanism of transposition. Using the transposon Tn3 (Figure 13-11), researchers grouped the mutations that prevent transposition into two categories. A trans-recessive class maps in the gene that encodes the transposase enzyme, a catalyst of transposition. A second class of cis-dominant mutations results in the buildup of an intermediate in the transposition process. Figure 13-12 diagrams the transposition pathway in the Tn3 transposition from one plasmid to another. The intermediate is a double plasmid, with both donor and recipient plasmid being fused together. The combined circle resulting from the fusion of two circular elements is termed a cointegrate. Apparently, the mutations in this second class delete a region on the transposon at which a recombination event takes place that resolves cointegrates into two smaller circles. This region, called the internal resolution site (IRS), appears in Figure 13-11.

The finding of a cointegrate structure as an intermediate in transposition helped establish a replicative mode of transposition for certain elements. In Figure 13-12, note how the transposable element is duplicated during the fusion event and how the recombination event that resolves the cointegrate into two smaller circles leaves one copy of the transposable element in each plasmid.

Conservative Transposition

Some transposons, such as Tn10, excise from the chromosome and integrate into the target DNA. In these cases, DNA replication of the element does not occur, and the element is lost from the site of the original chromosome.

Molecular Consequences of Transposition

The molecular consequences of transposition reveal an additional piece of evidence concerning the mechanism of transposition: on integration into a new target site, transposable elements generate a repeated sequence of the target DNA in both replicative and conservative transposition. Figure 13-13 depicts the integration of IS1 into a gene. In the example shown, the integration event results in the repetition of a 9-bp target sequence. Analysis of many integration events reveals that the repeated sequence does not result from reciprocal site-specific recombination (as is the case in phage l integration; see page 287); rather, it is generated during the process of integration itself. The number of base pairs is a characteristic of each element. In bacteria, 9-bp and 5-bp repeats are most common.

The preceding observations have been incorporated into somewhat complicated models of transposition. Most models postulate that staggered cleavages are made at the target site and at the ends of the transposable element by a transposase enzyme that is encoded by the element. One end of the transposable element is then attached by a single strand to each protruding end of the staggered cut. Subsequent steps depend on which mode of transposition occurs (replicative or conservative).

MESSAGE
In prokaryotes, transposition occurs by at least two different pathways. Some transposable elements can replicate a copy of the element into a target site, leaving one copy behind at the original site. In other cases, transposition comprises the direct excision of the element and its reinsertion into a new site.

© 1999 by W. H. Freeman and Company. All rights reserved.