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| Inducible Operon |
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| This is a type of operon which is switched on when a chemical, called inducer, is present. The inducer is almost always a substrate. This operon has following components. |
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| These genes produce mRNAs (transcription), which in turn synthesize specific polypeptides on the ribosomes (translation). The polypeptides so formed act as enzymes to catalyse some metabolic reaction in the cell. An operon may have one or more structural genes. The lactose or lac operon of E. coli has 3 structural genes: Z, Y, A required for breakdown of glucose. These transcribe a polycistronic mRNA which the ribosomes and tRNA translate into three polypeptides or enzymes: -galactosidase, lactose permease and transacetylase. |
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| The operator gene controls the functioning of a specific series of structural genes. It lies adjacent to the structural genes which it controls. The cistrons are expressed when the operator gene is turned on by an inducer, and are not expressed when the operator gene is turned off by a repressor. The repressor physically blocks the binding of RNA polymerase to the promoter and so keeps the gene turned off. |
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| This gene is located adjacent to the operator gene. It has two functions. |
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| 1.The promoter gene marks the site at which the RNA polymerase enzyme binds. When the operator gene is turned on, the enzyme moves over the operator gene to reach the structural genes and the transcription of mRNA starts. |
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| 2. The base sequence of promoter gene determines which DNA strand acts as a template for mRNA transcription. |
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| fig. 23.1 - Operon Model - Inducible System |
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| This gene controls the operator gene in cooperation with a chemical substance called inducer present in the cytoplasm. A regulator gene is not necessarily adjacent to the operator gene which it controls. The regulator gene codes for and produces a protein called repressor. The repressor protein binds with the operator gene to repress (stop) its action. |
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| A repressor is a protein produced by a regulator gene. It is called a regulatory protein as it serves to turn off the operator gene by binding with the latter. With this binding, the transcription of mRNA by a structural gene stops. |
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| Inducer is a chemical (most often a substrate or any other metabolite) which inactivates the repressor of an operon. The inducer binds with the repressor, forming a repressor-inducer complex. This complex prevents the repressor from binding with the operator gene of the operon. This frees the operator gene so that the RNA polymerase can move from the promoter to the structural genes. The structural genes are depressed and are then transcribed, forming a piece of polycistronic mRNA. The latter is translated by ribosomes and tRNAs into enzymes (Polypeptides). The entire process is called enzyme induction. The enzyme produced in response to the presence of an inducer is called inducible enzyme. |
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| When lactose is added to a culture of E. coli, a few lactose molecules are carried into the cell by the enzyme lactose permease, as a small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are converted into an active form which acts as an inducer and binds with the repressor, a product of the regulator gene. The repressor-inducer complex fails to join with the operator gene, which is turned on. The three structural genes are expressed as three enzymes to metabolise lactose to galactose and glucose. Lactose, thus, acts as the inducer for its own breakdown. |
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| When the concentration of inducer falls, having been metabolised by the enzymes, the operator is blocked again by the repressor so that the structural genes are repressed once again. This is an example of a negative feedback mechanism. |
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| The inducible operons generally function in catabolic pathways. |
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