A transcriptional barrier in the regulatory region of the tryptophan operon of Escherichia coli: its role in the regulation of repressor-independent RNA synthesis.

A study was made of the influence of the growth rate of Escherichia coli bacteria on the transcription of the tryptophan (trp) operon, in various trp repressor negative strains. Selective measurement of the levels of RNA transcribed from the regulatory region (reg) of this operon and from the structural genes, respectively, has revealed that the increase of the rate of trpRNA synthesis with bacterial growth rate (Rose and Yanofsky, 1972) is due to a decrease of the frequency of termination of transcription at the transcriptional barrier in the regulatory region of the operon. In a mutant strain of E. coli with a deletion covering the promotor distal part of the regulatory region of the trp operon where the barrier is located, the rate of trpRNA synthesis is not affected by the growth rate. In suA- strains, in which Rho factor activity is reduced the bacterial growth rate does not affect the rate of synthesis of trpRNA. This result suggests that in wild type bacteria Rho factor contributes to the control of the transcription of the trp operon. In bacteria with a mutation rendering Tryptophanyl-tRNA synthetase (TRSase) inactive (trpS- strains) the rate of trpRNA synthesis is affected by the growth rate in the same way as in the isogenic wild type bacteria. This result indicates that TRSase plays no obligatory role in the control of trpRNA synthesis through a mechanism of termination and anti-termination of transcription, at least not in the studied strains, which carried a relA mutation.