Crucial elements that maintain the interactions between the regulatory TnaC peptide and the ribosome exit tunnel responsible for Trp inhibition of ribosome function

Translation of the TnaC nascent peptide inhibits ribosomal activity in the presence of l-tryptophan, inducing expression of the tnaCAB operon in Escherichia coli. Using chemical methylation, this work reveals how interactions between TnaC and the ribosome are affected by mutations in both molecules. The presence of the TnaC-tRNA(Pro) peptidyl-tRNA within the ribosome protects the 23S rRNA nucleotide U2609 against chemical methylation. Such protection was not observed in mutant ribosomes containing changes in 23S rRNA nucleotides of the A748-A752 region. Nucleotides A752 and U2609 establish a base-pair interaction. Most replacements of either A752 or U2609 affected Trp induction of a TnaC-regulated LacZ reporter. However, the single change A752G, or the dual replacements A752G and U2609C, maintained Trp induction. Replacements at the conserved TnaC residues W12 and D16 also abolished the protection of U2609 by TnaC-tRNA(Pro) against chemical methylation. These data indicate that the TnaC nascent peptide in the ribosome exit tunnel interacts with the U2609 nucleotide when the ribosome is Trp responsive. This interaction is affected by mutational changes in exit tunnel nucleotides of 23S rRNA, as well as in conserved TnaC residues, suggesting that they affect the structure of the exit tunnel and/or the nascent peptide configuration in the tunnel.