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.

Verified step by step guidance

1. <span>Identify the role of the lac operon in E. coli, which is responsible for the metabolism of lactose when glucose is not available.</span>
2. <span>Understand that the lac operon is regulated by two main mechanisms: the presence of lactose and the absence of glucose.</span>
3. <span>When both glucose and lactose are present, the lac operon is not fully activated due to catabolite repression, where the presence of glucose inhibits the activation of the operon through the cAMP-CAP complex.</span>
4. <span>When glucose is absent and lactose is present, the lac operon is fully activated. The absence of glucose leads to an increase in cAMP levels, which binds to CAP, allowing it to bind to the promoter and enhance transcription. Lactose, when present, binds to the repressor, preventing it from binding to the operator and thus allowing transcription.</span>
5. <span>Compare the transcription levels: in the presence of both glucose and lactose, transcription is low due to catabolite repression, whereas in the absence of glucose and presence of lactose, transcription is high due to the activation by the cAMP-CAP complex and the removal of the repressor by lactose.</span>

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.