Repressible operons require that the a product binds to the repressor protein before it can bind to the operator.
The repressor binds to the operator gene and prevents it from initiating the synthesis of the protein called for by the operon. The presence or absence of certain repressor molecules determines whether the operon is off or on. 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. The repressor protein works by binding to the promoter region of the gene(s), which prevents the production of messenger RNA (mRNA)
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