Question

What will be the status of transcription of the lac operon in (a) the presence of glucose and (b) the absence of glucose, in each case with lactose present, if there is a mutation that produces an inactive adenylate cyclase?

Short answer

Answer: The transcription status of the lac operon will be minimal or not occurring in both scenarios due to the inactive adenylate cyclase preventing cAMP production and CAP activation, regardless of the presence or absence of glucose.

Step-by-step solution

Understanding Lac Operon Regulation

The lac operon is a group of genes responsible for metabolizing lactose in bacteria. It contains three structural genes: lacZ, lacY, and lacA. The lac operon is regulated by two proteins: the lac repressor (LacI) and the catabolite activator protein (CAP). The lac repressor binds to the operator region and inhibits transcription when lactose is absent. In the presence of lactose, an inducer molecule (allolactose) binds to the lac repressor and inactivates it, allowing transcription to occur. The catabolite activator protein (CAP) is activated when cyclic AMP (cAMP) levels are high. When glucose is absent, cAMP levels rise, and CAP stimulates the transcription by binding to a specific site on the DNA near the lac operon.

Effect of Inactive Adenylate Cyclase

Adenylate cyclase is an enzyme responsible for converting ATP into cyclic AMP (cAMP). An inactive adenylate cyclase in this case means that cAMP production will be significantly reduced or absent, regardless of the presence or absence of glucose.

Scenario (a): Presence of Glucose and Lactose

In this scenario, glucose is present while lactose is also present. Under normal circumstances, the presence of glucose would suppress cAMP production. However, since adenylate cyclase is inactive, there will be no or very low cAMP production. Thus, CAP cannot be activated and will not bind to the DNA to stimulate transcription, even though lactose is present to inactivate the lac repressor. As a result, transcription of the lac operon will be minimal or not occurring.

Scenario (b): Absence of Glucose and Presence of Lactose

In this scenario, glucose is absent, and lactose is present. Normally, this would lead to an increase in cAMP levels, activating CAP to stimulate transcription. However, because of the inactive adenylate cyclase, cAMP production will be negligible or nonexistent. This means CAP will not be activated, and it will not bind to the DNA to stimulate transcription. Therefore, even though lactose is present to inactivate the lac repressor, transcription of the lac operon will be minimal or not occurring due to the lack of CAP activation. In summary, the transcription status of the lac operon will be minimal or not occurring in both scenarios due to the inactive adenylate cyclase preventing cAMP production and CAP activation, regardless of the presence or absence of glucose.