EVIDENCE SUGGESTING CIS ACTION BY THE TNAC LEADER PEPTIDE IN REGULATING TRANSCRIPTION ATTENUATION IN THE TRYPTOPHANASE OPERON OF ESCHERICHIA-COLI

Expression of the tryptophanase (tna) operon in Escherichia coli is re gulated by catabolite repression and transcription attenuation. Elevat ed levels of tryptophan induce transcription antitermination at one or more Rho factor-dependent termination sites in the leader region of t he operon. Induction requires translation of a 24-residue coding regio n, tnaC, located in the 319-nucleotide transcribed leader region prece ding tnaA, the structural gene for tryptophanase. In the present paper , we show that two bacterial species that lack tryptophanase activity, Enterobacter aerogenes and Salmonella typhimurium, allow tryptophanas e induction and tna operon regulation when they early a plasmid contai ning the E. coli tna operon. The role of tnaC in induction was examine d by introducing mutations in a 24-nucleotide segment of tnaC off. col i surrounding and including the crucial Trp codon 12. Some mutations r esulted in a noninducible phenotype; these mostly introduced nonconser vative amino acid substitutions in TnaC. Other mutations had little or no effect; these generally were in third positions of codons or intro duced conservative amino acid replacements. A tryptophan-inserting, UG A-reading glutamine suppressor tRNA was observed to restore partial re gulation when Trp codon 12 of tnaC was changed to UGA. Stop codons int roduced downstream of Trp codon 12 in all three reading frames establi shed that induction requires translation in the natural tnaC reading f rame. Our findings suggest that the TnaC leader peptide acts in cis to prevent Rho-dependent termination.