Which of these acts as an inducer of the lac operon?

The correct option is Allolactose.

Step-by-Step Explanation:

1. Understanding the Lac Operon:
The lactose (lac) operon is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria. It consists of three structural genes (lacZ, lacY, and lacA), a promoter, a terminator, and an operator. By default, the operon is kept in an "off" state by a lac repressor protein that binds to the operator and blocks transcription.

2. The Role of an Inducer:
An inducer is a molecule that regulates gene expression. In the case of the lac operon, the inducer binds to the lac repressor. When the inducer binds to the repressor, it causes a conformational change in the repressor protein, reducing its affinity for the operator DNA. As a result, the repressor dissociates from the operator, allowing RNA polymerase to transcribe the structural genes.

3. Lactose vs. Allolactose:
While lactose is the substrate for the pathway, it is not the actual molecule that binds directly to the repressor. When lactose enters the cell, a small amount of it is converted by the enzyme β-galactosidase into its isomer, allolactose. It is this allolactose molecule that functions as the true physiological inducer by binding to the repressor and releasing the inhibition, thereby turning the operon "on".