Final answer:
When lactose levels are low, transcription of the lac operon is minimal because the repressor protein binds to the operator, blocking transcription since the inducer (allolactose) is not present to release the repressor.
Step-by-step explanation:
When lactose levels are low, the lac operon of E. coli behaves in a specific manner in terms of transcription from the lacI gene. The lac repressor protein, which is a product of the lacI gene, binds to the operator sequence in the absence of lactose, preventing transcription of the operon. Since the expression of the I gene is not regulated and the repressor is always present, transcription of the lac operon is minimal.
Given that lactose is not available to act as an inducer, there is no allolactose formed to inhibit the repressor, ensuring that the operon remains off. However, this dynamic changes when glucose is also low, as the CAP protein can then bind to the promoter sequence to enhance transcription of the operon, but only if lactose is also present. Otherwise, the lac operon remains inactive.