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Activator (genetics)
Encyclopedia
An activator is a DNA-binding protein
that regulates one or more gene
s by increasing the rate of transcription
. The activator may increase transcription by virtue of a connected domain which assists in the formation of the RNA polymerase
holoenzyme, or may operate through a coactivator
. A coactivator binds the DNA-binding activator and contains the domain assisting holoenzyme formation. A particular activator may bind one or more specific coactivators.
(RNAP) is usually bound to the promoter region on the gene which creates a complex that sometimes undergoes a transition that allows transcription
to take place. An activator essentially recruits the RNAP to its promoter region, by binding to the activator binding site itself which serves as a liaison between the RNA polymerase and the DNA. Sometimes, the activator is required for the RNA polymerase to change DNA conformation through allostery to initiate transcription. RNAP does not spontaneously transform into the open complex and requires the activator.
of the bacteria Escherichia coli
, the Lac repressor
is constitutively expressed and always bound to the operator region of the promoter, interfering with the ability of RNAP to bind to the promoter and transcribe the lac operon. In the presence of lactose
, the repressor changes conformation and falls off the operator and RNAP is able to bind to the promoter.
The catabolite activator protein (CAP) is an example of an activator for this operon, present in prokaryotic systems. Cyclic adenosine monophosphate
(cAMP) is present in low levels of glucose, and binds to CAP which changes to an active conformation. Only then, can CAP act as an activator and help recruit RNAP to its binding site.
In other words, for maximum expression of the lac genes, there needs to be a high level of lactose (for the repressor to fall off) and a low level of glucose (for CAP to recruit RNAP) in the cell. Otherwise, either the repressor will bind to the operator (no genes transcribed), or there is no cooperative binding by CAP thus depending on a spontaneous binding of RNAP (low levels of genes transcribed).
DNA-binding protein
DNA-binding proteins are proteins that are composed of DNA-binding domains and thus have a specific or general affinity for either single or double stranded DNA. Sequence-specific DNA-binding proteins generally interact with the major groove of B-DNA, because it exposes more functional groups that...
that regulates one or more gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s by increasing the rate of transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
. The activator may increase transcription by virtue of a connected domain which assists in the formation of the RNA polymerase
RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
holoenzyme, or may operate through a coactivator
Coactivator (genetics)
A coactivator is a protein that increases gene expression by binding to an activator which contains a DNA binding domain. The coactivator is unable to bind DNA by itself....
. A coactivator binds the DNA-binding activator and contains the domain assisting holoenzyme formation. A particular activator may bind one or more specific coactivators.
Role in transcriptional regulation
RNA polymeraseRNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
(RNAP) is usually bound to the promoter region on the gene which creates a complex that sometimes undergoes a transition that allows transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
to take place. An activator essentially recruits the RNAP to its promoter region, by binding to the activator binding site itself which serves as a liaison between the RNA polymerase and the DNA. Sometimes, the activator is required for the RNA polymerase to change DNA conformation through allostery to initiate transcription. RNAP does not spontaneously transform into the open complex and requires the activator.
Examples
In the lac operonLac operon
The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It consists of three adjacent structural genes, lacZ, lacY and lacA. The lac operon is regulated by several factors including the availability of glucose and of...
of the bacteria Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
, the Lac repressor
Repressor
In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...
is constitutively expressed and always bound to the operator region of the promoter, interfering with the ability of RNAP to bind to the promoter and transcribe the lac operon. In the presence of lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
, the repressor changes conformation and falls off the operator and RNAP is able to bind to the promoter.
The catabolite activator protein (CAP) is an example of an activator for this operon, present in prokaryotic systems. Cyclic adenosine monophosphate
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
(cAMP) is present in low levels of glucose, and binds to CAP which changes to an active conformation. Only then, can CAP act as an activator and help recruit RNAP to its binding site.
In other words, for maximum expression of the lac genes, there needs to be a high level of lactose (for the repressor to fall off) and a low level of glucose (for CAP to recruit RNAP) in the cell. Otherwise, either the repressor will bind to the operator (no genes transcribed), or there is no cooperative binding by CAP thus depending on a spontaneous binding of RNAP (low levels of genes transcribed).
See also
- Promoter (biology)
- RepressorRepressorIn molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...
- OperonOperonIn genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...
- Regulation of gene expressionRegulation of gene expressionGene modulation redirects here. For information on therapeutic regulation of gene expression, see therapeutic gene modulation.Regulation of gene expression includes the processes that cells and viruses use to regulate the way that the information in genes is turned into gene products...
- Transcription factorTranscription factorIn molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
- Glossary of gene expression termsGlossary of gene expression termsSee also: gene expression, List of Glossaries, List of Natural Sciences Glossaries-A:*Activator - protein that binds to an enhancer and activates transcription from nearby promoter....