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1. BRD MEDICAL COLLEGE GORAKHPUR DEPARTMENT OF BIOCHEMISTRY TOPIC -LAC OPERON CONCEPT NAME-PRAJWAL KESARWANI ROLL NO. - 92

2. CONTENT THE LAC OPERON REPRESSION OF LAC OPERON DEREPRESSION or INDUCTION OF LAC OPERON CLINICAL

3. THE LAC OPERON Operon is a unit of gene expression. The concept was described by Jacob and Monad. It deals with lactose metabolism in E.coli (bacteria). It includes structural genes , control elements ,regulator/inhibitor gene , promoter and operator areas. In bacterial cell,the Z , Y , A gene code for the structure of proteins, they are called structural genes. These 3 genes are present as contiguous segments of DNA .

4. In the bacterial cell ,the Z gene encodes beta- galactosidase , the enzyme which hydrolyses lactose to glucose and galactose . The Y gene is responsible for production of a permease which transports lactose and galactose into the cell. The A gene codes for thiogalactoside transacetylase . The transcription of these genes start from a common promoter (P),located close to the Z gene. The RNA Polymerase binds to the promoter and transcribes these 3 structural genes as a single mRNA.

5. TRANSCRIPTION IS NORMALLY REPRESSED Transcription of the structural gene is under the control of another regulator or the “i” (inhibitor) gene. Regulatory genes produces a repressor molecule. The repressor has strong affinity to the operator site and binds to it . The operator site is between the promoter and structural genes.

6. When the RNA Polymerase identifies the promoter sequence and moves toward the structural genes. It is stopped by the hindrance produced by repressor molecule. Hence the RNAP cannot move further and and transcribe the structural genes. Thus when lactose is not available,the lactose utilizing enzymes are not synthesized.

7. DEREPRESSION OF LAC OPERON When lactose is introduced into the medium,it binds to the repressor. Repressor-lactose complex is inactive and does not bind to the operator region. RNAP can transcribe the structural genes,which are then translated. Lactose switches the genes “on”. Lactose induces the synthesis of lactose utilizing enzymes. Hence it is an inducer of these genes and the mechanism is said to be derepression of the gene.

8. In the case of mutation in lac operon ,the product is not able to bind the operator site . Mutation in the lac operator region will not allow binding of normal repressor molecule.

9. CLINICAL APPLICATIONS OF DEREPRESSION Lactase in human intestine is an inducible enzyme. In humans , examples of derepression include- induction of tryptophan pyrrolase and transaminases by glucocorticoids Aminolevulinic acid(ALA) synthase and glucuronyl transferase by barbiturates.

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