Discussion
A hybrid operon was constructed by fusing the structural genes of trp operon and promoter region of lac operon. Efficient expression of this chimeric operon will require :
Answer: D
Expression needs the presence of lactose and absence of glucose.
The structural genes come from the trp operon, but their transcription is now controlled by the lac promoter region. That region responds to two signals: lactose (as allolactose) removes the lac repressor, and the absence of glucose raises cAMP so that CAP-cAMP can activate transcription.
Glucose keeps cAMP low and so blocks efficient expression, and without lactose the repressor stays bound. Tryptophan is irrelevant, because the trp promoter and operator that respond to it are no longer present.
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