In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?
Mutation of the lacI gene affecting the DNA-binding site of the protein

Verified step by step guidance

1. Understand the role of the lacI gene: The lacI gene encodes the lac repressor protein, which binds to the operator region of the lac operon to inhibit transcription in the absence of lactose.
2. Identify the mutation's effect: A mutation in the DNA-binding site of the lacI protein would likely impair its ability to bind to the operator region of the lac operon.
3. Predict the impact on transcription: If the lacI protein cannot bind to the operator, the operon will remain transcriptionally active, even in the absence of lactose, because the repressor cannot block RNA polymerase from initiating transcription.
4. Consider the role of lactose: Normally, lactose (or allolactose) binds to the lac repressor, causing it to release from the operator. In this case, the mutation renders the repressor ineffective regardless of lactose presence.
5. Conclude the overall effect: The mutation would likely result in constitutive expression of the lac operon genes, meaning they would be transcribed continuously, regardless of environmental lactose levels.

Key Concepts

Lac Operon Structure

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of three structural genes (lacZ, lacY, and lacA) and regulatory elements, including the promoter and operator. The operon is controlled by the lacI gene, which encodes the repressor protein that binds to the operator to inhibit transcription in the absence of lactose.

Role of the LacI Repressor

The lacI gene produces the lac repressor, a protein that binds to the operator region of the lac operon, preventing RNA polymerase from transcribing the downstream genes. When lactose is present, it binds to the repressor, causing a conformational change that releases the repressor from the operator, allowing transcription to occur. Mutations in the lacI gene can disrupt this regulatory mechanism.

Effects of Mutations on Gene Expression

Mutations in the lacI gene that affect the DNA-binding site of the repressor can lead to either a loss of function or altered binding affinity. If the repressor cannot bind to the operator, the operon will be constitutively expressed, leading to continuous transcription of the lac genes regardless of lactose presence. Conversely, a mutation that enhances binding could prevent transcription even when lactose is available.