Abstract
Among the transposable DNA elements that have been detected in bacterial genomes and plasmids1–3, the small insertion sequence (IS) elements have been recognized as a class of autonomous, basic units of transposition. IS elements are not only fully competent of transposition and able to cause several other related chromosomal aberrations such as deletion of adjacent DNA segments4,5 and integration of circular plasmid DNAs6, but have also proved to be the active units of transposition in the larger composite transposon (Tn) elements1–3,7–9. In addition, IS and Tn elements interfere with local transcription patterns in the host genome by causing polarity 10,11. Structural and functional analysis of the IS elements should therefore elucidate both the basis for autonomy of a genetic unit as small as 1,000 base pairs (bp), and the transposition mechanism, including its inherent step of DNA duplication12. We have now determined the 1,195 bp sequence of the transposable DNA element IS5 in phage λKH100. Near-terminal signal structures appear to protect the central region from outside-in transcriptions and translations, and render IS polarity as a direct consequence of IS autonomy. The nucleotide structure shows an unusual, highly compact organization of two completely overlapping, antiparallel genes and correlated control sequences.
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