Mutations can alter the function of the lac operon (see Module 11.1). Predict how the following mutations would affect the function of the operon in the presence and absence of lactose:
a. Mutation of the regulatory gene; repressor cannot bind to lactose.
b. Mutation of operator; repressor will not bind to operator.
c. Mutation of regulatory gene; repressor will not bind to operator.
d. Mutation of promoter; RNA polymerase will not attach to promoter.

Verified step by step guidance

1. Step 1: Understand the lac operon system. The lac operon is a group of genes in prokaryotes (like E. coli) that are involved in lactose metabolism. It includes structural genes (lacZ, lacY, lacA), a promoter (where RNA polymerase binds), an operator (where the repressor binds), and a regulatory gene (which produces the repressor protein). The operon is regulated by the presence or absence of lactose and the ability of the repressor to bind to the operator or lactose.
2. Step 2: Analyze mutation (a): Mutation of the regulatory gene; repressor cannot bind to lactose. If the repressor cannot bind to lactose, it will remain bound to the operator even in the presence of lactose. This would prevent RNA polymerase from transcribing the structural genes, meaning the operon would remain off regardless of lactose presence.
3. Step 3: Analyze mutation (b): Mutation of the operator; repressor will not bind to the operator. If the repressor cannot bind to the operator, the operon will remain on regardless of lactose presence. RNA polymerase will continuously transcribe the structural genes, leading to constant production of enzymes for lactose metabolism, even when lactose is absent.
4. Step 4: Analyze mutation (c): Mutation of the regulatory gene; repressor will not bind to the operator. If the repressor cannot bind to the operator, it is similar to mutation (b). The operon will remain on, and RNA polymerase will continuously transcribe the structural genes, leading to constant enzyme production regardless of lactose presence.
5. Step 5: Analyze mutation (d): Mutation of the promoter; RNA polymerase will not attach to the promoter. If RNA polymerase cannot bind to the promoter, transcription of the structural genes will not occur. This means the operon will remain off, and the enzymes for lactose metabolism will not be produced, regardless of lactose presence.

Key Concepts

Lac Operon Structure and Function

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of structural genes (lacZ, lacY, and lacA), a promoter, an operator, and regulatory genes. The operon is regulated by the presence of lactose, which binds to the repressor protein, allowing transcription of the genes necessary for lactose utilization when lactose is available.

Repressor Protein and Its Role

The repressor protein is a key regulatory element in the lac operon that binds to the operator region to inhibit transcription. In the absence of lactose, the repressor binds to the operator, preventing RNA polymerase from transcribing the operon. When lactose is present, it binds to the repressor, causing a conformational change that releases the repressor from the operator, allowing gene expression.

Mutations and Their Effects on Gene Regulation

Mutations in the lac operon can significantly impact its regulation and function. For example, mutations in the regulatory gene can affect the repressor's ability to bind to lactose or the operator, while mutations in the promoter can hinder RNA polymerase's ability to initiate transcription. Understanding these mutations helps predict how the operon will respond to the presence or absence of lactose.