The simplest and shortest transposons, known as insertion sequences, were first found in bacteria. They are designated IS1, IS2 etc. Typical insertion sequences are 750 to 1,500 base pairs (bp) long with terminal inverted repeats of 10 to 40 bp (Table 15.02). Often the inverted repeats at the ends of insertion sequences are not quite exact The simplest transposable repeats. For example, the inverted repeats of IS1 match in 20 out of 23 positions.
elements contain overlapping Insertion sequences only encode a single enzyme, the transposase, the enzyme genes for a transposase and a needed for movement. Between the inverted repeats is a region that actually contains regulatory protein boated two open reading frames, orfA and orfB. The transposase itself is derived from both between inverted repeats. open reading frames by a frameshift that occurs during translation (Fig. 15.03). The low frequency of such frameshifting results in the transposase only being expressed at low levels. Unregulated transposition would lead to massive damage to the host chromosome. Consequently, synthesis of transposase needs to be carefully regulated. When no frameshift occurs, the first open reading frame, orfA, is expressed. This gene product is a transcriptional regulator that controls the production of transposase and of itself.
Insertion sequences are found in the chromosomes of bacteria and also in the DNA of their plasmids and viruses. For example, several copies each of the insertion sequences IS1, IS2, and IS3 are found in the chromosome of E. coli. The F-plasmid has zero copies of IS1, one copy of IS2, and two copies of IS3. When the F-plasmid and chromosome possess identical IS sequences, the plasmid can insert itself into the host chromosome. This, in turn, allows transfer of chromosomal genes by the F-plasmid as explained in Chapter 18.
Insertion sequences contain no genes that provide a convenient phenotype. Originally their presence was recognized because movement of the insertion sequence inactivated genes with a noticeable phenotype. Such insertion mutations usually abolish gene function completely and are often polar to downstream genes of the operon. In addition, they revert only at very low frequencies (see Ch. 13).
insertion sequence A simple transposon consisting only of inverted repeats surrounding a gene encoding transposase
Movement by Conservative Transposition 401
Insertion sequences, like all transposons, are flanked by inverted repeats. Two open reading frames within the insertion sequence encode the transposase gene. When a frameshift occurs during translation, transposase is produced and the insertion sequence "jumps" to a new location. If the frameshift does not occur, then only the orfA gene product is expressed. This protein regulates transposition by turning off transcription of orfA and orfB at the promoter.
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