The Tn10-encoded tetR mRNA has heterogeneous 5' ends in vivo and in vitro.

The 5' ends of the Tn10-encoded mRNAs have been analyzed by quantitative primer extensions of the in vivo synthesized RNA and in vitro run-off transcription. The gene is transcribed by a tandem promoter system consisting of PR1 and PR2. While PR1 contributes about 25% of the total PR activity in vitro, more than 95% of the in vivo-produced transcripts originate from PR2. Transcription from PR2 shows extensive heterogeneity at the 5' ends of the mRNAs in vitro and most likely also in vivo. A deletional and oligodeoxyribonucleotide-directed mutational analysis of the PR2 transcript reveals that length heterogeneity results from reiterative copying of a run of five A's at the transcription start point. In vivo transcription leads to longer reiteration products than in vitro transcription.