Regulation of basal level expression of the tryptophan operon of Escherichia coli

Mutations in prfB, encoding release factor 2 (UGA- and UAA-specific), increase transcription termination at the trp operon attenuator in Escherichia coli strains grown in the presence of tryptophan. The prfB mutations have no effect on basal level expression in strains in which the natural trp leader peptide stop codon UGA was replaced by either UAG or UAA. The effect of introducing prfB mutations into mutant strains containing altered trp leader regions that influence basal level transcription readthrough was also determined. Our findings support a model for basal level control of trp operon expression in which ribosome release from the leader peptide stop codon, formation of alternative transcript secondary structures, and the position of the transcribing RNA polymerase regulate expression.