The nucleotide sequence of the insertion element IS200 has been determined partially, including the junctions between the element and the host chromosome at the insertion site. At most, two bases (A-A) are found repeated at the junctions and could be duplications of host sequences generated by the insertion of the element. No obvious sequence repeats, either direct or inverted, have been detected between the sequences just within the two ends of the element. The element is an extremely strong block to host transcription across the insertion site. A sequence similar to known transcription termination signals was found just within the element near the right end. Removal of less than 50 base-pairs at the right end of the element abolishes the transcription block. The putative terminator sequence is located within this 50 base-pair region. Genetic studies suggest that the element contains a promoter located more than 93 base-pairs from its left end. The proposed promoter and terminator are in proper orientation to form an internal transcription unit.
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