Regulation of Gene expression

1. Regulation of Gene expression by E. Börje Lindström This learning object has been funded by the European Commissions FP6 BioMinE project

2. 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

3. 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

4. Biosynthetic reactions, cont. • Feedback inhibition: - The biosynthesis of tryptophan occurs in several steps: E1 E2 E3 E4 E5 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

5. 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

6. P O att E1 E2 E3 E4 E5 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

7. 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

8. Biosynthetic reactions, cont.

9. Biosynthetic reactions, cont.

10. Biosynthetic reactions, cont.

11. Biosynthetic reactions, cont.

12. R P O lacZ lacY lacA 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

13. Catabolic reactions, cont. • Mechanism: + lactose: • transported into the cell  transformed into allo-lactose (inducer) • allo-lactose + repressor  [allo-lactose-repressor]- 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

14. Catabolic reactions, cont.

15. Catabolic reactions, cont.

16. Log OD time Catabolic repression (glucose-effect) • Works in bacteria and other prokaryotes (here in E. Coli K12) • Diauxi: • growth on two energy sourcesglucose + lactose  • two-step growth curve Growth on lactose lactose Growth on glucose glucose

17. 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

18. 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

19. Catabolic repression (glucose-effect), cont. • Conclusions: • Katabolite repression – a very useful function in bacteria • forces the bacteria to usethe best energy source first

20. 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)