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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Transposons

A frightening ability of pathogenic bacteria was discovered in Japanese hospitals in the 1950s. Bacterial dysentery is caused by bacteria of the genus Shigella. This bacterium initially proved to be sensitive to a wide array of antibiotics that were used to control the disease. In the Japanese hospitals, however, Shigella isolated from patients with dysentery proved to be simultaneously resistant to many of these drugs, including penicillin, tetracycline, sulfanilamide, streptomycin, and chloramphenicol. This multiple drug-resistance phenotype was inherited as a single genetic package, and it could be transmitted in an infectious mannernot only to other sensitive Shigella strains, but also to other related species of bacteria. This talent is an extraordinarily useful one for the pathogenic bacterium, and its implications for medical science were terrifying. From the point of view of the geneticist, however, the situation is very interesting. The vector carrying these resistances from one cell to another proved to be a self-replicating element similar to the F factor. These R factors (for "resistance") are transferred rapidly on cell conjugation, much like the F particle in E. coli (Chapter 9).

In fact, these R factors proved to be just the first of many similar F-like factors to be discovered. These elements, which exist in the plasmid state in the cytoplasm, have been found to carry many different kinds of genes in bacteria. Table 13-2 lists just some of the characteristics that can be borne by plasmids. What is the mode of action of these plasmids? How do they acquire their new genetic abilities? How do they carry them from cell to cell? The discovery of transposons made it possible to answer some of these questions.

Physical Structure of Transposons

If the DNA of a plasmid conferring drug resistance (carrying the genes for kanamycin resistance, for example) is denatured to single-stranded forms and then allowed to renature slowly, some of the strands form an unusual shape under the electron microscope: a large, circular DNA ring is attached to a "lollipop"-shaped structure (Figure 13-6). The "stick" of the lollipop is double-stranded DNA, which has formed through the annealing of two inverted repeat (IR) sequences in the plasmid (Figure 13-7). Subsequent studies have shown that the IR sequences are a pair of IS elements in many cases. For instance, IS10 is present at the ends of the region carrying the genes for tetracycline resistance (Figure 13-8). In some cases, however, the IR sequences are much smaller.

The genes for drug resistance or other genetic abilities carried by the plasmid are located between the IR sequences in the lollipop head. The IR sequences together with their contained genes have been collectively called a transposon (Tn). (Transposons are therefore longer than IS elements, since they contain extra protein-coding genes.) The remainder of the plasmid, bearing the genes coding for resistance-transfer functions (RTF), is called the RTF region (Figure 13-9). Table 13-3 lists some of the known transposons.

Movement of Transposons

A transposon can jump from a plasmid to a bacterial chromosome or from one plasmid to another plasmid. In this manner, multiple drug-resistant plasmids are generated. Figure 13-10 is a composite diagram of an R plasmid, indicating the various places at which transposons can be located.

MESSAGE
Transposons were originally detected as mobile genetic elements that confer drug resistance. Many of these elements consist of recognizable IS elements flanking a gene that encodes drug resistance. IS elements and transposons are now grouped together under the single term transposable elements.

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