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Gene Regulation

Gene Regulation. Chapter 14. Learning Objective 1. Why do bacterial and eukaryotic cells have different mechanisms of gene regulation?. Prokaryotes. Bacterial cells grow rapidly have a short life span Transcriptional-level control usually regulates gene expression. Eukaryotic Cells.

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Gene Regulation

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  1. Gene Regulation Chapter 14

  2. Learning Objective 1 • Why do bacterial and eukaryotic cells have different mechanisms of gene regulation?

  3. Prokaryotes • Bacterial cells • grow rapidly • have a short life span • Transcriptional-level control • usually regulates gene expression

  4. Eukaryotic Cells • Have long life span • respond to many different stimuli • One gene • may be regulated in different ways • Transcriptional-level control • and control at other levels of gene expression

  5. KEY CONCEPTS • Cells can synthesize thousands of proteins • but not all proteins are required in all cells • Cells regulate which parts of the genome will be expressed, and when

  6. Learning Objective 2 • What is an operon? • What are the functions of the operator and promoter regions?

  7. Operon • A gene complex • structural genes with related functions • controlled by closely linked DNA sequences • Regulated genes in bacteria • are organized into operons

  8. PromoterRegion • Each operon has a promoterregion • upstream from protein-coding regions • where RNA polymerase binds to DNA before transcription

  9. Operator (1) • Regulatory switch for transcriptional-level control of operon • Repressor protein • binds to operator sequence • prevents transcription

  10. Operator (2) • RNA polymerase • bound to promoter • is blocked from transcribing structural genes • If repressor is not bound to operator • transcription proceeds

  11. Learning Objective 3 • What is the difference between inducible, repressible, and constitutive genes?

  12. Inducible Genes (1) • An inducible operon • such as lac operon • is normally turned off • Repressor protein • is synthesized in active form • binds to operator

  13. Inducible Genes (2) • If lactose is present • is converted to allolactose (inducer) • binds to repressor protein • changes repressor’s shape • Altered repressor • cannot bind to operator • operon is transcribed

  14. The lac Operon

  15. lac operon Repressor gene Promoter Operator lac Z lac Y lac A DNA Repressor protein Transcription mRNA Translation Ribosome Fig. 14-2a, p. 307

  16. lac operon Repressor gene Promoter Operator lac Z lac Y lac A RNA polymerase Transcription mRNA mRNA Translation Transacetylase Inducer (allolactose) Lactose permease β-galactosidase Repressor protein (inactive) Enzymes for lactose metabolism Fig. 14-2b, p. 307

  17. Repressible Genes (1) • A repressible operon (trp operon) • is normally turned on • Repressor protein • is synthesized in inactive form • cannot bind to operator • A metabolite (metabolic end product) • acts as corepressor

  18. Repressible Genes (2) • With high intracellular corepressor levels • corepressor molecule binds to repressor • changes repressor’s shape • Altered repressor • binds to operator • turns off transcription of operon

  19. The trp Operon

  20. trp operon Repressor gene Operator trp E trp D trp C trp B trp A Promoter DNA RNA polymerase Transcription mRNA mRNA Translation Repressor protein (inactive) Enzymes of the tryptophan biosynthetic pathway Tryptophan (a) Intracellular tryptophan levels low. Fig. 14-4a, p. 310

  21. trp operon Repressor gene Promoter Operator trp E trp D trp C trp B trp A DNA Active repressor – corepressor complex mRNA Inactive repressor protein Tryptophan (corepressor) (b) Intracellular tryptophan levels high. Fig. 14-4b, p. 310

  22. Constitutive Genes (1) • Are neither inducible nor repressible • active at all times • Regulatory proteins • produced constitutively • catabolite activator protein (CAP) • repressor proteins

  23. Constitutive Genes (2) • Regulatory proteins • recognize and bind to specific base sequences in DNA • Activity of constitutive genes • controlled by binding RNA polymerase to promoter regions

  24. Learning Objective 4 • What is the difference between positive and negative control? • How do both types of control operate in regulating the lac operon?

  25. Negative Control • Repressible and inducible operons are under negative control • When repressor protein binds to operator • transcription of operon is turned off

  26. Positive Control (1) • Some inducible operons are under positive control • Activator protein binds to DNA • stimulates transcription of gene

  27. Positive Control (2) • CAP activates lac operon • binds to promoter region • stimulates transcription by tightly binding RNA polymerase • To bind to lac operon • CAP requires cyclic AMP (cAMP) • cAMP levels increase • as glucose levels decrease

  28. Positive Control

  29. Promoter CAP- binding site RNA polymerase – binding site Repressor gene Operator lac Z lac Y lac A DNA mRNA RNA polymerase binds poorly CAP (inactive) Allolactose Repressor protein (inactive) (a) Lactose high, glucose high, cAMP low. Fig. 14-5a, p. 311

  30. Promoter CAP- binding site RNA polymerase – binding site Repressor gene Operator lac Z lac Y lac A DNA RNA polymerase binds efficiently Transcription mRNA mRNA CAP Translation Galactoside transacetylase cAMP Lactose permease β -galactosidase Allolactose Enzymes for lactose metabolism Repressor protein (inactive) (b) Lactose high, glucose low, cAMP high. Fig. 14-5b, p. 311

  31. Binding CAP

  32. DNA cAMP CAP dimer Fig. 14-6, p. 312

  33. Learning Objective 5 • What are the types of posttranscriptional control in bacteria?

  34. Posttranscriptional Controls in Bacteria • Translational control • regulates translation rate of particular mRNA • Posttranslational controls • include feedback inhibition of key enzymes in metabolic pathways

  35. KEY CONCEPTS • Prokaryotes regulate gene expression in response to environmental stimuli

  36. KEY CONCEPTS • Gene regulation in prokaryotes occurs primarily at the transcription level

  37. Learning Objective 6 • Discuss the structure of a typical eukaryotic gene and the DNA sequences involved in regulating that gene

  38. Eukaryotic Genes • Are not normally organized into operons • Regulation occurs at levels of • Transcription • mRNA processing • Translation • Modifications of protein product

  39. Transcription • Requires • Transcription initiation site • where transcription begins • Promoter • to which RNA polymerase binds • In multicellular eukaryotes • RNA polymerase binds to promoter (TATA box)

  40. Transcription

  41. TATA box Transcription initiation site T T TATA A UPE A A pre-mRNA (a) Eukaryotic promoter elements. Fig. 14-9a, p. 316

  42. TATA box Transcription initiation site T T TATA A UPE A A pre-mRNA (b) A weak eukaryotic promoter. Fig. 14-9b, p. 316

  43. Transcription initiation site TATA box T T TATA A A A UPE UPE UPE UPE pre-mRNA (c) A strong eukaryotic promoter. Fig. 14-9c, p. 316

  44. TATA box Transcription initiation site T T TATA A Enhancer UPE UPE A A pre-mRNA (d) A strong eukaryotic promoter plus an enhancer. Fig. 14-9d, p. 316

  45. Regulated Eukaryotic Gene • Promoter • RNA polymerase-binding site • short DNA sequences (upstream promoter elements (UPEs) or proximal control elements) • UPEs • number and types within promoter region determine efficiency of promoter

  46. Enhancers (1) • Located far away from promoter • control some eukaryotic genes • Help form active transcription initiation complex

  47. Enhancers (2) • Specific regulatory proteins • bind to enhancer elements • activate transcription by interacting with proteins bound to promoters

  48. Enhancers

  49. Enhancer Target proteins RNA polymerase TATA box DNA (a) Little or no transcription. Fig. 14-11a, p. 317

  50. Enhancer Activator (transcription factor) TATA box DNA (b) High rate of transcription. Fig. 14-11b, p. 317

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