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The transposable genetic element Tn9 consists of two direct repeats of the insertion sequence IS1 flanking a region of 1,102 base pairs which determines chloramphenicol resistance. Transposition of Tn9 leads to the duplication of a 9-base pair sequence which preexists at the site of insertion. One copy of this sequence is found at each end of the inserted element. The chloramphenicol resistance determined by Tn9, and by various other R plasmids, is due to the synthesis of the enzyme chloramphenicol acetyl transferase (CAT). This enzyme catalyses the formation of acetylated derivatives of chloramphenicol which are inactive as inhibitors of protein synthesis. By using the chain termination technique of DNA sequencing, we have now determined the nucleotide sequence of the 1,102 base pair region between the directly repeated IS1 sequence in the bacterial transposon Tn9 (encoding chloramphenicol resistance). The amino acid sequence of CAT predicted from the nucleotide sequence is identical to that determined by Shaw and coworkers. An analysis of the sequence suggests that the internal 1,102 base pair region is not directly involved in transposition.

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