Lac Operon PDF

## Lac Operon ### Operon Hypothesis - An operon is a group of closely linked genes (structural and control genes), which regulate the metabolic pathway in prokaryotes. - The operon hypothesis was put forward by ___Jacob and Jacques Monad___ while studying the catabolism of lactose in *E. coli*. -...

## Lac Operon ### Operon Hypothesis - An operon is a group of closely linked genes (structural and control genes), which regulate the metabolic pathway in prokaryotes. - The operon hypothesis was put forward by ___Jacob and Jacques Monad___ while studying the catabolism of lactose in *E. coli*. - For this discovery, they were awarded with the Nobel prize in 1965. - A bacterium contains thousands of genes, when all genes function at the same time, the cell will be flooded with enzymes and proteins. - So, the genes for the required enzymes at that particular time are "Switched on" and the other genes are "Switched off". - This on and off mechanism is explained by the **Operon model**. - The action of the genes is regulated at the transcriptional level by **induction** and **repression** phenomena. ### Lac operon can be explained using TWO phenomena: - **Induction and Repression Phenomena** #### Induction: - A set of genes will be switched on when there is necessity to metabolize a new substrate. - When these genes are switched on, enzymes are produced. - This phenomenon is called **induction**. #### Repression: - When __a metabolite____ needed by a bacterium is synthesized in excess in the medium, bacterium stops its synthesis, and the genes associated with its metabolism are turned off. - This phenomenon is called **feed back repression** or **end-product inhibition**. ### Lactose Operon - Lac operon is a set of genes responsible for the metabolism of lactose in *E. coli*. - The lac operon was discovered by Jacob and Monad in 1961. ### Structure of an Operon - The genes composing operon are classified into 2 categories; 1. **Structural genes** - Lac Z,Y,A 2. **Control genes or Regulatory genes** #### I. Structural Genes: - The structural genes are segments of DNA that are associated with the synthesis of enzymes needed for the catabolism of lactose. It includes, - **Lac Z gene** - Codes for B-Galactosidase enzyme - **Lac Y gene** - Codes for galactose permease enzyme - **Lac A gene** - Codes for thiogalactoside transacetylase - These gene products are produced from a single polycistronic mRNA molecule. - The structural genes are responsible for the synthesis of three enzymes, namely B-galactosidase by the gene Z, galactoside permease by the gene Y and thiogalactoside transacetylase by the gene X. #### II. Control genes or regulatory genes: - These genes control the activity of structural genes either by induction or suppression. - They are present immediately adjacent to the structure genes. - There are three control genes namely, ##### Operator Gene (O) - It is the segment of DNA, overlaps with the promoter sequence. - It controls the transcription of mRNA from structure genes. - The repressor protein produced by lac L gene, binds with the operator gene. ##### Promoter gene (Plac) - It is the segment of DNA, present immediately adjacent to the operator gene. - RNA polymerase binds specifically to the promoter region and initiates the transcription of the structure genes. - It controls the rate of mRNA synthesis. ##### Repressor or Regulator gene (L) - Lac repressor is a tetrameric regulatory protein. - It lies outside the operon and produces a repressor molecule. - Repressor gene has its own promoter. - This repressor protein molecule binds to the operator gene and suppresses the transcription of three structural genes. ### Regulation of Lac Operon in Escherichia Coli #### I). Negative regulation: (Lac operon is switched OFF) ##### Conditions: - When Lactose is absent; - When Glucose is more in the cell; - When lactose is present in the medium and glucose level is also more in the cell. ##### Events: - Repressor is active - Repressor bound to the operator/Promoter region - Operon is repressed - Transcription is blocked and - No mRNA is synthesized - No gene product is produced - Lac operon is switched off This repressor molecule acts as a negative regulator of gene expression. ___ #### II). Positive regulation of lac operon: (Lac operon is switched ON) ##### Conditions: - When Lactose is present/added in the medium; - When glucose level is low in the medium ##### Events: - Repressor bound to the lactose, so repressor no longer binds to the operator - Operon is depressed - Transcription begins and - mRNA synthesis occur - Synthesis of enzymes takes place When lactose (inducer) is introduced into the medium, it diffuses into the cell and binds to the repressor protein to form an inactive inducer - repressor complex. The inactive inducer - repressor complex cannot bind to the operator gene. The operator is therefore, free and now the RNA polymerase binds to the initiation site on the promoter and transcribe the cistrons into polycistronic mRNA which codes for three enzymes, necessary for lactose catabolism. Lactose --> B-gal. --> Glucose + Galactose ### Effect of glucose on lac operon: - Effect of glucose on the activity of the lac operon: - The cells of *E. coli* utilize glucose in preference to lactose, when both of them are present in the medium. - After the depletion of glucose in the medium, utilization of lactose begins. - In the presence of glucose, the induction of lac operon is inhibited. - Glucose + Galactose --> No lac mRNA --> No B-galactosidase. - The attachment of RNA polymerase to the promoter site requires the presence of a Catabolite Gene Activator (CAP) protein. - CAP-CAP complex is functioning as a regulatory element of lac operon. - The CAP-CAP binds to a base sequence in the DNA of the promoter region in order to start transcription. - Thus the CAP-CAP Complex acts as a positive regulator. - The presence of glucose lowers the intracellular concentration of CAMP. - No glucose --> High CAMP - More glucose --> Very low camp. | Carbon source | CAMP concentration | |---|---| | Glucose | Low | | Glyceral | High | | Lactose | High | | Lactose + Glucose | Low | | Lactose + Glycerol | High | ### Summary | Carbohydrates | Activator protein | Repressor protein | RNA polymerase | lac Operon | |---|---|---|---|---| | + GLUCOSE + LACTOSE | Not bound to DNA | Lifted off operator site | Keeps falling off promoter site | No transcription | | + GLUCOSE - LACTOSE | Not bound to DNA | Bound to operator site | Blocked by the repressor | No transcription | | - GLUCOSE - LACTOSE | Bound to DNA | Bound to operator site | Blocked by the repressor | No transcription | | - GLUCOSE + LACTOSE | Bound to DNA | Lifted off operator site | Sits on the promoter site | Transcription |