Leader region of trp mRNA. (A) The nucleotide sequence of the 5′ end of trp mRNA includes a short open reading frame that encodes a peptide comprising 14 amino acids; the leader encodes two tryptophan residues and has an untranslated attenuator region that includes a region that can form a terminator structure (red and blue). (B and C) The attenuator region can adopt either of two distinct stem-