A composite transposon consists of two inverted repeats from two separate transposons moving together as one unit and carrying the DNA between them (Fig. 15.10). For example, consider a segment of DNA flanked at both ends by two identical insertion sequences. The transposase will move any segment of DNA surrounded by a pair of the inverted repeats that it recognizes. Consequently, when transposition occurs there are several possibilities (Fig. 15.10). First, each of the insertion sequences may move independently. Second, the whole structure between the two outermost inverted repeats may move as a unit, i.e., as a composite transposon.
Many of the well-known bacterial transposons that carry genes for antibiotic resistance or other useful properties are composite transposons. Three of the best known are Tn5 (kanamycin resistance), Tn9 (chloramphenicol resistance) and Tn10 (tetracycline resistance). Usually the pair of insertion sequences at the ends of the transposon are inverted relative to each other, as in Tn5 and Tn10. The IS elements of Tn5 and Tn10 have not been found alone and are named IS50 and IS10 respectively. Less often composite transposon A transposon that consists of two insertion sequences surrounding a central block of genes
Several steps in conservative and replicative transposition are very similar at the mechanistic level.
Transposable elements may be built up in modular fashion from simpler transposons plus entrapped DNA.
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