Final answer:
During attenuation, the formation of stem-loop structures is regulated by the presence or absence of tryptophan. When tryptophan levels are high, the terminator stem-loop forms and transcription terminates. When tryptophan levels are low, the antiterminator stem-loop forms and RNA polymerase can transcribe the structural genes of the trp operon.
Step-by-step explanation:
During attenuation, when tryptophan levels are high, the terminator stem-loop forms and transcription terminates. On the other hand, when tryptophan levels are low, the antiterminator stem-loop forms and RNA polymerase can transcribe the structural genes of the trp operon. The formation of the stem-loop structures is regulated by the presence or absence of tryptophan, which affects the binding of the trp repressor to the trp operator.