Question

Mechanism for regulation of the trp operon is described. Which is incorrect?

A. Transcription is down-regulated by the trp repressor, which is activated upon binding of Trp.

B. Ribosome stalled due to a low concentration of Trp induces an alternative mRNA structure, resulting in continued transcription.

C. Activity of the Trp synthesis operon is doubly controlled by the Trp concentrations.

D. All of these.

E. None of these.

Answer 1

Answer - A. Transcription is down-regulated by the trp repressor, which is activated upon binding of Trp.

Explanation :

The trp operon is controlled by both a repressor protein binding to the operator region as well as by translation-induced transcriptional attenuation. The trp repressor binds the operator region of the trp operon only when bound to tryptophan. This makes tryptophan a co-repressor of the operon. The trpL gene encodes a non-functional leader peptide which contains several adjacent trp codons. The tructural genes of the operon responsible for tryptophan biosynthesis are trpE, D, C, B and A. When trptophan level are high some binds to the repressor which then binds to the operator region and inhibits transcription. The mechanism of attenuation of the trp operon is diagrammed below.