The evolution of DNA sequences inEscherichia coli

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Abstract
It is proposed that certain families of transposable elements originally evolved in plasmids and functioned in forming replicon fusions to aid in the horizontal transmission of non-conjugational plasmids. This hypothesis is supported by the finding that the transposable elements Tn3and γδ are found almost exclusively in plasmids, and also by the distribution of the unrelated insertion sequences IS4 and IS5among a reference collection of 67 natural isolates ofEscherichia coli.Each insertion sequence was found to be present in only about one-third of the strains. Among the ten strains found to contain both insertion sequences, the number of copies of the elements was negatively correlated. With respect to IS5, approximately half of the strains containing a chromosomal copy of the insertion element also contained copies within the plasmid complement of the strain.