Xii

Transposase gene

-23-bp terminal-

inverted repeat

^-9-bp flanking direct repeat-J

111.22 Insertion sequences are simple transposable elements found in bacteria.

transposable elements that contain DNA sequences not directly related to transposition.

Insertion sequences The simplest type of transposable element in bacterial chromosomes and plasmids is an insertion sequence (IS). This type of element carries only the genetic information necessary for its movement. Insertion sequences are common constituents of bacteria and plas-mids. They are designated by IS, followed by an identifying number. For example, IS1 is a common insertion sequence found in E. coli.

Insertion sequences are typically from 800 to 2000 bp in length and possess the two hallmarks of transposable elements: terminal inverted repeats and the generation of flanking direct repeats at the site of insertion. Most insertion sequences contain one or two genes that code for transposase. IS1, a typical insertion sequence, is 768 nucleotide pairs long and has terminal inverted repeats of 23 bp at each end ( FIGURE 11.22). The flanking direct repeats created by IS1 are each 9 bp long — the most common length for flanking direct repeats. Table 11.4 summarizes these features for several bacterial insertion sequences.

Composite transposons Any segment of DNA that becomes flanked by two copies of an insertion sequence may itself transpose and is called a composite transpo-son. Each type of composite transposon is designated by the abbreviation Tn, followed by a number. Tn10 is a composite transposon of about 9300 bp that carries a gene (about 6500 bp) for tetracycline resistance between two IS10 insertion sequences ( FIGURE 11.23). The insertion sequences have terminal inverted repeats; so the composite transposon also ends in inverted repeats. Composite transposons also generate flanking direct repeats at their sites of insertion (see Figure 11.23). The insertion sequences at the ends of a composite transposon may be in the same orientation or they may be inverted relative to one other (as in Tn10).

The insertion sequences at the ends of a composite transposon are responsible for transposition. The DNA between the insertion sequences is not required for movement and may carry additional information (such as antibiotic resistance). Presumably, composite transposons evolve when one insertion sequence transposes to a location close to another of the same type. The transposase produced by one of the IS sequences catalyzes the transposition of both insertions sequences, allowing them to move together and carry along the DNA that lies between

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