IS1397 is active for transposition into the chromosome of Escherichia coliK-12 and inserts specifically into palindromic units of bacterial interspersed mosaic elements

We demonstrate that IS1397, a putative mobile genetic element discovered in natural isolates of Escherichia coli, is active for transposition into the chromosome off. coli K-12 and inserts specifically into palindromic units, also called repetitive extragenic palindromes, the basic element of bacter ial interspersed mosaic elements (BIMEs), which are found in intergenic reg ions of enterobacteria closely related to E. coli and Salmonella. We could not detect transposition onto a plasmid carrying BIMEs. This unprecedented specificity of insertion into a well-characterized chromosomal intergenic r epeated element and its evolutionary implications are discussed.