Question

The \(E\). coli lacZYA region will be upregulated if A. there is a defect in binding of the inducer to the product of the Lad gene. B. glucose and lactose are both present, but the cell cannot bind the CAP protein. C. glucose and lactose are both readily available in the growth medium. D. the operator has mutated so it can no longer bind the repressor. E. the Lac corepressor is not present.

Short answer

Answer: The lacZYA region will be upregulated when the operator has mutated so it can no longer bind the repressor.

Step-by-step solution

Option A

A defect in binding of the inducer to the product of the Lad gene does not result in upregulation of the lacZYA region, as proper inducer binding is necessary for this process.

Option B

When glucose and lactose are both present but the cell cannot bind the CAP protein, the lacZYA region won't be upregulated. The CAP protein is required to facilitate transcription when glucose is low, so inability to bind it will prevent upregulation.

Option C

The lacZYA region is not upregulated when both glucose and lactose are readily available in the growth medium. The presence of glucose triggers the catabolite repression, inhibiting the transcription of the lac operon even when lactose is present.

Option D

When the operator has mutated so it can no longer bind the repressor, the lacZYA region will be upregulated. Lack of binding of the repressor to the operator allows the RNA polymerase to initiate transcription, leading to upregulation.

Option E

The absence of the Lac corepressor does not upregulate the lacZYA region by itself, as the presence or absence of glucose and lactose also plays an important role in regulation. Based on this analysis, the correct answer is: D. The operator has mutated so it can no longer bind the repressor.

Key concepts

Catabolite Repression

Catabolite repression is a regulatory mechanism by which the presence of a preferred energy source, such as glucose, suppresses the expression of genes involved in the metabolism of alternative energy sources, like lactose in the case of the lac operon in Escherichia coli (E. coli). This process ensures that the bacterial cell uses the most efficient energy source available.

When glucose is present, the levels of cyclic AMP (cAMP) in the cell are low. This low level of cAMP affects the binding of the catabolite activator protein (CAP) to the promoter region of genes such as those in the lac operon. CAP requires cAMP to bind DNA effectively; without this complex, RNA polymerase is less likely to bind to the promoter, thus decreasing transcription of lac operon genes. This efficient use of resources is particularly important in prokaryotes like E. coli, where energy conservation can be crucial for survival.

Therefore, when glucose is abundant in the growth medium, even if lactose is also present, catabolite repression inhibits the expression of the lac operon genes, avoiding unnecessary production of enzymes that metabolize lactose.

Gene Expression in Prokaryotes

Understanding gene expression in prokaryotes is fundamental in molecular biology. Prokaryotes, such as bacteria, do not have a nucleus to separate transcription and translation processes. Instead, these organisms often regulate gene expression at the transcriptional level via operons.

An operon is a cluster of genes under the control of a single promoter and is typically involved in a common metabolic pathway. The lac operon in E. coli is a prime example of how gene expression can be regulated. When lactose is present and glucose is absent or scarce, the lac operon is activated to produce enzymes needed for the breakdown of lactose. The process is regulated by proteins that can either enhance (activators) or inhibit (repressors) the binding of RNA polymerase to the operon's promoter.

In the case of the lac operon, the regulatory system involves both an activator, the catabolite activator protein (CAP), and a repressor protein. The CAP, when bound with cAMP, enhances transcription, while the lac repressor, when bound with lactose, allows transcription. This system provides a responsive and efficient method of gene expression based on the cell's environmental conditions.

Repressor Protein Function

Repressor proteins are pivotal in the control of gene expression. A repressor binds to specific DNA sequences known as operators, which are located near the promoters of target genes. In doing so, repressors can block the binding of RNA polymerase to the promoter, thereby preventing the initiation of transcription and subsequent gene expression.

In the context of the lac operon, the lac repressor protein binds to the operator in the absence of lactose, effectively shutting down the operon. When lactose is present, it serves as an inducer by binding to the repressor, causing a conformational change that prevents the repressor from binding to the operator. This release allows RNA polymerase access to the promoter, leading to the transcription of the lacZYA genes, which are necessary for lactose metabolism.

However, mutations in the operator sequence can prevent repressor binding, resulting in the lac operon being perpetually active, regardless of lactose presence. As mentioned in our exercise, such a mutation would lead to the upregulation of the lacZYA region, underscoring the significant role repressor proteins play in bacterial gene regulation.