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Tryptophan Operon
A repressible operon is an operon that is normally "on" but can be turned "off" under certain circumstances. A common example of a repressible operon in bacterial cells is the tryptophan operon (also known as Trp operon). The tryptophan operon is used to regulate the gene expression of the enzymes needed to synthesize tryptophan. When the cell has a low supply of the amino acid tryptophan, the inactive form of the repressor protein cannot bind to the operator and so the RNA polymerase binds to the promoter and synthesizes the mRNA molecules needed to produce the enzymes that are used by the cell to synthesize tryptophan molecules. However, when the intracellular concentration of tryptophan is high, then the tryptophan can act as a co-repressor. At high concentrations, tryptophan binds to the repressor protein and activates it. The active form of the repressor protein can now bind onto the operator region of the operon, thereby blocking the ability of RNA polymerase to bind to the promotor and express the gene. In such a case, the genes will not be expressed and tryptophan is not produced. In this manner, the bacterial cell can regulate the gene expression for these particular genes.
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