key term - Repression

Definition

Repression is a fundamental mechanism of gene regulation in which a regulatory protein, called a repressor, binds to a specific DNA sequence near a gene and blocks the transcription of that gene. This process is central to the operon theory, which describes how genes are coordinately expressed in bacteria.

5 Must Know Facts For Your Next Test

1. Repression is a key mechanism in the lac operon, where the lac repressor binds to the operator and blocks transcription of the lac genes in the absence of lactose.
2. Repressors can be inactivated by the binding of a specific inducer molecule, relieving repression and allowing transcription to occur.
3. Repression can be either constitutive, where the repressor is always bound, or inducible, where the repressor is only bound in the absence of an inducer.
4. The strength of repression is determined by the affinity of the repressor for the operator sequence and the concentration of the repressor protein.
5. Repression is a highly efficient way for cells to conserve resources by only expressing genes when they are needed.

Review Questions

• Explain how repression functions in the lac operon to control the expression of the lac genes.
  • In the lac operon, the lac repressor protein binds to the operator sequence, blocking the RNA polymerase from transcribing the lac genes (lacZ, lacY, and lacA). This repression keeps the lac genes turned off when lactose is not present. When lactose is available, it binds to the lac repressor, causing a conformational change that releases the repressor from the operator. This relieves the repression, allowing the lac genes to be transcribed and the enzymes for lactose metabolism to be produced.
• Describe the differences between constitutive and inducible repression, and provide examples of each.
  • Constitutive repression occurs when the repressor is always bound to the operator, keeping the associated genes permanently repressed. This is seen in the trp operon, where the trp repressor binds to the operator even in the presence of tryptophan. Inducible repression, on the other hand, is where the repressor is only bound to the operator in the absence of a specific inducer molecule. The lac operon is an example of inducible repression, as the lac repressor only binds to the operator when lactose is not present to act as an inducer and relieve the repression.
• Analyze how the strength of repression can be modulated by factors such as repressor affinity and concentration, and explain the biological significance of this regulation.
  • The strength of repression is determined by the affinity of the repressor protein for the operator sequence and the concentration of the repressor. Higher affinity repressors and higher concentrations of the repressor will result in stronger repression, making it more difficult for RNA polymerase to bind and initiate transcription. This fine-tuning of repression strength allows cells to precisely control the expression of genes based on their metabolic needs. Stronger repression conserves cellular resources by preventing the unnecessary production of gene products, while weaker repression allows for rapid induction of gene expression when the associated proteins are required. This flexibility in repression strength is crucial for the efficient regulation of metabolic pathways and other cellular processes.

Related terms

Operon:

A group of genes that are transcribed together as a single unit and are under the control of a shared promoter and regulatory sequences.

Repressor Protein: A regulatory protein that binds to an operator sequence and blocks the transcription of a gene or set of genes.

Operator:

A DNA sequence to which a repressor protein binds, preventing the transcription of the associated genes.