Answer:
Lactose is the inducer for lac operon and the substrate for the enzyme \[\beta \]- galactosidase-. If lactose is provided as the carbon source in the growth medium, in the absence of the preferred carbon source such as glucose, the lactose is transported into the cells by the action of enzyme permease. The active form of lactose binds to the repressor protein and brings about a conformational change in the repressor. As a result the represser is inactive, i.e. it cannot bind to the operator. This provides access of the RNA polymerase to structural genes and transcription and production of enzymes continue and metabolism of lactose takes place. In its absence the repressor is active and binds to the operator, thereby switching off the process. Therefore, regulation of lac operon can be visualised as regulation of enzyme systhesis by its substrate.
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