In bacteria, the genes that encode and regulates the enzymes of a metabolic pathway are usually clustered together. This set of genes responsible for encoding and regulating a metabolic pathway in prokaryotes is called OPERON.
The concept of operon was first described by two scientists Francois Jacob and Jacques Monod In E. coli. Each operon consists of 4 components:
Promoter:- The promoter region is a part of DNA that acts as binding site for the enzyme RNA polymerase. The RNA polymerase after binding to the promoter region can initiate the transcription of the genes associated with it.
Operator:- The operator serves as a binding site for the repressor protein coded by the regulatory gene. Usually the operator is located adjacent to or overlaps the promoter region of the structural gene.
Regulatory gene:- The regulatory gene codes for Repressor protein. The repressor protein is a gene regulatory protein which binds to the operator region of DNA with high affinity.
Structural gene:- The main genes responsible for coding enzymes that are associated with a particular metabolic pathway are called the structural genes. All the structural genes are located adjacent to each other having a common promoter and operator region. The RNA polymerase on binding with the promoter region transcribes all the structural genes into a single polycistronic mRNA.
[Polycistronic- When a single mRNA codes for more than one polypeptide chain.]
lac operon refers to the set of genes responsible for metabolism of the disaccharide, Lactose. Jacob and Monod observed that when an E.coli is grown in a medium containing Glucose as carbon source, the enzymes for breakdown of Glucose (related to glycolytic pathway) are synthesised but the bacteria lacked the enzymes required for lactose breakdown. However, when these bacteria were transferred to a medium containing Lactose as carbon source, within a few minutes the bacteria started producing enzymes required to metabolise Lactose.
This experiment proved that the bacteria contains the information in DNA required for enzymes related to lactose metabolism but these genes are only expressed when lactose is available to be utilised. It was found that the presence of lactose itself acts as inducer for the lac operon which activates the structural genes associated to it.
1) Lactose- Absent
The RNA polymerase binds to the promoter of Regulatory gene and transcribes a mRNA called repressor mRNA
The repressor mRNA is translated to Repressor protein by ribosomes.
The repressor protein binds to the operator region located upstream of the structural gene.
RNA polymerase cannot transcribe the structural gene due to binding of repressor protein to the operator.
Structural genes are not transcribed, as a result no enzymes are produced by the structural gene.
Thus, in absence of lactose bacteria and conserve its energy by not synthesising the enzymes required for lactose metabolism
2) Lactose- Present
The RNA polymerase binds to the promoter of Regulatory gene and transcribes a repressor mRNA.
Repressor mRNA is translated to Repressor protein.
Lactose binds to the repressor proteins causing its inactivation.
The inactive repressor cannot bind to the operator region.
RNA polymerase binds to the promoter of the structural gene and transcribes all the structural genes into a single polycistronic mRNA.
There are three structural genes of lac operon located adjacent to each other i.e. lac Z, lac Y and lac A.
lac Z gene codes for the enzyme B-galactosidase which catalyses the hydrolysis of bonds present between glucose and galactose in lactose molecule
lac Y gene codes for the enzyme Permease which increases the permeability of the cell membrane for Lactose.
lac A gene codes for the enzyme Transacetylase. The exact function of transcetylase is not known but according to some theories it transfers the acetyl group to a toxic molecule resulting in its inactivation.
These 3 enzymes breakdown the Lactose molecules enabling the bacteria to use it as an energy source.
Thus, in presence of Lactose enzymes required for breakdown of Lactose is produced in the cell.