Several transposable elements are able to promote replicon fusions in Escherichia coli. We show here that the transposon, Tn5, also has this property. Tn5 is able to promote fusions between the plasmid pBR322 (on which it resides) and bacteriophage λ at 2% of the frequency of Tn5 transposition onto λ. Plasmid/phage replicon fusions formed in strains that are totally deficient in homologous and integrative recombination always consist of direct duplications of the insertion element at the junctions between λ and pBR322. In order to define those regions within the Tn5 necessary for causing replicon fusions, a series of deletions of Tn5 generated in vitro and in vivo were constructed. Deletion derivatives that remove all but the 1500-base-pair right inverted repetition (stem) are competent to promote fusions. The left inverted repetition is unable to supply this activity, but can be complemented in trans to form replicon fusions. We conclude that both repeats have the sites necessary for replicon fusions, but only the right stem has the required protein(s). This establishes that the right repeat is an insertion sequence.
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