Which of the following features of the trp operon is most essential to the process of attenuation independent of the repressor protein?

the order of the structural genes, e, d, c, b, a
allosteric modifications as a result of tryptophan binding.
the operator sequence (trpo)
the ability of sequences within the leader mrna to pair with one another and form stem-lopp structures

The ability of sequences within the leader mRNA to pair with one another and form stem-lopp structures

Explanation:

A form of feedback regulation controls the  trp operon by regulating transcription termination, a mechanism referred to as Attenuation.

-In the presence of high concentrations of tryptophan, RNA polymerase ceases transcription  shortly after initiation in a region called the leader sequence.

-If  the concentration of tryptophan is low, transcription does not  terminate until the entire operon is transcribed. The mechanism of attenuation links alternative RNA secondary structures  to transcription termination.

-Immediately after transcription,  RNA from the leader region folds into one of two alternative  secondary structures.

-One of these structures is a transcription  termination signal that stops RNA polymerase from continuing  to the end of the operon.

-The alternative structure does not  contain a transcription termination signal and allows transcription of a single mRNA that encodes all the structural genes.  

-The decision as to which of the two alternative RNA structures  formed is regulated by the concentration of tryptophan.

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