UNIMPAIRED REPRESSIBILITY OF TRANSCRIPTION OF OPERATOR-PROXIMAL EIGHTH OF FIRST (E) GENE OF TRYPTOPHAN OPERON IN ESCHERICHIA-COLI

A deletion mutant, strain OAE14, retains the operator-proximal eighth of the first gene of the tryptophan operon (or approximately 3 percent the length of the entire operon). Strain OAE14 is effectively transcribed with producing the small trp messenger RNA (6S), the size of which is estimated to correspond to the length of the trp operon segment retained in this mutant. The conclusion is derived from kinetic analysis that the distance between the sites of initiation and termination of transcription coincides with the length of the trp operon segment in this mutant. Transcription of strain OAE14 is completely repressed following the addition of tryptophan. According to the current idea that deletions entering the operator region of an operon (and presumably leaving the promoter intact) diminish repressibility on the operon, the retention of normal repressibility in strain OAE14 indicates that an intact functional operator region is present in this strain. On the basis of this conclusion, the hypothesis is proposed that, if transcription begins ahead of the operator region, the region must be smaller than about 3 percent the length of the entire operon, and that, if the operator region is large, the region is not to be transcribed. © 1969 Springer-Verlag.