Question

The presence of ______ in a bacterium's environment prevents CAP from binding to the DNA, resulting in ______ in transcription of the \(\operatorname{lac}\) operon. a. lactose, an increase b. glucose, an increase c. cAMP, a decrease d. glucose, a decrease e. lactose, a decrease

Short answer

The correct answer is: d. glucose, a decrease.

Step-by-step solution

Understanding the role of glucose.

The lac operon is transcribed when lactose is present and glucose is absent. Glucose negatively regulates the lac operon's transcription. When glucose levels are high, cAMP levels are low and vice versa. High levels of glucose consequently prevent CAP from binding to the DNA.

Connecting glucose presence to transcription level.

From step 1, it is understood that the presence of glucose would prevent CAP from binding to the DNA. However, this lack of binding results in reduced or decreased transcription of the lac operon, not an increase.

Identifying the correct option.

By connecting the presence of glucose with decreased transcription of the lac operon, the correct choice becomes clear. The option that says 'glucose' and 'a decrease' is therefore the correct answer.

Key concepts

CAP Binding

CAP, which stands for Catabolite Activator Protein, is a regulatory protein in bacteria that plays a crucial role in the expression of the lac operon. The lac operon is responsible for the breakdown of lactose into simpler sugars that bacteria can use as an energy source. CAP works in conjunction with cyclic AMP (cAMP) to increase the rate of transcription of the lac operon in response to the nutritional state of the cell.

When glucose levels are low, cAMP concentrations increase. cAMP then binds to CAP, changing its shape and allowing it to bind to a specific site on the DNA near the lac operon. This CAP-cAMP complex enhances the binding of RNA polymerase to the operon's promoter, facilitating the transcription of genes needed for lactose metabolism. Conversely, when glucose is abundant, cAMP levels drop, CAP does not bind to the DNA, and transcription of the lac operon diminishes.

Glucose and cAMP Levels in Bacteria

Glucose is the preferred energy source for bacteria, and its presence affects the level of cAMP, an important signaling molecule. How does this work? In the context of the lac operon, cAMP acts as a signal of low energy levels. When glucose is scarce, the adenylate cyclase enzyme becomes more active, converting ATP to cAMP. Thus, increased cAMP levels indicate a reduced availability of glucose.

• If glucose is present: cAMP levels decrease, reducing CAP binding and lac operon transcription.
• If glucose is absent: cAMP levels surge, enhancing CAP binding and thereby promoting lac operon transcription.

Bacteria use this elegant system to ensure that energy production is optimized based on their environment's nutrient profile.

Transcription Regulation

The lac operon is a prime example of transcription regulation in bacteria, coordinating the cell's response to the presence of lactose and glucose. Essentially, the lac operon's transcription is switched 'on' when lactose is present (to be used as an energy source) and glucose (the preferable energy source) is scarce.

Several key players are involved in this regulation:

• The repressor protein, which binds to the operator sequence of the DNA, blocking transcription in the absence of lactose.
• When lactose is available, it binds to the repressor, causing a conformational change that prevents the repressor from binding to the operator, allowing transcription to proceed.
• As explained previously, the CAP-cAMP complex enhances RNA polymerase binding only when glucose levels are low, thus integrating the signals of lactose and glucose availability.

Through interactions between these proteins and small molecules, the cell exquisitely controls the lac operon's transcription in response to changing environmental conditions.