Inhibition of the progress of transcription on the tryptophan operon of Escherichia coli

It has been reported that when l-tryptophan is added to tryptophan-deprived cells of Escherichia coli, transcription which is in progress over the most operator-proximal region of trpE, the first structural gene of the tryptophan operon (trpO-E-D-C-B-A), is inhibited immediately (Imamoto, 1968, 1970). In this paper we show that transcription inhibition is not restricted to the initial segment of trpE; rather, there is a gradient of inhibition throughout the operon, with transcription of trpE inhibited most severely. We also find that this inhibition of the progression of transcription occurs in a repressor-negative amber mutant (trpR); thus the inhibition does not involve the operon-specific repression system. In a strain in which trpE is deleted, severe inhibition of transcription is observed for trpD, the gene which is now located immediately adjacent to the operator. The inhibition is independent of the normal function of the RNA synthesis control gene, rel, and is not affected by a polarity relief suppressor, suA. Attempts to show that abnormally rapid degradation is responsible for the reduction in detectable trp mRNA were unsuccessful.