Why is the transcription rate of the lac operon low when both glucose and lactose are absent?

• A. The lac operon is activated by the presence of lactose, which is absent.
• B. The presence of glucose inhibits the lac operon through catabolite repression.
• C. The lac operon requires high levels of cAMP, which is abundant when glucose is present.
• D. The lac operon is repressed by the lac repressor binding to the operator.

Answer: (D)

Verified step by step guidance

1. Understand the role of the lac operon: The lac operon is a set of genes responsible for the metabolism of lactose in bacteria. It is regulated by the presence or absence of lactose and glucose.
2. Identify the role of lactose: Lactose acts as an inducer by binding to the lac repressor, causing it to release from the operator region of the operon, allowing transcription to occur.
3. Consider the absence of lactose: When lactose is absent, the lac repressor remains bound to the operator, preventing RNA polymerase from transcribing the operon genes, thus keeping the transcription rate low.
4. Examine the effect of glucose: In the presence of glucose, catabolite repression occurs, which further inhibits the lac operon by reducing the levels of cyclic AMP (cAMP), a molecule that enhances the binding of the cAMP receptor protein (CRP) to the promoter, facilitating transcription.
5. Conclude the combined effect: With both glucose and lactose absent, the lac repressor remains bound to the operator, and the low levels of cAMP due to the absence of glucose further ensure that the transcription rate of the lac operon remains low.