Simone Lawson
In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. Repressors are commonly found in prokaryotes and are rare in eukaryotes. The benefit of the repressor being constitutively produced is that it acts as a master switch to regulate gene expression, allowing unnecessary genes to be turned off when their products are not required for a metabolic process.
| Characteristics | Values |
|---|---|
| Definition | A repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. |
| Repressor function | Blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA. |
| Repressor types | DNA-binding repressor, RNA-binding repressor, co-repressor, aporepressor, active repressor |
| Repressor examples | trp repressor, LacI gene, λ repressor |
| Repressor regulation | Repressor dissociates from the operator site when an inducer is present, allowing gene transcription to occur. |
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What You'll Learn
The repressor protein acts as a lactose sensor
The operator is a small fragment of DNA that acts as a negative regulatory site, bound by the lac repressor protein. The lac repressor protein binds to sites called operators, which surround the promoter and overlap with the +1 site of the operon. When there is no lactose present, the repressor binds to the operators, blocking transcription.
When lactose is present, the repressor releases the operators, and transcription can occur. This is because when lactose is present, it is converted into allolactose, which binds to the lac repressor, causing an allosteric change in its shape. In its changed state, the lac repressor is unable to bind tightly to its cognate operator.
The repressor protein is, therefore, a lactose sensor, acting as a master switch to regulate the expression of genes involved in the metabolism of lactose.
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Repressors can alter DNA or RNA structure
Repressors are DNA- or RNA-binding proteins that can inhibit gene expression by binding to the operator or associated silencers. They are more commonly found in prokaryotes and are rare in eukaryotes. While repressors do not alter DNA structure, they can alter RNA structure by preventing the translation of mRNA into proteins.
A DNA-binding repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA. This blocking or reduction of expression is called repression. An RNA-binding repressor, on the other hand, binds to the mRNA and prevents translation.
Repressors can also have two binding sites: one for the silencer region and one for the promoter. This causes chromosome looping, bringing the promoter region and the silencer region close to each other.
The Lac operon is an example of a repressor that is constitutively expressed and usually bound to the operator region of the promoter. It interferes with the ability of RNA polymerase to initiate transcription. In the presence of the inducer allolactose, the Lac operon changes conformation, reducing its DNA-binding strength and dissociating from the operator DNA sequence. RNA polymerase can then bind to the promoter and begin transcription.
Another example is the L-arabinose operon, which houses genes coding for arabinose-digesting enzymes. When the repressor gene (araC) is present, it binds to the araI region to form a loop, preventing polymerases from binding to the promoter and transcribing the structural genes into proteins. When both arabinose and araC are present, a conformational change occurs in the araC, and it acts as an activator, promoting RNA polymerase recruitment.
The benefit of the repressor being constitutively produced is that it allows for the regulation of gene expression. By binding to the operator or associated silencers, repressors can prevent the transcription of specific genes, ensuring that certain proteins are not synthesized when they are not needed by the cell. This helps to conserve resources and energy for the cell.
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Repressors are more common in prokaryotes
In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. A DNA-binding repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA. An RNA-binding repressor binds to the mRNA and prevents translation of the mRNA into protein. This blocking or reduction of expression is called repression.
Repressors can also have two binding sites: one for the silencer region and one for the promoter. This causes chromosome looping, allowing the promoter region and the silencer region to come into close proximity to each other. The lacZYA operon, for example, houses genes that encode proteins needed for lactose breakdown. The lacI gene codes for a protein called "the repressor" or "lac repressor", which functions as a repressor of the lac operon.
The repressor molecule, produced by the LacI gene, binds to the operator site and blocks the transcription of a set of structural genes (Lac Z) that follow the operator site. The repressor, when bound to an inducer, decreases its affinity for the operator and allows transcription of the structural genes. The number of repressor molecules in a bacterial cell is usually small.
The L-arabinose operon houses genes that code for arabinose-digesting enzymes. These enzymes function to break down arabinose as an alternative energy source when glucose is low or absent. Once produced, araC acts as a repressor by binding to the araI region to form a loop, which prevents polymerases from binding to the promoter and transcribing the structural genes into proteins.
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Repressor genes can be isolated and studied
In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. Repressor proteins are essential for the regulation of gene expression in cells.
Benno Muller Hill, working with Walter Gilbert, also identified a potential lac repressor protein by isolating cell fractions that bound a radioactively labelled inducer. Gilbert and Muller-Hill's work with negative and positive controls was another important step in studying repressor genes. They examined the extracts from super-repressed and constitutive mutants, which provided insights into the repressor's ability to bind to inducers.
Additionally, Gilbert and Muller-Hill's work with Mark Ptashne's simultaneous isolation of the repressor in the phage λ system confirmed the applicability of Mill's methods and Platt's strong interference. These experiments were significant for the field of regulatory genetics and provided a molecular understanding of the control process.
Isolating repressor genes is challenging due to the hypothesized low concentration of repressors in the system. However, the successful isolation of repressor genes allows for critical experiments to be performed, contributing to our understanding of gene regulation.
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Repressors can be regarded as anti-activators
Repressors are DNA- or RNA-binding proteins that inhibit the expression of one or more genes by binding to the operator or associated silencers. They block the attachment of RNA polymerase to the promoter, preventing the transcription of genes into messenger RNA.
The Lac operon, for example, responds to an inducer that causes the repressor to dissociate from the operator, derepressing the operon. The repressor protein acts as a lactose sensor, recruiting RNA polymerase only in the presence of lactose.
In the trp operon, the repressor-tryptophan complex binds to the trp operator when tryptophan is plentiful. This prevents the unnecessary synthesis of tryptophan when it is not required.
The L-arabinose operon contains a regulatory repressor gene (araC) that, once produced, acts as a repressor by binding to the araI region, forming a loop that prevents polymerases from binding to the promoter and transcribing the structural genes into proteins. When SAM is present, it binds to the MetJ protein, increasing its affinity for its cognate operator site and halting the transcription of genes involved in methionine synthesis.
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Frequently asked questions
A repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers.
The benefit of the repressor being constitutively produced is that it allows for the regulation of gene expression. This is useful for the organism as it allows unnecessary genes to be turned off when the products of those genes are not required for a metabolic process.
The trp operon of E. coli codes for the enzymes that the bacterium needs to make the amino acid tryptophan. The trp operon is a negative control mechanism that responds to a repressor protein that binds to two molecules of tryptophan.
The repressor regulates gene expression by binding to operator sites, which are short DNA sequences located upstream of the structural gene. This binding blocks the attachment of RNA polymerase to the promoter, preventing transcription of the genes into messenger RNA.
