Gene regulation

A detail on gene expression, operon and lac operon

1. GENE REGULATION Dr Neethu Asokan

2. Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products. Gene regulation is the process of turning genes on and off. Gene regulation ensures that the appropriate genes are expressed at the proper times. Gene regulation can also help an organism respond to its environment. Dr Neethu Asokan I NTRODUCTION

3. G ene regulation can take place at various steps of gene expression which includes the following: Replication level – Any error in copying the DNA may result in an altered expression. Transcriptional level – During transcription, any error in the polymerization may again lead to a change in expression of the gene. Post-transcriptional level – During the post-transcriptional modification i.e., RNA splicing, there may be some changes. Translational level – During translation, if there is an error in the attachment of mRNA to the tRNA molecules, there may arise some changes. Dr Neethu Asokan

4. Operon, genetic regulatory system found in bacteria and their viruses in which genes coding for functionally related proteins are clustered along the DNA. This feature allows protein synthesis to be controlled coordinately in response to the needs of the cell. By providing the means to produce proteins only when and where they are required, the operon allows the cell to conserve energy. Dr Neethu Asokan OPERON

5. The regulation usually takes place in the expression of the RNA polymerase at the promoter site. This affects the accessory proteins which bind to the recognition sites. These accessory proteins can regulate the promoter site in two ways: Positive regulation by activators Negative regulation by repressors Dr Neethu Asokan

6. Two regulatory proteins communicate these signals with the genes: Positive regulators (activators): Activator increases transcription of the regulated genes. In lac operon, activator ( called CAP) acts as a glucose sensor. It activates the transcription of the operon when glucose is absent/low. Negative regulators (repressor): Repressor decreases or eliminates transcription of genes. In lac operon, repressor acts as a lactose sensor. Dr Neethu Asokan

7. The lactose or lac operon of Escherichia coli is a cluster of three structural genes encoding proteins involved in lactose metabolism and the sites on the DNA involved in the regulation of the operon. It was Jacob and Monod in 1961 who proposed the operon model for the regulation of transcription. Dr Neethu Asokan LAC OPERON

8. The lac operon consists of: Regulatory gene i – It codes for the repressor protein. z gene – It codes for beta- galactosidase which catalyzes the hydrolysis of lactose into glucose and galactose . y gene – It codes for permease which regulates the lactose permeability in the cell. a gene – It codes for transacetylase which assists the enzyme beta- galactosidase . Hence, all these genes help in lactose metabolism. In lac operon, lactose acts as an inducer. If lactose is provided in the medium for the bacteria, the regulatory gene is activated. Dr Neethu Asokan

9. When lactose is absent The lac repressor senses the presence of lactose (more precisely allolactose -an isomer of lactose) in the medium. In the absence of allolactose (A) the repressor protein (R) binds to the operator region (O) and blocks the RNA polymerase from transcribing the structural genes. Dr Neethu Asokan Mechanism

10. When lactose is present When lactose is present in a medium, some of it will be converted to allolactose . This allolactose binds to the repressor protein. Binding of allolactose with repressor protein changes the shape of repressor protein so it can no longer binds to the operator region. This allows the RNA polymerase to bind to the promoter site, starting the initiation of transcription of the structural genes lacZ , lacY , and lacA to produce mRNA. Once the mRNA is produced, it is translated to produce enzymes . The lacZ gene encodes for the production of the enzyme β- galactosidase (B), lacY gene for the production of the enzyme permease (P) and the lacA gene is responsible for the production of the enzyme thiogalactoside transacetylase . Dr Neethu Asokan

11. When glucose levels are high When glucose levels are high, no cAMP is made, the CAP– cAMP complex does not form. CAP cannot bind DNA without cAMP and RNA polymerase cannot bind to the promoter efficiently. In this case, transcription occurs only at a low level.   Dr Neethu Asokan

12. For the transcription of structural genes by the lac operon, two requirements must be met: Glucose must be absent: The level of cyclic AMP must be high enough so that the CAP protein binds to the CAP binding site. Bound CAP helps to attach RNA polymerase efficiently to the lac operon promoter. Lactose must be present: There must be an inducer (such as lactose) so that lactose repressor does not block transcription by binding to the operator. Dr Neethu Asokan

13. Glucose Binding of CAP Lactose Binding of Repressor Level of Transcription + – – + No transcription + – + – Low level transcription – + – + No transcription – + + – High level transcription Dr Neethu Asokan

14. Dr Neethu Asokan

15. Dr Neethu Asokan

16. Dr Neethu Asokan THANK YOU…