Bacterial operons — trp operon control: A mutation that prevents tryptophan from binding its repressor would most likely lead to which expression state?

Introduction / Context:The Escherichia coli trp operon is a repressible system. The aporepressor requires tryptophan (corepressor) to bind the operator and block transcription. Mutations that disrupt this interaction alter regulatory logic.

Given Data / Assumptions:

• The repressor must bind tryptophan to adopt a DNA-binding conformation.
• Mutation specifically abolishes the tryptophan-binding site on the repressor.
• Promoter, operator, and structural genes are otherwise intact.

Concept / Approach:Without tryptophan binding, the repressor remains inactive and cannot bind the operator. Therefore, transcription proceeds regardless of intracellular tryptophan levels, yielding constitutive expression of the trp biosynthetic genes.

Step-by-Step Solution:

Verification / Alternative check:Classical genetics shows that repressor mutations in the corepressor-binding domain lead to constitutive synthesis of enzymes such as anthranilate synthase.

Why Other Options Are Wrong:

• Inducible expression applies to lac-type systems or repression relieved by absence of tryptophan; here repression can never occur.
• No expression would require promoter/operator loss or super-repressor, not loss of corepressor binding.
• “None” is unnecessary because a clear prediction exists.

Common Pitfalls:Confusing co-repressible (trp) with inducible (lac) logic. Remember: trp repressor needs tryptophan to repress.

Final Answer:Constitutive trp operon expression