Required for tryptophan-regulated expression of the tna operon. In the presence of free L-Trp release of this nascent peptide by release factor 2 is inhibited and the ribosome stalls with the last amino acid in the P site and a UGA stop codon in the A site. This prevent transcripiton termination factor Rho binding, and thus allows transcription and translation of TnaA and TnaB.