A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of transcription. This blocking of expression is called repression. DNA-binding proteins are a broad class of protein molecules that possess certain structural motifs (e. ... Gene expression, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ...

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. A regulator gene is a gene that codes for a protein or RNA involved in regulating the expression of other genes. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... An operator is a segment of DNA that regulates the activity of the structural genes of an operon it is linked to, by interacting with a specific repressor or activator. ... This article does not cite any references or sources. ... The life cycle of an mRNA in a eukaryotic cell. ...

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). An inducer is a molecule in biology that starts gene expression. ...

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 is a homodimer consisting of two monomers which each provide 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. Methionine is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2SCH3. ... Sucrose, or common table sugar, is composed of glucose and fructose. ... In chemistry, a monomer (from Greek mono one and meros part) is a small molecule that may become chemically bonded to other monomers to form a polymer. ... Side view of an α-helix of alanine residues in atomic detail. ... Two antiparallel molecules run side-by-side in opposite directions. ... Diagram of β-pleated sheet with H-bonding between protein strands The β sheet (also β-pleated sheet) is the second form of regular secondary structure in proteins — the first is the alpha helix — consisting of beta strands connected laterally by three or more hydrogen bonds, forming a generally twisted, pleated sheet. ... In mathematics, an operator is a function that performs some sort of operation on a number, variable, or function. ... Sucrose, or common table sugar, is composed of glucose and fructose. ...

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-purine step that becomes over-twisted 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. For the movie, see Palindromes (film). ... In genetics, dyad symmetry refers to two areas of a DNA molecule whose base pair sequences are repeats of each other, inverted relative to each other, or are palindromes. ...

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. A cofactor is the following: In mathematics a cofactor is the minor of an element of a square matrix. ... This article does not cite its references or sources. ...

See also

• lac repressor

Lac Repressor molecule more details. ...

External links

• MeSH Repressor+Proteins