Gene regulation

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Imagine a situation when a cell starts producing enzymes required for metabolism and those required for cell death (apoptosis) at the same time. The cell will be in a confused state and will not know which function to perform first. The needs of the body keep changing with time and cell has to tune itself to perform the desired set of activities. Gene regulation helps a unicellular organism to adapt well to the environment.

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Gene regulation

  1. 1. Gene regulation
  2. 2. Necessity of Gene Regulation <ul><li>Unicellular organisms </li></ul><ul><ul><li>Depending on the needs of the body some genes have to be transcribed whereas the rest have to be switched off </li></ul></ul><ul><ul><li>Helps to adapt to the changes in environment </li></ul></ul><ul><li>Multicellular organisms </li></ul><ul><ul><li>Helps in differentiation of cells </li></ul></ul><ul><ul><li>Performance of various functions in the body </li></ul></ul>
  3. 3. Gene Regulation Mechanisms in Prokaryotes and Eukaryotes <ul><li>In prokaryotes the primary control point is the process of transcription initiation </li></ul><ul><li>In eukaryotes expression of gene into proteins can be controlled at various locations. </li></ul>
  4. 4. Gene Regulation in Prokaryotes <ul><li>Operon hypothesis- Proposed by two French microbiologists, Francois Jacob and Jacques Monod in 1961 for which they won the nobel prize in 1965. </li></ul><ul><li>Operon -In prokaryotes the linked genes are clustered into units known as operons. The genes in a single operon affect the same biochemical pathway that is either they are expressed or repressed under similar condition. </li></ul><ul><li>Polycistronic RNA – In prokaryotes a single operon gets transcribed into polycistronic mRNA which can be translated into multiple proteins. </li></ul>
  5. 5. Control of Transcriptional Initiation <ul><li>Promoter sequences –Help the enzyme RNA polymerase to recognize the transcriptional initiation sites </li></ul><ul><ul><li>-35 position is TTGACA </li></ul></ul><ul><ul><li>-10 position is TATAAT </li></ul></ul><ul><li>Accessory or regulatory proteins –Control the ability to recognize the transcriptional initiation sites </li></ul><ul><ul><li>Activators </li></ul></ul><ul><ul><li>Repressors </li></ul></ul><ul><li>Operators - The interaction of the regulatory proteins with the operators modulates the accessibility of promoter regions of prokaryotic DNA. </li></ul>
  6. 6. Transcriptional Regulation in E.coli <ul><li>In E.coli the following two kinds of operons exist and in both the cases the gene regulation is carried out by repressor proteins. </li></ul><ul><ul><li>Catabolite-regulated operons -They are the operons which produce gene products necessary for the utilization of energy. Example lac operon. </li></ul></ul><ul><ul><li>Attenuated operons – These operons produce gene products necessary for the synthesis of small biomolecules such as amino acids. These operons are typically attenuated by the sequences within the transcribed RNA. Example trp operon </li></ul></ul>
  7. 7. The lac operon <ul><li>Components of lac operon </li></ul><ul><li>Regulatory gene </li></ul><ul><ul><li>The regulatory gene is the i gene that codes for the repressor protein of the lac operon. </li></ul></ul><ul><ul><li>This i gene is expressed all the time hence it is also known as a constitutive gene. </li></ul></ul><ul><ul><li>The lac repressor protein has two functional domains or regions one that binds the operator sequence and the other that binds the lactose sugar. </li></ul></ul>
  8. 8. Components of lac Operon <ul><li>Structural genes </li></ul><ul><ul><li>lac Z codes for β-galactosidase (β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. </li></ul></ul><ul><ul><li>Lac Y codes for permease, which transports lactose into the cell. </li></ul></ul><ul><ul><li>Lac A codes for transacetylase whose function is not considered here </li></ul></ul><ul><li>Operator </li></ul><ul><li>Promoter </li></ul>
  9. 9. Structure of a lac operon Image reference - http:// www.emunix.emich.edu/~rwinning/genetics/proreg.htm
  10. 10. In the presence of an inducer that is lactose <ul><li>Lactose binds repressor </li></ul><ul><li>Repressor undergoes conformational change </li></ul><ul><li>Repressor does not bind operator </li></ul><ul><li>Lac operon is transcribed </li></ul>Image reference - http:// www.emunix.emich.edu/~rwinning/genetics/proreg.htm
  11. 11. In the absence of an inducer that is lactose <ul><li>Repressor protein is constitutively produced </li></ul><ul><li>Enzymes required for the lactose metabolism are not produced </li></ul>Image reference - http:// www.emunix.emich.edu/~rwinning/genetics/proreg.htm
  12. 12. Catabolite Repression <ul><li>Feed back control of lac operon through Catabolite repression </li></ul><ul><ul><li>Transcription of lac operon takes place with the help of another protein named catabolite activator protein (CAP for short). </li></ul></ul><ul><ul><li>When a small molecule called cyclic AMP (cAMP) binds CAP it is able to bind the promoter region of the lac operon </li></ul></ul><ul><ul><li>In the absence of cAMP, CAP fails to bind to the promoter region and hence no transcription takes place. </li></ul></ul><ul><ul><li>cAMP is produced by an enzyme called adenylcyclase </li></ul></ul><ul><ul><li>In the presence of glucose in the environment the following changes take place- </li></ul></ul><ul><ul><ul><li>Synthesis of adenylcyclase is inhibited </li></ul></ul></ul><ul><ul><ul><li>cAMP production drops down </li></ul></ul></ul><ul><ul><ul><li>(cAMP – CAP) complex does not form </li></ul></ul></ul><ul><ul><ul><li>CAP fails to bind to the promoter sequence </li></ul></ul></ul><ul><ul><ul><li>Transcription of lac operon does not take place </li></ul></ul></ul><ul><ul><li>Acts like a feed back mechanism </li></ul></ul><ul><ul><ul><li>Lactose ------- Beta-galactosidase -----> Glucose ↑ + Galactosidase </li></ul></ul></ul><ul><ul><ul><li>Transcription of lac operon is inhibited </li></ul></ul></ul>
  13. 13. Inducible and Repressible Operons <ul><li>Inducible operon - the effector molecule interacts with the repressor protein such that it can not bind to the operator. </li></ul><ul><ul><li>Lac operon is an inducible system as this operon is always turned off except in the presence of an inducer that is lactose. </li></ul></ul><ul><li>Repressible operon - the effector molecule interacts with the repressor protein such that it can bind to the operator </li></ul><ul><ul><li>trp Operon is an example of repressible system which means that it is automatically switched on and stops only when a repressor becomes active and binds it </li></ul></ul>
  14. 14. The trp Operon <ul><li>Components of the trp operon are the promoter, operator and five structural genes which are responsible for tryptophan biosynthesis that is trpE, D, C, B and A . trpL gene lies between operator and trpE gene. </li></ul><ul><li>The trp operon is controlled by two mechanisms </li></ul><ul><ul><li>Negative control system </li></ul></ul><ul><ul><li>Translation-induced transcriptional attenuation </li></ul></ul>
  15. 15. Negative control system <ul><li>Presence of Co-repressor that is tryptophan molecule </li></ul>
  16. 16. Translation-induced transcriptional attenuation <ul><li>Role of trpL </li></ul><ul><li>GeneL which is situated immediately at the 5' of the trpE gene is 160 bp in length and controls the expression of the trp operon by the process of attenuation. </li></ul><ul><li>The geneL codes for the leader region of RNA and this region is located at the 5' end of RNA. </li></ul><ul><li>This region of RNA is capable of forming different stable stem loop structures. </li></ul>
  17. 17. Characteristics of the leader region of the mRNA <ul><li>The leader region coded by gene L is made up of 1-4 domains and contains tandem tryptophan codons. </li></ul><ul><li>The domain 3 of mRNA can base pair with either domain 2 or domain 4. </li></ul><ul><li>Domain 4 is known as attenuator as its presence is required to stop the transcription. </li></ul><ul><li>The domain1 of the leader region of mRNA codes for a chain of 14 amino acids which have two tryptophan residues. </li></ul>
  18. 18. Under high levels of Tryptophan <ul><li>As the cellular levels of tryptophan are high the levels of tryptophan tRNA are also high. </li></ul><ul><li>The ribosome complex moves through the domain 1 translating it into a small peptide. </li></ul><ul><li>The ribosome complex quickly moves to the domain 2 as there is an abundance of tryptophan tRNA </li></ul><ul><li>When the ribosome complex lies associated with the domain 2 the domain 3 and 4 are free to form a stem loop structure. </li></ul><ul><li>This stem loop structure is an intrinsic transcription terminator which does not allow the RNA polymerase to move further </li></ul><ul><li>This stops the transcription and related translation </li></ul>
  19. 19. Under low levels of Tryptophan <ul><li>As the cellular levels of tryptophan are low the levels of tryptophan tRNAs are also low making the translation of domain 1 slow. </li></ul><ul><li>Hence domain 2 is free to get associated with domain 3 </li></ul><ul><li>This also forms a stem loop structure but it is not a terminator of transcription </li></ul><ul><li>This allows the continued transcription of the operon. </li></ul><ul><li>Thus trp E –A are translated and the enzymes necessary for the synthesis of tryptophan are produced . </li></ul>
  20. 20. Image reference - http://www.ndsu.edu/pubweb/~mcclean/plsc431/prokaryo/prokaryo3.htm
  21. 21. <ul><li>Thank you </li></ul>

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