Regulation of Gene expression

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Regulation of Gene expression

• by
• E. Börje Lindström

This learning object has been funded by the
European Commissions FP6 BioMinE project
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Introduction

• Biosynthetic reactions consume energy

? Sophisticated control mechanisms in bacteria

• Available energy is limited in Nature

? Production of as much cell material per energy
as possible

• The environment is important

• the nutrient in the medium is used first
• rapid and drastic changes in the nutrients ?
• reversible control reactions needed

• Two types of model systems

- Biosynthetic - Catabolic
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Biosynthetic reactions
Tryptophan is chosen as a model system

• Tryptophan is an essential amino acid
• Tryptophan is missing in some plant proteins ?
• of industrial importance

• The bacterial cells are controlling the
  biosynthesis of tryptophan in three ways

• feedback inhibition
• end product repression
• attenuation

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Biosynthetic reactions, cont.

• Feedback inhibition

- The biosynthesis of tryptophan occurs in
several steps
E5
E4
E3
E2
E1
Chorismate glutamine ? antranilic acid ? B ? C
? D ? tryptophan
Mechanism

• - enzyme E1 (the first enzyme) is an allosteric
  protein with
• - a binding site for for the substrate
• a binding site for the effectors (inhibitor
  try)

• E1 try ? E1-try-complex that is inactive
• the complete biosynthesis of try is stopped

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Biosynthetic reactions, cont.

• End product repression (EPR)

• In spite of end product inhibition ?
• loss of energy due to enzymes E2-E5 are still
  synthesized

• another regulation is needed
• end product repression

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Biosynthetic reactions, cont.
Mechanism
P promoter O operator att attenuator
E1 E5 structural genes for the enzymes E1-E5.

• RNA polymerase binds to P

? Initiation of mRNA synthesis

• The repressor is an allosteric protein
• - inactive without tryptophan (does not bind to
  the operator)

• tryptophan acts as co-repressor

• binds to the repressor
• makes the repressor active

? Blocks the RNA polymerase movement

• The repressor binds to O

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Biosynthetic reactions, cont.

• Attenuator region

- barrier for the RNA polymerase
1) try ? the polymerase removed from the DNA
2) - try ? the polymerase continues into the
structural genes

• EPR inhibits all enzymes in tryptophan
  biosynthesis
• ? save energy

• however, a slow total inhibition does not
  effect already existing enzymes
• high specificity only the tryptophan operon is
  effected

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Biosynthetic reactions, cont.
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Biosynthetic reactions, cont.
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Biosynthetic reactions, cont.
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Biosynthetic reactions, cont.
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Catabolic reactions

• Catabolic systems are inducible

• The inducer is the available carbon/energy
  source

• Model system lactose operon in E. coli

• Where

• gene R repressor protein active without the
  inducer
• ? blocks mRNA polymerase
• gene lacZ b-galactosidase splits lactose
  into glycose galactose
• gene lacY permease transport lactose into the
  cell
• no attenuator sequence in catabolic systems

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Catabolic reactions, cont.

• Mechanism

lactose

• transported into the cell ? transformed into
  allo-lactose (inducer)
• allo-lactose repressor ? allo-lactose-represso
  r- complex ? inactive
• RNA polymerase starts transcription of lactose
  operon
• ? b-galactosidase is produced ? break down of
  lactose

- lactose

• allo-lactose-repressor- complex disintegrate
• the repressor binds to O and blocks further
  transcription of the operon

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Catabolic reactions, cont.
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Catabolic reactions, cont.
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Catabolic repression (glucose-effect)

• Works in bacteria and other prokaryotes (here in
  E. Coli K12)

• Diauxi

• growth on two energy sources glucose lactose
  ?
• two-step growth curve

Growth on lactose
lactose
Growth on glucose
glucose
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Catabolic repression (glucose-effect)

• Mechanism

• cAMP an important substance
• required for initiation of transcription of many
  inducible systems
• global regulation
• glucose present ? cAMP ? (decreases)

- CAP (katabolite activator protein) an
allosteric protein
- cAMP-CAP-complex binds to the promoter ?
promotes transcription

• production of b-galactosidase ?

• 1) lactose present
• 2) cAMP-CAP-complex present

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Catabolic repression (glucose-effect), cont.

• glucose

• no cAMP-CAP-complex ?
• no transcription of lactose operon
• no b-galactosidase production

• - glucose

• cAMP-CAP-complex present ?
• transcription of lactose operon
• b-galactosidase production
• brake down of lactose

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Catabolic repression (glucose-effect), cont.

• Conclusions

• Katabolite repression a very useful function
  in bacteria
• forces the bacteria to use the best energy
  source first

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Other types of Regulations

• Constitutive systems

• no regulation
• always present

• Enzymes that are needed during all types of
  growth
• e.g. those involved in glycolysis

• mRNA

• Unstable
• half-life 2 min ? sub-units
• ? new mRNA

• polycistronic mRNA

- one operator for several genes

• monocistronic mRNA

- one operator per gene (in eukaryotes)