If the operator region (lacO) of the lac operon were mutated so that the repressor could no longer bind, what effect would this have on lactose metabolism in Escherichia coli?

• A. There would be no effect on lactose metabolism.
• B. Lactose metabolism would only occur when glucose is present.
• C. The genes for lactose metabolism would be continuously expressed, even in the absence of lactose.
• D. The genes for lactose metabolism would never be expressed, even in the presence of lactose.

Answer: (C)

Verified step by step guidance

1. Understand the lac operon system: The lac operon is a set of genes in Escherichia coli that are responsible for lactose metabolism. It includes structural genes (lacZ, lacY, and lacA), a promoter, an operator (lacO), and a regulatory gene (lacI) that produces the repressor protein.
2. Learn the role of the operator region (lacO): The operator is a DNA sequence where the repressor protein binds to block transcription of the structural genes. When the repressor is bound to the operator, RNA polymerase cannot transcribe the genes for lactose metabolism.
3. Consider the mutation in the operator region: If the operator region is mutated so that the repressor can no longer bind, the regulatory control of the lac operon is disrupted. The repressor protein will be unable to prevent transcription, regardless of whether lactose is present or absent.
4. Analyze the effect on gene expression: Without the repressor binding to the operator, RNA polymerase will have continuous access to the structural genes. This means the genes for lactose metabolism (lacZ, lacY, and lacA) will be transcribed and expressed continuously.
5. Conclude the impact on lactose metabolism: The continuous expression of the genes for lactose metabolism will occur even in the absence of lactose, leading to unnecessary production of enzymes involved in lactose metabolism. This is inefficient for the cell but is the direct result of the mutation in the operator region.