Answer: Low, High, Lac repressor
Explanation:
The expression of the lac structural genes is low when lactose is absent from the culture medium and is high when lactose is added, because lactose binds to the lac repressor and inactivates it.
When Lactose is present the lac genes is expressed because the substrate lactose act as inducer and binds to the lac repressor and inactivate it. So the gene is expressed.
But in the absence of lactose, the lac repressor binds the DNA and prevent the expression of the lac genes
