The trp operon in E. coli controls synthesis of the amino acid tryptophan. What happens to the trp operon when tryptophan is already present in the environment?
AThe operon switches on to make even more tryptophan
BThe operon switches off; more tryptophan is not needed
CThe operon is permanently deleted from the chromosome
DThe operon turns into an inducer instead of a repressor
Answer & Solution
Correct answer: B. The operon switches off; more tryptophan is not needed
1. E. coli can synthesize tryptophan using enzymes encoded by five genes arranged in the trp operon.
2. If tryptophan is present in the environment, E. coli does not need to synthesize it.
3. In that case, the switch controlling activation of the trp operon genes is switched off.
4. Deleting the operon or converting it into an inducer operon are not the outcomes described here.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 16 "Gene Expression", section The trp Operon: A Repressor Operon_
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