Regulation of Gene expression

• Transcriptional Control


  Occurs in all cell types, both prokaryotes and eukaryotes

  
• We will only be studying prokaryotes since the system is simpler to understand
• Operon


  A group of genes that share a single promoter
• Structural Genes


  Genes to be transcribed by RNAP, have related functions

  
• Eukaryotic vs Prokaryotic Gene

  • Monocistronic: Eukaryotic genes specify a single protein
  • Polycistronic: Prokaryotic genes with related function are situated in tandem on the DNA

  
• Promoter


  Region on DNA where RNAP binds to start transcription, acts as the on/off switch for genes

  
• Regulatory Sequences


  Segments on DNA where regulatory proteins bind, capable of increasing or decreasing the expression of a gene (transcription)

  
• Enhancer


  Activator binds to increase transcription
• Operator


  Repressor binds to decrease transcription
• Positive Gene Regulation


  Active form of the regulatory protein turns on or increases the transcription of the operon

  
• Negative Gene Regulation


  Operons are switched off by the active form of the regulatory protein (repressor)

  
• Repressible Operon

  Gene expression is normally ON, operon can be turned OFF by a corepressor activating a repressor

  
• Inducible Operon


  Gene expression is normally OFF, operon can be turned ON by an inducer inactivating a repressor

  
• Types of Gene Expression

  • Constitutive
  • Inducible
  • Repressible

  
• Constitutive


  Genes are always ON, unregulated
• Inducible


  Genes are only turned on as needed, regulated
• Repressible


  Genes are only turned off as needed, regulated

  
• Anabolic pathways


  Have genes that are repressible

  
• Catabolic pathways


  Have genes that are inducible

  
• Anabolism: synthesis of molecules from simpler subunits, if an essential nutrient is not present, bacteria must be able to synthesize it
• Catabolism: breakdown of complex molecules into simpler units, it would be a waste of energy for bacteria to make enzymes for the breakdown of a substance if that substance does not exist
• Pathway

  • Anabolic
  • Catabolic
• Anabolic Pathway


  Repressible gene expression, always on, turns off as needed

  
• Catabolic Pathway


  Inducible gene expression, always off, turns on as needed

  
• Polypeptides that make up enzymes for lactose hydrolysis and uptake are examples of inducible gene expression
• Components of Operon Regulation

  • Regulatory Gene (constitutive)
  • Regulatory Protein (repressor, activator)
  • Effector (corepressor, inducer)
  • Operon
  • Promoter
  • Regulatory sequence (operator, enhancer)
  • Structural Genes (repressible, inducible)

  
• Regulatory Gene


  Region on DNA that codes for the production of the regulatory protein, upstream of the operon, constitutive expression

  
• Regulatory Protein


  Allosteric, has an allosteric site which can bind an effector, two forms: effector bound or unbound, thus alternates between active and inactive forms
• Repressor


  Active form binds to DNA and blocks RNAP function by binding to operator
• Activator


  Active form binds to DNA and enhances RNAP's function by binding to enhancer
• Models of Negative Gene Regulation

  • Repressible Operon (e.g. trp): Repressor made in inactive form, activated by corepressor, turns gene off
  • Inducible Operon (e.g. lac): Repressor made in active form, inactivated by inducer, turns gene on

  
• The trp operon regulatory protein is a repressor

  
• The trp repressor is coded by a regulatory gene (trpR) with constitutive expression, continuously making the trp repressor

  
• The trp repressor is made in the inactive form
• When tryptophan (an essential amino acid) is present in the environment


  The bacteria want to switch the trp repressor from the inactive to the active form to turn off transcription of the trp operon
• Switching the trp repressor from inactive to active form

  Effector (corepressor) activates the repressor
• The effector that activates the trp repressor is a corepressor
• The corepressor causes the regulatory protein (trp repressor) to bind to the operator, inactivating gene expression

  
• Inactive trp operon Regulation

  How do you switch from the inactive form of the repressor (regulatory protein) to the active form?

  
• Inactive trp operon Regulation


  How do you turn on the repressor (regulatory protein)?