| Popis: |
The attenuator of the tryptophan (trp) operon of Escherichia coli, located between the promoter and the first major structural gene of the operon, functions as a site of regulated transcription termination (Bertrand et al., 1975). Discrete in vivo RNA transcripts, corresponding to the initial 137 to 141 nucleotides of trp mRNA, were isolated and characterized. RNase T1 digests of these transcripts yielded a series of 3′-OH oligonucleotides with the general sequence CUn-OH (n = 4 to 8), suggesting that transcription terminates in vivo at any of five adjacent base-pairs in the A + T-rich portion of the attenuator region. These 3′-OH termini are essentially identical to the 3′-OH termini of the major trp mRNA species produced in vitro (Lee et al., 1976). Internal deletions within the trp operon, with one terminus in the immediate vicinity of the attenuator, were characterized as to their effects on (1) the production of attenuator-terminated transcripts, (2) the rho-dependence of attenuator termination, and (3) the relief of attenuator termination accompanying tryptophan starvation. We find that the nucleotide sequence essential for normal termination and its control extends no more than 11 base-pairs beyond the site of termination in vivo. Deletion of a portion of the A + T-rich sequence of the attenuator reduces the frequency of termination in vivo, and eliminates termination in this region in vitro. |
