:"For other uses, see
repression"A repressor is a DNA-binding proteinthat regulates the expression of one or more genes by decreasing the rate of transcription. This blocking of expression is called repression.
Repressor proteins are coded for by
regulator genes. Repressor proteins then attach to a DNAsegment known as the operator. By binding to the operator, the repressor protein prevents the RNA polymerasefrom creating messenger RNA.
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
corepressoris a molecule that can bind to repressor and make it binds to operator tightly, which decreases transcription.
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).
An example of a repressor protein is the methionine repressor
MetJ. MetJ is a homodimerconsisting of two monomerswhich each provide a beta ribbonand an alpha helix. Together, the beta ribbons of each monomer come together to form an antiparallel beta-sheetwhich binds to the DNA operator("Met box") in its major groove. Once bound the MetJ dimerinteracts with another MetJ dimer bound to the complementary strand of the operator via its alpha helices.
The Met box has the sequence AGACGTCT which is a
palindrome(it shows dyad symmetry) allowing the same sequence to be recognised on either strand of the DNA. The junction between c and g in the middle of the Met box contains a pyramidine-purinestep that becomes over-twistedforming a kink in the phosphodiesterbackbone. This is how the protein checks for the recognition site as it allows the DNA duplex to follow the shape of the protein.
Each MetJ dimer contains two binding sites for the
cofactor S-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.
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