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| Repressible Operon |
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| Another type of operon system is the repressible operon which is regulated by a chemical substance called co-repressor. It is almost always the end product of a metabolic reaction. The operon is switched off when the co-repressor, is present. |
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| Tryptophan operon of E. coli is an example of repressible operon. It has the following components. |
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| These genes transcribe mRNAs, which are translated by ribosomes and tRNAs into the polypeptides. The latter function as enzymes. The trytophan operon comprises 5 structural genes denoted as E, D, C, B, and A. |
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| This gene controls a specific series of structural genes, and lies adjacent to them. It remains turned on because it is not bound by the repressor protein produced by the regulator gene. Therefore, the cistrons keep functioning, producing enzymes. The latter catalyse the synthesis of tryptophan from the raw materials. |
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| If tryptophan is added to the culture and it is taken up by the bacterium, it acts as a co-repressor and binds to the repressor. The repressor co-repressor complex so formed binds with the operator gene, which gets turned off. This stops the expression of the structural genes. When the organism gets tryptophan in excess from outside, it conserves its raw materials and energy by suspending the synthesis of this compound in its own body. This phenomenon is called feedback repression. |
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| fig. 23.3 - Repressible Operon |
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| This gene lies adjacent to the operator gene. It marks the site where the RNA polymerases enzyme binds. When the operator gene is turned on, this enzyme moves over the operator gene to the structural genes, and transcription starts. |
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| It controls the operator gene in cooperation with a substance called co repressor present in the cytoplasm. The regulator gene codes for a protein named aporepressor. The latter combines with the co-repressor, and aporepressor-co-repressor complex joins the operator gene, which is turned off. The structural genes now stop transcription, and enzyme synthesis comes to an end. |
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| In the tryptophan operon, the end product, i.e., tryptophan acts as a co- repressor. |
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| The repressible operon generally functions in the anabolic pathways. |
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| The co-repressor is a non-protein compound, which may come from outside or from metabolism within the cell. Its concentration controls transcription in the cell. If concentration of a co repressor rises, apo-repressor-co-repressor complex is formed, the complex binds with the operator gene and further production of co-repressor stops. If the co-repressor level falls, aporepressor does not join the operator gene. This restarts the formation of co-repressor. |
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| The end product is generally utilized in some other metabolic pathway so that it does not accumulate and fails to act as a co-repressor. If somehow it accumulates or is received from outside, it combines with a repressor and stops transcription. |
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