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Types of transposable elements which comprise large discrete segments of deoxyribonucleic acid (DNA) capable of moving from one chromosome site to a new location. In bacteria, the transposable elements can be grouped into two classes, the insertion sequences and the transposons. The ability of transposable elements to insert into plasmid or bacterial virus (bacteriophage) which is transmissible from one organism to another allows for their rapid spread. See Bacteriophage, Plasmid
The insertion sequences were first identified by their ability to induce unusual mutations in the structural gene for a protein involved in sugar metabolism. These insertion sequences are relatively small (about 500–1500 nucleotide pairs) and can only be followed by their ability to induce these mutations. Most bacterial chromosomes contain several copies of such insertion sequence elements.
The transposons are larger segments of DNA (2000–10,000 base pairs) that encode several proteins, usually one or two required for the movement of the element and often an additional protein that imparts a selective advantage to the host containing a copy of that element. The structure of many transposons suggests they may have evolved from the simpler insertion sequence elements.
All transposable elements, both the simple insertion sequence elements and the more complex transposons, have a similar structure and genetic organization. The ends of the element represent recognition sites and define the segment of DNA undergoing transposition. A short sequence present at one end of the element is repeated in an inverted fashion at the other end. These terminal inverted repeats are characteristic for each element.
Members of a widespread group of transposons, the Tn3 family, all have a similar structure and appear to move by a similar mechanism. Transposase, one protein encoded by the element, promotes the formation of intermediates called cointegrates, in which the element has been duplicated by replication. A second element-encoded protein, resolvase, completes the process by converting the cointegrates into the end products of transposition, a transposon inserted into a new site. A third protein encoded by the Tn3 element imparts resistance to the antibiotic ampicillin.
Transposons are known that encode resistances to almost all antibiotics as well as many toxic metals and chemicals. In addition, some transposons have acquired the ability to direct the synthesis of proteins that metabolize carbohydrates, petroleum, and pesticides. Other transposable elements produce enterotoxins that cause travelers to become ill from drinking water contaminated with bacteria carrying the element. The broad spectrum of activities encoded by the transposable elements demonstrates the strong selective advantage that has accompanied their evolution.
Transposable elements are not restricted to prokaryotes. Yeast as well as higher eukaryotes have DNA segments that move and cause mutations. The eukaryotic elements have much in common with their prokaryotic counterparts: the termini of the elements are composed of inverted repeats, and many of the larger elements are composed of two small insertion sequence-like regions flanking a unique central region. One class of eukaryotic virus, the ribonucleic acid (RNA) retrovirus, also has this structure and is thought to integrate into the host chromosome through a transpositionlike mechanism. See Antibiotic, Gene, Gene action, Retrovirus, Virus