Question

In E. coli, three enzymes, β-galactosidase, permease, and transacetylase, began to be produced as soon as lactose was added. Explain why the enzymes were not forming in the absence of lactose.

Answer 1

In E. coli, these enzymes are not produced in the absence of lactose due to a regulatory system called the lac operon. This system ensures the cell doesn’t waste energy making enzymes for a fuel source that isn’t available.

• Repressor Binding: A regulatory gene (the i gene) continuously produces a repressor protein. In the absence of lactose, this protein binds tightly to a DNA segment called the operator.
• Transcriptional Block: When the repressor is attached to the operator, it physically blocks RNA polymerase from moving forward along the DNA.
• No mRNA Production: Because RNA polymerase is blocked, it cannot transcribe the structural genes (lacZ, lacY, and lacA) into mRNA. Without mRNA, the cell cannot produce the enzymes β-galactosidase, permease, and transacetylase.

When lactose (the inducer) is finally added, it binds to the repressor, changing its shape so it can no longer stick to the operator. This “switches on” the operon, allowing the enzymes to be produced immediately.