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Control of Gene Expression

This chapter explores the control of gene expression through transcription and the role of regulatory proteins in blocking or stimulating transcription. The regulation of gene expression in prokaryotic and eukaryotic organisms is discussed, along with the various DNA-binding motifs of regulatory proteins. The prokaryotic regulation of transcription initiation and the organization of operons are also examined.

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Control of Gene Expression

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  1. Control of Gene Expression Chapter 16

  2. replication(mutation!) genes DNA Nucleic acids ~ “software” (nucleotides) transcription messages RNA (nucleotides) nucleus ribosome (cytoplasm) translation Protein “hardware” (amino acids)

  3. Fig. 16.1

  4. Helix-Turn-Helix Motif

  5. Homeodomain Motif

  6. Zinc Finger Motif

  7. Leucine Zipper Motif

  8. lactose The Lac Operon: repressor repressor LacI LacZ LacY LacA promoter operator DNA RNA-P mRNA primary RNA transcript start codon stop codon stop codon stop codon protein products of the operon: beta- galactosidase permease acetylase

  9. + -- + -- lacI+ Z+ Y+ + + + + lacIc Z+ Y+ -- -- -- -- lacIS Z+ Y+ + + + + lacOc Z+ Y+ + -- + -- lacI+ Z-- Y-- lacIc Z+ Y+ + -- + -- lacI+ Z+ Y+ lacIc Z-- Y-- -- -- -- -- lacI-- Z+ Y+ lacI+ Z+ Y+ + + + + lacOc Z+ Y+ lacO+ Z-- Y-- + + + -- lacOc Z+ Y-- lacO+ Z-- Y+ + + + + lacI+ lacO+ Z+ Y+ lacI-- lacOc Z+ Y+ Data of Jacob and Monod, 1961 phenotype for: genotype B-gal Permease +IPTG --IPTG +IPTG --IPTG Lac+ Lacc Lac- Lacc } Lac+ lacI+ dominant in cis and trans Lac+ } lacI-- dominant Lac- } Lacc lacOc dominant in cis Lacc /Lac+ lacOc dominant in cis even in presence of lacI-- } Lacc

  10. + -- + -- lacI+ Z+ Y+ + + + + lacIc Z+ Y+ -- -- -- -- lacIS Z+ Y+ + + + + lacOc Z+ Y+ + -- + -- lacI+ Z-- Y-- lacIc Z+ Y+ + -- + -- lacI+ Z+ Y+ lacIc Z-- Y-- -- -- -- -- lacI-- Z+ Y+ lacI+ Z+ Y+ + + + + lacOc Z+ Y+ lacO+ Z-- Y-- + + + -- lacOc Z+ Y-- lacO+ Z-- Y+ + + + + lacI+ lacO+ Z+ Y+ lacI-- lacOc Z+ Y+ Data of Jacob and Monod, 1961 phenotype for: genotype B-gal Permease +IPTG --IPTG +IPTG --IPTG Lac+ Lacc Lac- Lacc } Lac+ lacI+ dominant in cis and trans Lac+ } lacI-- dominant Lac- } Lacc lacOc dominant in cis Lacc /Lac+ lacOc dominant in cis even in presence of lacI-- } Lacc

  11. + -- + -- lacI+ Z+ Y+ + + + + lacIc Z+ Y+ -- -- -- -- lacIS Z+ Y+ + + + + lacOc Z+ Y+ + -- + -- lacI+ Z-- Y-- lacIc Z+ Y+ + -- + -- lacI+ Z+ Y+ lacIc Z-- Y-- -- -- -- -- lacIS Z+ Y+ lacI+ Z+ Y+ + + + + lacOc Z+ Y+ lacO+ Z-- Y-- + + + -- lacOc Z+ Y-- lacO+ Z-- Y+ + + + + lacI+ lacO+ Z+ Y+ lacI-- lacOc Z+ Y+ Data of Jacob and Monod, 1961 phenotype for: genotype B-gal Permease +IPTG --IPTG +IPTG --IPTG Lac+ Lacc Lac- Lacc } Lac+ lacI+ dominant in cis and trans Lac+ } lacIS dominant Lac- } Lacc lacOc dominant in cis Lacc /Lac+ lacOc dominant in cis even in presence of lacI-- } Lacc

  12. + -- + -- lacI+ Z+ Y+ + + + + lacIc Z+ Y+ -- -- -- -- lacIS Z+ Y+ + + + + lacOc Z+ Y+ + -- + -- lacI+ Z-- Y-- lacIc Z+ Y+ + -- + -- lacI+ Z+ Y+ lacIc Z-- Y-- -- -- -- -- lacIS Z+ Y+ lacI+ Z+ Y+ + + + + lacOc Z+ Y+ lacO+ Z-- Y-- + + + -- lacOc Z+ Y-- lacO+ Z-- Y+ + + + + lacI+ lacO+ Z+ Y+ lacI-- lacOc Z+ Y+ Data of Jacob and Monod, 1961 phenotype for: genotype B-gal Permease +IPTG --IPTG +IPTG --IPTG Lac+ Lacc Lac- Lacc } Lac+ lacI+ dominant in cis and trans Lac+ } lacIS dominant Lac- } Lacc lacOc dominant in cis Lacc /Lac+ lacOc dominant in cis even in presence of lacI-- } Lacc

  13. + -- + -- lacI+ Z+ Y+ + + + + lacIc Z+ Y+ -- -- -- -- lacIS Z+ Y+ + + + + lacOc Z+ Y+ + -- + -- lacI+ Z-- Y-- lacIc Z+ Y+ + -- + -- lacI+ Z+ Y+ lacIc Z-- Y-- -- -- -- -- lacIS Z+ Y+ lacI+ Z+ Y+ + + + + lacOc Z+ Y+ lacO+ Z-- Y-- + + + -- lacOc Z+ Y-- lacO+ Z-- Y+ + + + + lacI+ lacO+ Z+ Y+ lacIS lacOc Z+ Y+ Data of Jacob and Monod, 1961 phenotype for: genotype B-gal Permease +IPTG --IPTG +IPTG --IPTG Lac+ Lacc Lac- Lacc } Lac+ lacI+ dominant in cis and trans Lac+ } lacIS dominant Lac- } Lacc lacOc dominant in cis Lacc /Lac+ lacOc dominant in cis even in presence of lacIS } Lacc

  14. Fig. 16.7

  15. From: http://www.aw.com/mathews/ch26/c26lara.htm

  16. Attenuation of trp operon transcription

  17. Fig. 16.8

  18. Fig. 16.16

  19. Control of Gene Expression • Controlling gene expression is often accomplished by controlling transcription initiation. • Regulatory proteins bind to DNA to either block or stimulate transcription, depending on how they interact with RNA polymerase.

  20. Control of Gene Expression • Prokaryotic organisms regulate gene expression in response to their environment. • Eukaryotic cells regulate gene expression to maintain homeostasis in the organism.

  21. Regulatory Proteins • Gene expression is often controlled by regulatory proteins binding to specific DNA sequences. • regulatory proteins gain access to the bases of DNA at the major groove • regulatory proteins possess DNA-binding motifs

  22. Regulatory Proteins • DNA-binding motifs are regions of regulatory proteins which bind to DNA • helix-turn-helix motif • homeodomain motif • zinc finger motif • leucine zipper motif

  23. Prokaryotic Regulation • Control of transcription initiation can be: • positive control – increases transcription when activators bind DNA • negative control – reduces transcription when repressors bind to DNA regulatory regions called operators

  24. Prokaryotic Regulation • Prokaryotic cells often respond to their environment by changes in gene expression. • Genes involved in the same metabolic pathway are organized in operons. • Some operons are induced when the metabolic pathway is needed. • Some operons are repressed when the metabolic pathway is no longer needed.

  25. Prokaryotic Regulation • The lac operon contains genes for the use of lactose as an energy source. • Regulatory regions of the operon include the CAP binding site, promoter, and the operator. • The coding region contains genes for 3 enzymes: • b-galactosidase, permease, and transacetylase

  26. Prokaryotic Regulation • The lac operon is negatively regulated by a repressor protein: • lac repressor binds to the operator to block transcription • in the presence of lactose, an inducer molecule binds to the repressor protein • repressor can no longer bind to operator • transcription proceeds

  27. Prokaryotic Regulation • In the presence of both glucose and lactose, bacterial cells prefer to use glucose. • Glucose prevents induction of the lac operon. • binding of CAP – cAMP complex to the CAP binding site is required for induction of the lac operon • high glucose levels cause low cAMP levels • high glucose  low cAMP  no induction

  28. Prokaryotic Regulation • The trp operon encodes genes for the biosynthesis of tryptophan. • The operon is not expressed when the cell contains sufficient amounts of tryptophan. • The operon is expressed when levels of tryptophan are low.

  29. Prokaryotic Regulation • The trp operon is negatively regulated by the trp repressor protein • trp repressor binds to the operator to block transcription • binding of repressor to the operator requires a corepressor which is tryptophan • low levels of tryptophan prevent the repressor from binding to the operator

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