For a repressible operon to be transcribed, which of the following conditions must occur?

• A. The corepressor must be absent so that the repressor cannot bind to the operator.
• B. The inducer must be present to activate the repressor.
• C. The operator must be deleted from the operon.
• D. The repressor must be bound to the operator to block RNA polymerase.

Answer: (A)

Verified step by step guidance

1. Understand the concept of a repressible operon: A repressible operon is a type of gene regulation system in prokaryotes where transcription is typically active but can be turned off when a specific molecule (corepressor) is present.
2. Identify the role of the corepressor: The corepressor binds to the repressor protein, activating it. Once activated, the repressor binds to the operator region of the operon, blocking RNA polymerase and preventing transcription.
3. Analyze the condition for transcription: For the operon to be transcribed, the repressor must not be bound to the operator. This happens when the corepressor is absent, as the repressor remains inactive and cannot bind to the operator.
4. Evaluate the incorrect options: The inducer activating the repressor is incorrect because repressible operons do not use inducers; they use corepressors. Deleting the operator is not a natural regulatory mechanism, and the repressor binding to the operator blocks transcription rather than enabling it.
5. Conclude the correct condition: The corepressor must be absent so that the repressor cannot bind to the operator, allowing RNA polymerase to transcribe the operon.