Transposable element ISI intrinsic; duplications of variable length (IS elements/IS1 variant/transposon/transposition mechanism/DNI

Target duplication during transposition is one of the characteristics of mobile genetic elements. ISJ, a resident insertion element of Escherichia coli K-12, was known to generate a 9-base-pair target duplication, while an ISI variant, characterized by a nucleotide substitution in one of its terminal inverted repeats, was reported to duplicate 8 base pairs of its target during cointegration. We have constructed a series of transposons flanked by copies of either the normal or the variant ISI. The analysis of their transposition products revealed that transposons with normal termini as well as those with variant termini can intrinsically generate either 9- or 8-base-pair target duplications. We also observed that a normal ISJ from the host chromosome generated an 8-base- pair repeat. The possible relevance of the observation for the understanding of transposition processes and models to ex- plain the variable length of target duplications are discussed.