Repressor

In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers.

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.

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.

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.

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.

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] + The FLC operon is a conserved eukaryotic locus that is negatively associated with flowering via repression of genes needed for the development of the meristem to switch to a floral state in the plant species Arabidopsis thaliana.

FLC expression has been shown be regulated by the presence of FRIGIDA, and negatively correlates with decreases in temperature resulting in the prevention of vernalization.

[8] FLC genes also have a large number of homologues across species that allow for specific adaptations in a range of climates.

The lac operon : 1 : RNA Polymerase, 2 : lac repressor , 3 : Promoter, 4 : Operator, 5 : Lactose, 6 : lacZ, 7 : lacY, 8 : lacA. Top : The gene is essentially turned off. There is no lactose to inhibit the repressor, so the repressor binds to the operator, which obstructs the RNA polymerase from binding to the promoter and making lactase. Bottom : The gene is turned on. Lactose is inhibiting the repressor, allowing the RNA polymerase to bind with the promoter, and express the genes, which synthesize lactase. Eventually, the lactase will digest all of the lactose, until there is none to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.
Structure of L-arabinose operon of E. coli. The work was uploaded by Yiktingg1 in wikimedia commons. https://commons.wikimedia.org/wiki/File:L-arabinose_structure.png#filehistory