In a mutant that lacks adenylyl cyclase, the enzyme that synthesizes cAMP, predict which of the following conditions of extracellular lactose and glucose would cause regulation of the lac operon to differ from that of wild-type cells.
a. No lactose, no glucose
b. No lactose, abundant glucose
c. Abundant lactose, no glucose
d. Abundant lactose, abundant glucose

Verified step by step guidance

1. Understand the role of adenylyl cyclase and cAMP in the regulation of the lac operon. Adenylyl cyclase is responsible for synthesizing cyclic AMP (cAMP) from ATP. cAMP binds to the catabolite activator protein (CAP), forming a cAMP-CAP complex that enhances transcription of the lac operon when glucose levels are low.
2. Analyze the mutant condition. In the mutant lacking adenylyl cyclase, cAMP cannot be synthesized. This means the cAMP-CAP complex cannot form, and the lac operon will not be activated regardless of glucose levels.
3. Evaluate condition (a): no lactose, no glucose. In wild-type cells, the absence of lactose would prevent lac operon transcription because the repressor remains bound to the operator. In the mutant, the lack of cAMP would not affect this outcome since the operon is already repressed.
4. Evaluate condition (b): no lactose, abundant glucose. In wild-type cells, the presence of glucose would inhibit cAMP production, and the absence of lactose would keep the operon repressed. In the mutant, the lack of cAMP would not change this outcome, as the operon remains repressed due to the absence of lactose.
5. Evaluate conditions (c) and (d): abundant lactose, no glucose and abundant lactose, abundant glucose. In wild-type cells, abundant lactose would remove the repressor, and the presence or absence of glucose would determine the level of transcription via cAMP-CAP. In the mutant, even with lactose present, the lack of cAMP would prevent activation of the operon, leading to a difference in regulation compared to wild-type cells in these conditions.

Key Concepts

Lac Operon Regulation

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. Its regulation is influenced by the presence of lactose and glucose. When lactose is present, it binds to the repressor protein, allowing transcription of the operon. Conversely, high glucose levels inhibit the operon through catabolite repression, which prevents the synthesis of cAMP, a crucial signaling molecule.

cAMP and Adenylyl Cyclase

Cyclic AMP (cAMP) is a secondary messenger that plays a vital role in cellular signaling. It is synthesized from ATP by the enzyme adenylyl cyclase. In the context of the lac operon, cAMP levels are inversely related to glucose concentration; low glucose leads to high cAMP, which activates the CAP (catabolite activator protein) to enhance transcription of the lac operon. In the mutant lacking adenylyl cyclase, cAMP cannot be produced, affecting operon regulation.

Catabolite Repression

Catabolite repression is a regulatory mechanism that ensures bacteria preferentially utilize the most efficient energy source. In the presence of glucose, the synthesis of cAMP is inhibited, leading to reduced activation of the lac operon, even if lactose is available. This mechanism allows cells to conserve energy by prioritizing glucose metabolism over lactose, which is less efficient. Understanding this concept is crucial for predicting how the lac operon will behave in different nutrient conditions.