Lecture 11 - Transcription and Regulation

• Regulation Overview

  • The promoter/operator combo regulates transcription
  • Sometimes you need to turn on transcription of the Operon, other times you might need to shut it down
• Negative regulatory proteins inhibit transcription
• Positive regulatory proteins activate transcription
• Can have both inducible and reprecible operons
• Inducible operons are when transcription of the operon is normally off, and something happens that turns it on
• Repressible operons: are ones that are normally on, and something happens that it needs to be turned off
• Negative Inducible Operon

  • Transcription is normally turned off and must be turned on
  • Regulator protein is a repressor
  • Blocks the binding of the RNA polymerase to the promoter
  • Keeps transcription off
  • Repressor needs to be relived of its duties in order for transcription to proceed
• Negative Inducible Operon

  • A small molecule called inducer binds to the inhibitor/repressor and inactivates it
  • Repressor can no longer bind to DNA
  • RNA polymerase can activate transcription
  • Usually involved in the degradation (metabolism) of molecules
• lac Operon

  • Lactose found in milk, can be metabolized by e. coli
  • Needs to be transported across the bacterial cell membrane by the enzyme permease (lacY)
  • E. coli breaks lactose down into glucose and galactose that is catalyzed by the enzyme beta-galactosidase (lacZ) it can also convert lactose into allolactose
• lac operon:

  • In the absence of lactose, very little of the operon is transcribed (but not none)
  • If lactose is added to the medium instead of glucose, then the rate of synthesis of the lac operon is increased x1000 within 2-3 minutes
  • Absence of lactose, Repressor protein (lacl) binds to the operator (lacO) and inhibits transcription
  • Lactose is broken down into allolactose by the little bit of beta-galactosidase that is made
• lac operon

  • allolactose binds to the repressor and causes the repressor to be released from the operator
  • RNA polymerase then binds to the promoter and induces the expression of 3 structural genes (lac Z, lac Y, lac A)
  • Lactose is rapidly converted into glucose and galactose and some allolactose, which keeps the operon in the on position
  • Once the lactose is depleted no more allolactose is made, freeing up the repressor to the to the operation and inhibit trancription
• Allolactose


  Proper form to act as a ligand to bind to the repressor, relieving the repressor of its function
• Negative Inducible System

  Inhibitor/repressor binds to the promoter preventing RNA polymerase from binding
• RNA polymerase binds to the operator


  Initiates transcription
• Promoter


  Binds proteins that can inhibit or activate transcription
• Allolactose binds to the repressor


  Allows for the transcription of the lacZ, lacY and lacA structural genes
• Ligand appears


  Interacts with the inhibitor/repressor and inhibits its function
• Transcriptional regulation

  • Negative Inducible System
• RNA polymerase


  Only binds to the operator in the absence of an inhibitor binding to the promoter