Q.Regulation of lac operon by repressor is referred to as : (A) Inducible regulation (B) Repressible regulation (C) Negative regulation (D) Positive regulation
The regulation of the lac operon by its repressor protein is termed negative regulation because the repressor actively blocks transcription.
The lac operon in E. coli is a classic example of gene regulation, demonstrating how bacteria adapt to their environment by controlling gene expression. This operon is responsible for the metabolism of lactose, a disaccharide sugar. The core idea is that the genes for lactose metabolism are only expressed when lactose is available and glucose (the preferred energy source) is absent.
At the heart of this regulation, particularly concerning the repressor, is a mechanism designed to prevent the wasteful synthesis of enzymes when lactose is not present. The lac operon consists of a regulatory gene (i gene), a promoter (P), an operator (O), and three structural genes (z, y, a). The i gene codes for the lac repressor protein.
Here's how the repressor regulates the lac operon:
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Repressor Synthesis: The i gene is constitutively expressed, meaning it is always producing the lac repressor protein, albeit at a low level. This repressor protein is a tetramer.
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Repressor's Default Action (Absence of Lactose):
- In the absence of lactose, the repressor protein is in its active conformation.
- It binds tightly to the operator region (O) of the lac operon.
- The operator region is located immediately downstream of the promoter. When the repressor is bound to the operator, it physically blocks the movement of RNA polymerase.
- This blockage prevents RNA polymerase from transcribing the structural genes (z, y, a), which code for enzymes like beta-galactosidase, permease, and transacetylase, necessary for lactose metabolism.
- Thus, in the absence of lactose, the operon is switched off, and the enzymes are not produced.
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Repressor's Action in Presence of Lactose (Induction):
- When lactose is present in the medium, a small amount of it enters the bacterial cell.
- Lactose is then converted into allolactose, which acts as the actual inducer molecule.
- Allolactose binds to the repressor protein. This binding causes a conformational change in the repressor.
- Due to this change, the repressor protein can no longer bind to the operator region.
- With the operator free, RNA polymerase can now bind to the promoter and move along the DNA, transcribing the structural genes.
- This leads to the production of the enzymes required for lactose metabolism, allowing the bacterium to utilize lactose as an energy source.
The repressor protein's primary function is to block transcription. When it is active and bound to the operator, it exerts a negative control over gene expression.
Considering the options:
- (A) Inducible regulation: This describes the overall nature of the lac operon, where the presence of a substrate (lactose) induces or switches on the transcription of genes. While true for the operon as a whole, it doesn't specifically describe the repressor's action.
- (B) Repressible regulation: This type of regulation is typically seen in anabolic pathways (e.g., tryptophan operon), where the end product of a pathway acts as a corepressor to activate a repressor, thereby switching off gene expression. This is the opposite of the lac operon's mechanism.
- (C) Negative regulation: This term precisely describes the repressor's role. The repressor protein negatively controls gene expression by binding to the operator and preventing transcription. Its removal (by the inducer) allows transcription to proceed.
- (D) Positive regulation: This involves an activator protein that binds to DNA and promotes transcription. While the lac operon also exhibits positive regulation through Catabolite Activator Protein (CAP) in the absence of glucose, the question specifically asks about the regulation by the repressor.
The NCERT textbook clearly states that the regulation of the lac operon by its repressor is an example of negative regulation. The repressor acts as a negative regulator by preventing transcription.
The regulation of the lac operon by the repressor is referred to as (C) Negative regulation because the repressor protein actively blocks the transcription of the structural genes.
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