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.

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. Understanding this structure is essential for analyzing how the presence or absence of lactose and glucose 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. When lactose is present, 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 the transcription level 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-CAP complex to the promoter, which promotes transcription. This concept is vital for understanding the transcription levels in the specified growth medium.