Transcription termination: Nucleotide sequence at 3′ end of tryptophan operon in Escherichia coli

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RESUMO

We have determined the RNA and DNA sequences in the region specifying termination of transcription at the end of the tryptophan (trp) operon of Escherichia coli. A 3′-terminal mRNA fragment of about 150 nucleotides yielded oligonucleotide products that could be assigned to the end of trpA (the last structural gene in the operon) by correlation with the amino acid sequence of the protein product. Analysis of the DNA corresponding to this region served to align the few noncoding RNA oligonucleotide sequences and demonstrated that termination of trp transcription occurs in vivo at a site 36 nucleotides after trpA, with greater than 95% efficiency. In two different strains partially defective in the transcription termination factor rho, the purified transcript is much longer and more complex, suggesting that a significant amount of read-through occurs in these strains. This is consistent with evidence [Guarente, L. P., Mitchell, D. H. & Beckwith, J. (1977) J. Mol. Biol. 112, 423-436] that efficient termination in vivo at the end of the trp operon is a rho-dependent event. The trp terminator (trp t) shares several features with other known sites of transcription termination, including (i) a 3′-terminal RNA sequence of several uridine residues, C-A-U-U-U-UOH, (ii) a G·C-rich region in the DNA immediately preceding the site of termination, followed by an A·T-rich region, and (iii) a region of dyad symmetry in the DNA which, in the transcript, is capable of forming a stable hairpin containing seven G·C base pairs and one A·U base pair in its stem.

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