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 repression.

Repressor proteins are coded for by regulator genes. Repressor proteins then attach to a DNA segment known as the operator. By binding to the operator, the repressor protein prevents the RNA polymerase from 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 corepressor is a molecule that can bind to repressor and make it bind to the operator tightly, which decreases transcription. A repressor that binds with a corepressor is termed an aporepressor or inactive repressor. One type of aporepressor is the 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).

Examples
An example of a repressor protein is the methionine repressor MetJ.MetJ interacts with DNA bases via a pair of α-helices. MetJ is a homodimer consisting of two monomers, which each provides a beta ribbon and an alpha helix. Together, the beta ribbons of each monomer come together to form an antiparallel beta-sheet which binds to the DNA 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 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 pyrimidine-purine step that becomes positively supercoiled forming a kink in the 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.

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.

Another example is the lac operon 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. Lactose is called an inducer because it inactivates the repressor and induces transcription.