A bidirectionally active signal for termination of transcription is located between tetA and orfL on transposon Tn10

A terminator of transcription with bidirectional activity has been located between the translation termination codons of the genes tetA and orfL on Tn 10. These genes are transcribed towards each other. Each orientation of the intervening sequence is shown to reduce the expression of the lacZ and galK genes when cloned between the respective structural gene and its promotor. The 3′ends of the respective mRNAs were determined by S1 mapping. The results confirm that the same sequence capable of forming a stem-loop structure with a GC rich stem is the termination signal for both orientations. In the more efficient tetA orientation (99%–96% reduction of expression) this sequence is followed by a run of six thymines. In the less efficient orfL orientation (96%–78% reduction of expression) it is followed by an AT rich sequence with seven thymines out of eleven base pairs