Question

In the trp operon, attenuation occurs through the recognition of two tryptophan codons in the leader sequence. what would happen if these two codons were mutated to stop codons?
a. this operon will be insensitive to attenuation by tryptophan.
b. the structural genes will be transcribed in the absence of tryptophan.
c. the tryptophan biosynthetic enzymes will not be synthesized.
d. all of these

Answer 1

If a lot of tryptophan is present, the operon will be repressed.
Under intermediate amounts of tryptophan, the change to stop codons would cause the ribosome to stall and therefore would mimic as if there were no tryptophan present.
If no tryptophan present, the operon would be maximally expressed.

a - True. The ribosome would always be stalled in the leader sequence - so this wouldn't matter. However, the operon would still be transcribed because the repressor would be active (remember Trp operon is controlled both by attenuation and repression). True as long as there's not much tryptophan to activate the repressor!
b- True - With no tryptophan the repressor isn't engaged and the ribosome is stalled in the leader sequence.
c- Matters about how much tryptophan is available. So True if there's lots of tryptophan available.
d- I would choose this one as all of the above could be true depending on the levels of tryptophan.