FIGURE 1 Regulation of the B. subtilis trp operon. (a) In the absence of tryptophan, the structure of the trp leader RNA allows continued transcription of the trp operon. (b) In the presence of tryptophan, the TRAP protein binds tryptophan and associates with the trp operon leader through interaction with 11 GAG and UAG triplets. This leads to formation of a terminator structure in the RNA, which halts transcription.