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What is operon?
An operon is defined as the collection of genes that function together and are transcribed simultaneously from a promoter to a m-RNA. They usually exist in prokaryotes but certain operons exist in eukaryotes too.
Who discovered operon?
The operon was discovered by Jaques Monod and Francois Jacob in 1961. In bacteria, the first operon known as lac operon was discovered. During the early days of the invention, they found that the bacteria started producing enzymes for lactose metabolism only in the presence of lactose and absence of glucose. Then, they started studying the lac operon and its functioning with a keen vision of knowledge. Their discovery of lac operon still stands out to be an outstanding example of gene regulation.
The discovery of operon was accidental and it happened when two scientists were conducting research on the same organism but of different topics. Then, suddenly they realized that the combination of their findings would give a better and outstanding result. Thus, the operon is created which actually comes from the word “to operate”. The operator region and the repressor functions as the regulators of the operon. The structural genes code for the proteins essential for the regulation of the metabolic pathways and the promoter region is absolutely important for the expression of the gene.
Functioning of operon:
The operon has the promoter, operator and certain genes of structure.
➔ Promoter:
It is the region for the RNA polymerase to bind and transcribe the DNA sequences into RNA which is the first step of gene expression.
➔ Operator:
It is the region where the regulatory proteins such as activators and repressors bind and regulate the gene expression.
➔ Genes of structure:
These genes are involved in the transcription which form proteins which are actively involved in the metabolic pathways.
Types of operon:
The operon is basically divided into two main types namely, inducible and repressible operon.
➔ Inducible operon:
It is usually inactive until an inducer converts it to an active state. The repressor protein is bound by the inducer which in turn prevents the binding of the repressor to the operator region. It is normally present in the catabolic pathways where molecules are broken down to produce energy. E.g. Lac operon
➔ Repressible operon:
It is usually active and a repressor molecule converts it to an inactive state. The repressor molecule binds to the operator region and prevents the functioning of the operon. It is normally present in the anabolic pathways where molecules are formed to create an impact in energy. E.g. Trp operon
Lac operon:
It is an inducible operon and it has regions like promoter, operator, cAMP molecule in CAP , regulatory genes of lac I and structural genes of lac A, lacY and lac Z.
➔ Promoter:
It is the region for the RNA polymerase to bind to initiate transcription, which is necessary for the metabolic pathways. This site is very much needed for the selective operon to be active.
➔ Operator:
It is the region for the repressor molecules to bind which in turn switches off the operon. It is adjacent to the promoter site and inhibits the expression of the structural genes.
➔ cAMP molecule in CAP:
The cAMP molecule binds to the Catabolite Activator Protein (CAP) and positively regulates the lac operon. When there is low glucose in the body, the cAMP molecule binds to the CAP and the lac operon is induced to convert lactose into glucose and galactose.
➔ Regulatory gene of lac I:
In lac operon, lac I gene synthesizes the repressor protein which is later bound to the operator region to turn off the operon. It is known to follow negative regulation.
➔ Structural gene of lac A:
This gene codes for the protein transacetylase which is an enzyme. There is still research going on to understand the metabolic pathway followed by this specific enzyme.
➔ Structural gene of lac Y:
It codes for the protein lactose permease which is an enzyme for translocating lactose into the cell.
➔ Structural gene of lac Z:
This gene codes for the enzyme 𝜷-galactosidase which breaks down lactose into glucose and galactose
Trp operon:
It is a repressible operon and it has promoter, operator, regulatory gene of trp R and structural genes of trp A, trp B, trp C, trp D and trp E. It follows the process of attenuation.
➔ Promoter:
Its function is the same as that of the lac operon. The RNA Polymerase binds to the promoter region and initiates the gene expression.
➔ Operator:
It is also present adjacent to the promoter region, just like the lac operon.
➔ Regulatory gene of trp R:
The trp R gene codes for the repressor protein. Here, the trp repressor is bound to the tryptophan which in turn is bound to the operator region and the gene expression is turned off. It is known as negative regulation.
➔ Structural genes:
The structural genes of trp A, trp B, trp C, trp D and trp E code for the enzymes which produce tryptophan from chorismic acid.
