Control Mechanism - Repression

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Regulation of Protein Synthesis - Operon Structural Genes Operator Genes Promoter Gene Regulator Gene Control Mechanism - Induction Control Mechanism - Repression Inducible System Repressible System Negative Control of Protein Synthesis Positive Control of Protein Synthesis Mutation in the Operon

Control Mechanism - Repression

	Control Mechanism - Repression - The synthesis of an enzyme may be inhibited by a product of metabolism. In E. coli tryptophan synthetase is inhibited by tryptophan. Thus tryptophan synthetase is formed only if the bacterium is grown ill a tryptophan free medium. The amino acid histidine also acts as a repressor. This process is called enzyme repression. In the repressible system the protein produced by the regulator gene is an inactive repressor, also called the aporepressor.
The aporepressor on combining with a corepressor or effector molecule becomes activated. It undergoes a conformational change which enables it to bind to the operator gene. When the corepressor is absent the inactive aporepressor does not bind to the operator gene. Because the operator gene is not blocked, the structural genes transcribe mRNA, and protein synthesis takes place
When the corepressor is present the aporepressor combines with it to form a complex, and is activated. The complex blocks the operator gene. The structural genes are therefore unable to synthesize mRNA, and consequently no protein synthesis takes place. The corepressor may be a product of one of the enzymes produced by tbe structural genes.

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Regulation of Gene Activity in Eukaryotes Frenster's Model of Gene Regulation Nonhistone Protein as Derepressors Britten Davidson Operon Operator Model Sensor Genes Producer Genes Integrator Genes Receptor Genes