How does lactose presence influence Lac operon expression in E. coli?

How does lactose presence influence Lac operon expression in E. coli?

• A. Lactose directly binds RNA polymerase to start transcription.
• B. Lactose binds to the operator to block transcription.
• C. Lactose (via allolactose) binds to the repressor, inactivating it and allowing transcription.
• D. Lactose prevents repressor binding by altering DNA conformation.

Answer: (C)

This question tests how an inducer relieves repression in the lac operon. When lactose is present, it is converted by β-galactosidase into allolactose, which binds to the Lac repressor and changes its shape so it can no longer bind the operator. With the repressor inactivated, RNA polymerase can bind to the promoter and transcribe the lac operon genes, producing enzymes that enable lactose utilization. If lactose is absent, the repressor stays bound to the operator and blocks transcription. (Glucose levels can modulate expression via CAP–cAMP, but lactose presence specifically acts through allolactose inactivating the repressor.)

This question tests how an inducer relieves repression in the lac operon. When lactose is present, it is converted by β-galactosidase into allolactose, which binds to the Lac repressor and changes its shape so it can no longer bind the operator. With the repressor inactivated, RNA polymerase can bind to the promoter and transcribe the lac operon genes, producing enzymes that enable lactose utilization. If lactose is absent, the repressor stays bound to the operator and blocks transcription. (Glucose levels can modulate expression via CAP–cAMP, but lactose presence specifically acts through allolactose inactivating the repressor.)