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repressor protein

(Science: molecular biology) A protein that binds to an operator of a gene preventing the transcription of the gene. The binding affinity of repressors for the operator may be affected by other molecules. Inducers bind to repressors and decrease their binding to the operator, while co repressors increase the binding. The paradigm of repressor proteins is the lactose repressor protein that acts on the lac operon and for which the inducers are _ galactosides such as lactose, it is a polypeptide of 360 amino acids that is active as a tetramer. Other examples are the lambda repressor protein of lambda bacteriophage that prevents the transcription of the genes required for the lytic cycle leading to lysogeny and the cro protein, also of lambda, which represses the transcription of the lambda repressor protein establishing the lytic cycle. Both of these are active as dimers and have a common structural feature the helix turn helix motif that is thought to bind to dNA with the helices fitting into adjacent major grooves.