In lac operon regulation, which molecule binds the LacI repressor to relieve repression?

• A. Glucose
• B. Lactose
• C. Galactose
• D. Ribose

Answer: (B)

The repressor LacI blocks transcription by binding to the operator, preventing RNA polymerase from transcribing the lac operon. When an inducer binds LacI, the repressor changes shape and releases from the operator, lifting the block and allowing transcription to proceed. In this system, the inducer that binds LacI is allolactose, an isomer of lactose produced inside the cell from lactose. Among the given options, lactose is the closest and best match because it is converted into the actual inducer that relieves repression. The other sugars don’t interact with LacI to relieve repression, and glucose mainly affects regulation through the CAP-cAMP system rather than directly inactivating LacI.