Fate of donor insertion sequence IS1 during transposition.

The number of insertion sequence IS1 segments in Escherichia coli K-12 and 20 mutants in which an ISI had been inserted at a new site was measured by Southern blot hybridization analysis. The parent strain appeared to contain seven IS1 segments. Each of the mutants contained these seven IS1 and one additional IS1 corresponding to the new IS1 insertion. These results suggest that a copy of a donor ISI is inserted at the new site. A model for transposition is prevented that postulates that a reactive intermediate is formed by a tandem duplication of a transposable sequence.