Repressors

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. A DNA-binding repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA. An RNA-binding repressor binds to the mRNA and prevents translation of the mRNA into protein. This blocking of expression is called repression.

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). Within the Eukaryotic genome are regions of DNA known as silencers. These DNA sequences bind to repressors to partially or fully repress the expression of 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 the promoter. This causes chromosome looping, allowing the promoter region and the silencer region to come to close proximity.

lacZYA transcribes the proteins needed for lactose breakdown.lacI synthesizes the repressor of the lacZYA gene. The gene lacI is situated immediately upstream of lacZYA but is transcribed from a different promoter. The lacI gene synthesizes LacI repressor protein. The LacI repressor protein represses lacZYA by binding to the operator sequence lacO.

...
Wikipedia