Inmolecular genetics, arepressor is aDNA- or RNA-binding protein that inhibits theexpression of one or more genes by binding to theoperator or associatedsilencers. A DNA-binding repressor blocks the attachment ofRNA polymerase to thepromoter, thus preventingtranscription of the genes intomessenger RNA. An RNA-binding repressor binds to the mRNA and preventstranslation of the mRNA into protein. This blocking or reducing of expression is calledrepression.
If aninducer, 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 cantranscribe the message (expressing the gene). Aco-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 anaporepressor orinactive repressor. One type ofaporepressor is thetrp 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 specificinducer(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 assilencers. 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 thepromoter. This causes chromosome looping, allowing the promoter region and the silencer region to come in proximity of each other.
ThelacZYA operon houses genes encoding proteins needed for lactose breakdown.[2] ThelacI gene codes for a protein called "the repressor" or "the lac repressor", which functions to repressor of the lac operon.[2] The genelacI is situated immediately upstream oflacZYA but is transcribed from alacI promoter.[2] ThelacI gene synthesizes LacI repressor protein. The LacI repressor protein represseslacZYA by binding to the operator sequencelacO.[2]
Thelac repressor isconstitutively expressed and usually bound to theoperator region of thepromoter, which interferes with the ability ofRNA polymerase (RNAP) to begin transcription of thelac operon.[2] In the presence of theinducerallolactose, 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 thelacZYA gene.[2]
An example of a repressor protein is themethionine repressor MetJ. MetJ interacts withDNA bases via a ribbon-helix-helix (RHH) motif.[3] MetJ is ahomodimer consisting of twomonomers, which each provides abeta ribbon and analpha helix. Together, the beta ribbons of each monomer come together to form anantiparallelbeta-sheet which binds to the DNAoperator ("Met box") in its major groove. Once bound, the MetJdimer interacts with another MetJ dimer bound to the complementary strand of the operator via its alpha helices. AdoMet binds to a pocket in MetJ thatdoes not overlap the site of DNA binding.
The Met box has the DNA sequence AGACGTCT, apalindrome (it showsdyad symmetry) allowing the same sequence to be recognized on either strand of the DNA. The junction between C and G in the middle of the Met box contains apyrimidine-purine step that becomespositively supercoiled forming a kink in thephosphodiester 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.
Each MetJdimer contains two binding sites for thecofactorS-Adenosyl methionine (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.
TheL-arabinose operon houses genes coding for arabinose-digesting enzymes. These function to break down arabinose as an alternative source for energy when glucose is low or absent.[4] Theoperon consists of a regulatory repressor gene (araC), three control sites (ara02, ara01, araI1, and araI2), two promoters (Parac/ParaBAD) and three structural genes (araBAD). Once produced, araC acts as repressor by binding to the araI region to form a loop which preventspolymerases from binding to the promotor andtranscribing the structural genes into proteins.
In the absence ofArabinose 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 promotesRNA polymerase recruitment to the structural araBAD region.[4]
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TheFLC operon is a conserved eukaryotic locus that is negatively associated with flowering via repression of genes needed for the development of themeristem to switch to a floral state in the plant speciesArabidopsis thaliana. FLC expression has been shown be regulated by the presence ofFRIGIDA, and negatively correlates with decreases in temperature resulting in the prevention ofvernalization.[6] The degree to which expression decreases depends on the temperature and exposure time as seasons progress. After the downregulation of FLC expression, the potential for flowering is enabled. The regulation of FLC expression involves both genetic andepigenetic factors such ashistone methylation andDNA methylation.[7] Furthermore, a number of genes arecofactors 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]
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