Consider the transcription of genes of the lac operon under two conditions: (1) when both glucose and lactose are present and (2) when glucose is absent and lactose is present. Describe the comparative levels of transcription of lac operon genes under these conditions, and explain the molecular basis for the difference.

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 three structural genes (lacZ, lacY, and lacA) and regulatory elements that control their expression. The operon is activated in the presence of lactose and repressed when glucose is available, illustrating the concept of gene regulation in response to environmental conditions.

Catabolite Repression

Catabolite repression is a regulatory mechanism that prioritizes the use of glucose over other sugars, such as lactose. When glucose is present, it inhibits the production of cyclic AMP (cAMP), which is necessary for the activation of the lac operon. This results in low transcription levels of the lac genes when glucose is available, contrasting with high transcription levels when glucose is absent and lactose is present.

cAMP and CAP Interaction

Cyclic AMP (cAMP) binds to the catabolite activator protein (CAP), forming a complex that enhances the transcription of the lac operon. In the absence of glucose, cAMP levels rise, allowing the cAMP-CAP complex to bind to the promoter region of the lac operon, facilitating RNA polymerase binding and increasing transcription. This interaction is crucial for understanding how the presence or absence of glucose and lactose affects gene expression.