Variant insertion element IS1 generates 8-base pair duplications of the target sequence

Transposition in both eukaryotes and prokaryotes is characterized by the generation of short, direct duplications of the target DNA sequence at the integration site of the transposable element(1). For example, the bacterial insertion sequence IS1, or the IS1-flanked transposon Tn9, generates 9-base pair (bp) repeats during either transposition or cointegration(2-8). All the transposable elements so far sequenced are characterized by the presence at their ends, of a perfect, or near-perfect, inverted repeat sequence, which is similar to 30 bp long(1) and is thought to be important for recognition by enzymes during transposition. While studying IS1-mediated cointegration(9-12), we found an IS1 carrying a single base pair change in the terminal inverted repeat sequence. We now report that it generates 8-bp duplications of the target sequence, a finding which confirms the importance of the inverted repeat for transposition and which, because one end only of the IS1 is changed, has implications for the mechanism of transposition by IS1.