Info

i 1 1 1 1

i i 1 1 1 1 1 1 1 1

Target sequence

Recipient DNA molecule

(i.e. SINGLE INSERTION SEQUENCE MOVES)

FIGURE 15.10 Principle of the Composite Transposon

Two identical insertion sequences can move as two independent transposons or as one composite transposon. Composite transposition moves both insertion sequences (region a-b and c-d) as well as any intermediate DNA (region b-c) to the new location.

FIGURE 15.10 Principle of the Composite Transposon

Two identical insertion sequences can move as two independent transposons or as one composite transposon. Composite transposition moves both insertion sequences (region a-b and c-d) as well as any intermediate DNA (region b-c) to the new location.

the two IS elements face in the same direction, as in Tn9, which is flanked by direct repeats of IS1.

Once a useful composite transposon has assembled by chance, natural selection will act to keep the parts together. Mutations accumulate that inactivate the innermost pair of inverted repeats, which prevents the insertion sequences from jumping independently. Often, one of the two transposase genes is also lost. The result is that the two ends and the middle are now permanently associated and always move as a unit (Fig. 15.11). In practice, all stages from newly formed to fully fused composite transposons are found in bacteria. For that matter, novel composite transposons can be assembled in the laboratory by genetic manipulation.

Movement of transposons, or their sub-components, may cause rearrangements of the host DNA molecule.

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