Which of the following molecules binds to the operator region of a bacterial operon to regulate gene expression?

• A. Repressor protein
• B. Activator protein
• C. RNA polymerase
• D. Ribosome

Answer: (A)

Verified step by step guidance

1. Understand the structure of a bacterial operon, which typically includes the promoter, operator, and structural genes. The operator is a specific DNA sequence where regulatory proteins can bind to control gene expression.
2. Recall that gene expression in bacteria is regulated by proteins that either inhibit or enhance the transcription of genes. These proteins interact with the operator region to influence RNA polymerase activity.
3. Identify the role of the repressor protein: it binds directly to the operator region to block RNA polymerase from transcribing the downstream genes, effectively turning off gene expression.
4. Contrast this with activator proteins, which bind to other regulatory sites (not the operator) to increase transcription, and RNA polymerase, which binds to the promoter to initiate transcription but does not bind the operator.
5. Recognize that ribosomes are involved in translation, not transcription regulation, and therefore do not bind to the operator region.