4.7 Operon concept :
It is a transcriptional control mechanism of gene regulation.
Francois Jacob and Jacques Monod (1961) explained that metabolic pathways are regulated as a unit.
For example in E.coli, when lactose sugar is provided to the culture medium, cell induces production of three enzymes necessary for digestion of lactose.
i. β-galactosidase : Digests lactose into galactose and glucose.
ii. β-galactoside permease : Permits lactose molecules to enter into the cell.
iii. Transacetylase (β-Galactoside acetyltransferease) : Transfers an acetyl group from acetyl CO-A to galactoside.
Synthesis of these three enzymes, is controlled by a long segment of DNA known as Operon. It consists of an operator site O and three structural genes Z, Y and A .The action of structural genes is regulated by operator site with the help of a repressor protein.
Repressor protein is produced by the action of gene i (inhibitor) known as regulator gene. The gene expression depends on whether operator is switched on or switched off.
If the operator is switched on, the three genes z, y and a are transcribed by RNA Polymerase into a single m-RNA. Each structural gene is generally known as cistron and the transcribed long m-RNA covering various cistrons is known as Polycistronic.
Switching on or switching off of the operator is achieved (acomplished) by a protein called repressor. When this protein is attached to the operator and blocks it, the switch is turned off and structural genes are not expressed.
Lac operon :
Lactose or lac operon of E.coli is inducible operon. The operon is switched on when a chemical inducer- lactose is present in the medium. Lac operon consists of following components :
1. Regulator gene (repressor gene)
2. Promoter gene
3. Operator gene
4. Structural genes
5. Inducer - It is not a component of operon.
1. Regulator gene :
This gene controls the operator gene in cooperation with an inducer present in the cytoplasm.
Regulator gene preceeds the promoter gene. It may not be present immediately adjacent to operator gene.
Regulator gene produces a protein called repressor protein.
Repressor binds with operator gene and represses (stops) its action. It is called regulator protein.
2. Promoter gene :
This gene preceeds the operator gene. It is present adjacent to operator gene.
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 and transcription of structural genes starts.
Promoter gene base sequence determines which strand of DNA acts a template.
3. Operator gene :
It preceeds the structural genes. This controls the functioning of structural genes. It lies adjacent to the Structural genes. When operator gene is turned on by an inducer, the Structural genes produce m-RNA. Operator gene is turned off by a product of repressor gene.
4. Structural gene :
When lactose is added to the E.coli culture, the structural genes catalyse (produce) m-RNA which in turn produces polypeptides, on the ribosomes. The polypeptides formed, act as enzymes to caltalyse lactose in the cell. There are 3 structural genes in the sequence lac-Z, lac-Y and lac-A. Enzymes produced are β-galactosidase, β-galactoside permease and transacetylase respectively
5. Inducer :
It is a chemical in the cytoplasm (allolactose) which inactivates the repressor. When lac operon is switched on, then inducer joins with repressor protein preventing the binding of repressor to the operator gene. So the Operator gene is free and now enzyme RNA polymerase can move from promoter to structural genes via operator gene.
Role of lactose :
A few molecules of lactose enter into the cell by an enzyme permease. A small amount of this enzyme is present even when operon is switched off.
A few molecules of lactose, act as inducer and bind to repressor. This repressor–inducer complex fails to join with the operator gene, which is then turned on.
Structural genes produce all enzymes. Thus, lactose acts as an inducer of its own break down. When the inducer level falls, the operator is blocked again by repressor. So structural genes are repressed/inactivated again. This is negative feedback.
