Control of Gene Expression

Control of Gene Expression Chapter 16

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

Fig. 16.1

Helix-Turn-Helix Motif

Homeodomain Motif

Zinc Finger Motif

Leucine Zipper Motif

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

+ -- + -- 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

+ -- + -- 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

+ -- + -- 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

Fig. 16.7

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

Attenuation of trp operon transcription

Fig. 16.8

Fig. 16.16

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.

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.

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

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

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

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.

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

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

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

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.

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

Control of Gene Expression