Molecular Mechanisms of Gene Regulation
-
Upload
gray-shields -
Category
Documents
-
view
33 -
download
1
description
Transcript of Molecular Mechanisms of Gene Regulation
![Page 1: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/1.jpg)
1
10
Molecular Mechanisms of Gene Regulation
![Page 2: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/2.jpg)
Transcription Regulation: Prokaryotes
• Transcription regulation is common mechanism in prokaryotes
• Negative Regulation = repressor binds to regulatory site to block transcription of active gene
• Positive Regulation = Activator binds to regulatory site to stimulate transcription; gene is off in absence of activator
![Page 3: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/3.jpg)
![Page 4: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/4.jpg)
Operon Model of Regulation
• The operon consists of several structural genes required for lactose metabolism under the control of a single regulatory domain = coordinate regulation
• Repressor binding site = operator
• Site for transcriptional activation = promoter
![Page 5: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/5.jpg)
![Page 6: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/6.jpg)
Operon Model of Regulation
• Inducible operon is activated by small molecule inducers; mode of regulation in degradative (catabolic) pathways
• Repressible operon is shut off by small molecule co-repressors; mode of regulation in biosynthetic (anabolic) pathways
![Page 7: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/7.jpg)
Gene Regulation Mechanisms
Negative regulation: repressor protein blocks transcription:
• Inducible: inducer antagonizes repressor to allow transcription initiation
• Repressible: aporepessor combines with co-repressor to form functional repressor
![Page 8: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/8.jpg)
![Page 9: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/9.jpg)
Gene Regulation Mechanisms
• Positive Regulation: transcription occurs only if promoter is activated by transcriptional activator
• Negative regulation is more common in prokaryotes
• Positive regulation is more common in eukaryotes
• Autoregulation: protein regulates its own transcription
![Page 10: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/10.jpg)
Lac Operon
• Lactose degradation is regulated by the lactose (lac) operon
• The first regulatory mutations discovered affect lactose metabolism
• Structural proteins of the lac operon: B- galactosidase required for lactose metabolism and lactose permease required for lactose entry
![Page 11: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/11.jpg)
![Page 12: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/12.jpg)
Lac Operon
• Lac operon gene expression can be inducible or constitutive
• The repressor is expressed constitutively (continuously) from the i gene and binds to the operator to block transcription
• The operon is inducible since lactose binds and inactivates the repressor to permit transcription initiation
![Page 13: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/13.jpg)
Lac Operon
• Lactose operator is essential site for repression
• Operator mutations are cis-dominant because only genes on the same genetic unit are affected
• Lactose promoter is essential site for transcription
• Lac operon contains linked structural genes and regulatory sites
![Page 14: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/14.jpg)
Lac Operon
• Lactose operon is also subject to positive regulation
• Positive regulation of the lac operon involves cAMP-CRP ( cyclic AMP receptor protein) which binds to the promoter to activate transcription by RNA polymerase
• cAMP-CRP complex regulates the activity of the lac operon
![Page 15: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/15.jpg)
Trp Operon
• The trp operon contains the structural genes which encode enzymes required for the synthesis of the amino acid tryptophan
• The trp operon is transcriptionally active unless tryptophan is present
• The trp operon is an example of a repressible system regulated by a negative feedback loop
![Page 16: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/16.jpg)
![Page 17: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/17.jpg)
Trp Operon
• The trp operon is shut off when tryptophan binds to inactive aporepressor
• Tryptophan-repressor complex binds to operator to block transcription when tryptophan levels are high
• If tryptophan levels fall trp-repressor complex dissociates from operator
![Page 18: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/18.jpg)
Attenuation
• Attenuation is a very sensitive form of translational regulation of the trp operon
• The trp attenuator sequence consists of 5’ base sequence in mRNA which is complementary and can base pair to form a stem and loop structure
![Page 19: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/19.jpg)
![Page 20: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/20.jpg)
Attenuation
• Attenuation results in the premature termination of mRNA synthesis due to stem and loop formation in the 5’ region of mRNA
• If tRNA-trp is present , synthesis of the leader peptide results in pairing of mRNA which blocks the action of RNA polymerase
![Page 21: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/21.jpg)
![Page 22: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/22.jpg)
Attenuation
• At low concentrations of tRNA-trp, the ribosome stalls, and the mRNA opens so that transcription continues
• Attenuation permits the cell to respond to tryptophan levels by expressing the genes needed for its synthesis when needed
![Page 23: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/23.jpg)
Gene Regulation: Eukaryotes
Transcription regulation in eukaryotes differs from prokaryotes:
• The processes of transcription and translation are separate
• Many genes are “split” genes; alternative splicing may occur
• Each gene is regulated by a separate promoter
![Page 24: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/24.jpg)
Gene Regulation: Eukaryotes
Gene organization in eukaryotes:• DNA is bound to histones and non-histone
regulatory proteins• Significant fraction of DNA is moderately
or highly repetitive• Large fraction of DNA does not code for
proteins• Gene regulation can occur by DNA
rearrangements
![Page 25: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/25.jpg)
Gene Regulation: Eukaryotes
• Some genes are regulated by alteration of the DNA by amplification or rearrangement
• Gene amplification increases dosage and the amount of gene product
• Gene dosage refers to the ratios of amounts of gene products
• In gene amplification the number of genes increases
![Page 26: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/26.jpg)
Transcriptional Regulation
• Transcriptional control is a frequent mode of regulation in eukaryotes
• Housekeeping genes are essential for metabolic activity and are expressed constitutively
• Many specialized genes are under transcriptional control
• Genes controlling yeast mating type regulate transcription
![Page 27: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/27.jpg)
Transcriptional Enhancers
• Enhancers are DNA regulatory sequences that bind specific transcription factors to activate transcription by RNA polymerase
• Enhancers may be located in the 5’ or 3’ direction from a gene and may be found at great distances from a gene
![Page 28: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/28.jpg)
Transcriptional EnhancersTranscription activation:• Transcription activator binds
to enhancer• Transcription factor TFIID
which contains the TATA box binding protein (TBP) forms a complex with the transcription factor-enhancer site
• TBP binds to TATA box
![Page 29: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/29.jpg)
![Page 30: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/30.jpg)
Transcription Enhancers
• RNA polymerase joins the transcription complex and mRNA synthesis begins
• Housekeeping genes required for cell maintenance are constitutively expressed
• Differentiated functions are expressed due to selective gene activation
![Page 31: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/31.jpg)
Eukaryotic Transcription
• Eukaryotic transcription complex includes many proteins called
general transcription factors common to the promoters of many genes
• Transcriptional activation occurs by a mechanism called recruitment involving the interaction of transcription factors with promoter and enhancer elements
![Page 32: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/32.jpg)
![Page 33: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/33.jpg)
Transcription Regulation
• Combinatorial control means that a few genes can control many others
• Strategically placed enhancers can act as genetic switches
• One gene can have two or more promoters that are regulated differently
![Page 34: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/34.jpg)
![Page 35: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/35.jpg)
Genetic Switching
• Genetic switching involves the competition by two promoters for a single enhancer
• The presence of specific activator proteins will determine which promoter is activated and which gene product is expressed
![Page 36: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/36.jpg)
DNA Methylation
• DNA methylation occurs often at 5’-CG-3’ dinucleotides and is associated with a low level of transcriptional activity
• DNA methylases catalyze the the formation of 5-methylcytosine
• Methylation represents a form of transcriptional regulation
![Page 37: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/37.jpg)
Genomic Imprinting
• Unusual epigenetic silencing in mammals
• Occurs in germ line
• Imprints methylated differently in the sexes
• Evident in Prader-Willi and Angelman syndromes
![Page 38: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/38.jpg)
Alternative Splicing
• Alternative splicing is a form of gene regulation which results in the generation of alternative mRNAs from a single gene
• Different splice patterns may occur in different tissues resulting in tissue-specific gene expression
![Page 39: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/39.jpg)
![Page 40: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/40.jpg)
Translational Regulation
• Gene regulation can occur at the level of translation
Types of translational control include:• Absence of mRNA translation without
molecular signal• Regulation of mRNA half-life• Regulation of the rate of protein synthesis• Aborted translation
![Page 41: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/41.jpg)
DNA Rearrangements• Yeast cells carry out programmed DNA
rearrangements as part of a mating system called homothallism in which some cells undergo a conversion to opposite mating type
• In vertebrate immune system DNA splicing between variable region and constant region domains takes place in the formation of antibodies
![Page 42: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/42.jpg)
DNA Rearrangements
![Page 43: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/43.jpg)
Antibody Structure
![Page 44: Molecular Mechanisms of Gene Regulation](https://reader035.fdocuments.us/reader035/viewer/2022062720/568134d3550346895d9bfd2d/html5/thumbnails/44.jpg)
Antibody Gene Rearrangements