Final answer:
The secondary structure that causes attenuation of the trp operon is the stem-loop structure.
Step-by-step explanation:
The secondary structure that causes attenuation of structural genes of the trp operon is the stem-loop structure.
In the trp operon regulatory region, there is a leader sequence called trpL that contains four stretches of RNA.
These stretches can base pair with each other in different combinations.
When a terminator stem-loop forms, transcription terminates, and RNA polymerase is released from the mRNA.
However, when an antiterminator stem-loop forms, it prevents the formation of the terminator stem-loop, allowing transcription of the structural genes to continue.