The premature termination of transcription attenuation takes place in the trp operon depending on the cellular level of tryptophan

never forms, and so attenuation does not occur. RNA poly-merase continues along the DNA, past the 5' UTR, transcribing all the structural genes into mRNA, which is translated into the enzymes encoded by the trp operon. These enzymes then synthesize more tryptophan. Important events in the process of attenuation are summarized in Table 16.3.

Several additional points about attenuation need clarification. The key factor controlling attenuation is the number of tRNA molecules charged with tryptophan, because

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