For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.
Growth medium contains lactose but no glucose.

Verified step by step guidance

1. Understand the lac operon genotype: The given genotype I⁺ P⁺ O⁺ Z⁺ Y⁺ indicates that the lac operon is wild-type. This means the repressor protein (I⁺), promoter (P⁺), operator (O⁺), and structural genes (Z⁺ for β-galactosidase and Y⁺ for permease) are all functional.
2. Analyze the growth medium: The medium contains lactose but no glucose. Lactose acts as an inducer by binding to the repressor protein, preventing it from binding to the operator. The absence of glucose means that cyclic AMP (cAMP) levels will be high, allowing the cAMP-CAP complex to bind to the promoter and enhance transcription.
3. Determine the state of the repressor: Since lactose is present, it binds to the repressor protein, causing a conformational change that prevents the repressor from binding to the operator. This allows RNA polymerase to access the promoter.
4. Evaluate the role of cAMP-CAP: The absence of glucose leads to high cAMP levels. cAMP binds to the catabolite activator protein (CAP), forming the cAMP-CAP complex. This complex binds to the promoter region, increasing the affinity of RNA polymerase for the promoter and enhancing transcription.
5. Conclude the transcription level: With the repressor inactivated by lactose and the cAMP-CAP complex enhancing RNA polymerase binding, the operon genes (Z and Y) will be transcribed at an 'activated' level. This ensures efficient metabolism of lactose in the absence of glucose.

Key Concepts

Lac Operon Structure

The lac operon in E. coli consists of several key components: the promoter (P), operator (O), and structural genes (Z and Y) that code for enzymes involved in lactose metabolism. The I gene produces a repressor that can bind to the operator, blocking transcription when lactose is absent. Understanding this structure is essential for analyzing how the presence of lactose affects gene expression.

Inducible Gene Expression

The lac operon is an example of an inducible system, meaning that its transcription can be activated in response to specific signals—in this case, the presence of lactose. When lactose is available, it binds to the repressor, causing it to release from the operator, which allows RNA polymerase to transcribe the operon genes. This concept is crucial for determining transcription levels in different growth media.

Catabolite Repression

Catabolite repression is a regulatory mechanism in bacteria that prioritizes the use of glucose over other sugars, such as lactose. When glucose is present, the levels of cyclic AMP (cAMP) are low, leading to reduced activation of the lac operon. In the absence of glucose, cAMP levels rise, enhancing the binding of the cAMP-CRP complex to the promoter, which further activates transcription of the lac operon genes.