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 reducing of expression is called repression.
Function
If an
inducer, a molecule that initiates the gene expression, is present, then it can interact with the repressor protein and detach it from the operator. RNA polymerase then can transcribe the message (expressing the gene). A co-repressor
is a molecule that can bind to the repressor and make it bind to the operator tightly, which decreases transcription.
A repressor that binds with a co-repressor is termed an aporepressor or inactive repressor. One type of
trp repressor, an important metabolic protein in bacteria. The above mechanism of repression is a type of a feedback mechanism because it only allows transcription to occur if a certain condition is present: the presence of specific inducer
(s). In contrast, an active repressor binds directly to an operator to repress gene expression.
While repressors are more commonly found in prokaryotes, they are rare in eukaryotes. Furthermore, most known eukaryotic repressors are found in simple organisms (e.g., yeast), and act by interacting directly with activators.[1] This contrasts prokaryotic repressors which can also alter DNA or RNA structure.
Within the eukaryotic genome are regions of DNA known as
silencers. These are DNA sequences that bind to repressors to partially or fully repress a gene. Silencers can be located several bases upstream or downstream from the actual promoter of the gene. 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 in proximity of each other.
The lacZYA operon houses genes encoding proteins needed for lactose breakdown.[2] The lacI gene codes for a protein called "the repressor" or "the lac repressor", which functions to repressor of the lac operon.[2] The gene lacI is situated immediately upstream of lacZYA but is transcribed from a lacI promoter.[2] The lacI gene synthesizes LacI repressor protein. The LacI repressor protein represses lacZYA by binding to the operator sequence lacO.[2]
The lac repressor is
constitutively expressed and usually bound to the operator region of the promoter, which interferes with the ability of RNA polymerase (RNAP) to begin transcription of the lac operon.[2] In the presence of the inducerallolactose, the repressor changes conformation, reduces its DNA binding strength and dissociates from the operator DNA sequence in the promoter region of the lac operong. RNAP is then able to bind to the promoter and begin transcription of the lacZYA gene.[2]
met operon repressor
An example of a repressor protein is the
operator ("Met box") in its major groove. Once bound, the MetJ dimer
interacts with another MetJ dimer bound to the complementary strand of the operator via its alpha helices. AdoMet binds to a pocket in MetJ that does not overlap the site of DNA binding.
The Met box has the DNA sequence AGACGTCT, a
phosphodiester
backbone. This is how the protein checks for the recognition site as it allows the DNA duplex to follow the shape of the protein. In other words, recognition happens through indirect readout of the structural parameters of the DNA, rather than via specific base sequence recognition.
(SAM) which is a product in the biosynthesis of methionine. When SAM is present, it binds to the MetJ protein, increasing its affinity for its cognate operator site, which halts transcription of genes involved in methionine synthesis. When SAM concentration becomes low, the repressor dissociates from the operator site, allowing more methionine to be produced.
In the absence of Arabinose and araC (repressor), loop formation is not initiated and structural gene expression will be lower. In the absence of Arabinose but presence of araC, araC regions form dimers, and bind to bring ara02 and araI1 domains closer by loop formation.[5] In the presence of both Arabinose and araC, araC binds with the arabinose and acts as an activator. This conformational change in the araC no longer can form a loop, and the linear gene segment promotes RNA polymerase recruitment to the structural araBAD region.[4]
cofactors act as negative transcription factors for FLC genes.[8] FLC genes also have a large number of homologues across species that allow for specific adaptations in a range of climates.[9]