Fig. 17.1 Levels at which gene expression can be controlled ...
Study Guide/OutlineEukaryotic Gene Regulation Why is eukaryotic gene expression more complex than prokaryotic? Name six different levels at which gene expression might be controlled. What evidence has shown the role of chromosome packaging and histone proteins in gene regulation? What role does DNA methylation play? What are DNA binding motifs in transcription factor proteins? What are enhancers and silencers? How does RNA processing and stability contribute to gene regulation? What is alternative splicing? How is this used in the sexdetermination genes in Drosophila? What are the Homeotic genes? How are the Homeotic genes arranged on the chromosome and how does the chromosome location correspond to their function? How many sets of homeotic genes do Drosophila have? Mammals? Eukaryotic Gene Expression--more complex than prokaryotic expression because: 1. Same DNA in all cells, different gene expression pattern, depending upon cell differentiation e.g. RBC precursor cells hemoglobin 2. Eukaryotes have 700x more DNA, 20x more genes 3. Pancreatic cells insulin
Complex temporal expression (developmental stages) E.g. (hemoglobin Hbe, HbF, HbA0) Eukaryotic DNA is packaged into chromatin. Transcription of genes requires correct de-packaging and repackaging. Exons and introns allow new level of gene regulation and diversity unavailable for prokaryotes. Different levels of gene regulation in eukaryotic expression Levels at which eukaryotic gene expression is controlled Initiating or inhibiting Transcription [majority] Transcriptional activators, coactivators, repressors Promoters DNA response elements, enhancers, silencers DNA packaging and chromatin structure Histone acetylation, histone variation DNA methylation Initiating or inhibiting Translation mRNA processing, alternative splicing RNA stability RNA silencing Initiating or inhibiting protein activity (Posttranslational modification) Basal (General) Transcription in Eukaryotes RNA polymerase and general transcription factors
Activator protein Regulatory Transcription Factors act as activators or repressors of general transcription Enhancer Core promoter RNA transcription is increased. (a) Gene activation Repressor protein Silencer (b) Gene repression Brooker Fig 17.2 Core promoter RNA transcription is inhibited. How do different Transcription Factor proteins recognize specific promoter sequences? (DNA-Binding Motifs in Transcription Factors)
DNA-Binding Domains in Transcription Factors Loop Recognition helix Turn Recognition helix (a) Helixturnhelix motif (b) Helixloophelix motif Brooker, Fig 17.3 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display DNA-Binding Domains in Transcription Factors, cont. Zn2+ 1 Zinc finger Recognition helix Leu Leu Zn2+ 2
sheet 3 Zn2+ Leu Zn2+ Leu Leu Leucine side chains (zipper) Coiled coil Zinc ion Recognition helix 4 (c) Zinc finger motif Brooker, Fig 17.3 (d) Leucine zipper motif Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Regulatory Transcription Factors can interact directly with General Transcription Factors or
with a co-factor Activator protein ON Coactivator TFIID Enhancer TFIID Coding sequence Core promoter The activator/coactivator complex recruits TFIID to the core promoter and/or activates its function. Transcription will be activated. (a) Transcriptional activation via TFIID Brooker, Fig 17.4 OFF Repressor protein Coding sequence Silencer
Core promoter The repressor protein inhibits the binding of TFIID to the core promoter or inhibits its function. Transcription is repressed. (b) Transcriptional repression via TFIID Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display OR Mediator protein interacts between regulatory factors and general transcription factors (enhancers and silencers are distant) Core promoter ON Mediator TFIID RNA polymerase and general transcription factors Core promoter OFF Mediator TFIID RNA polymerase and general transcription
factors STOP Coding sequence Coding sequence Activator protein Repressor protein Enhancer Silencer The activator protein interacts with mediator. This enables RNA polymerase to form a preinitiation complex that can proceed to the elongation phase of transcription. (a) Transcriptional activation via mediator Brooker, Fig 17.5 The repressor protein interacts with mediator so that transcription is repressed. (b) Transcriptional repression via mediator Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hormone Transcription factor Response element Common ways to modulate
regulatory transcription factors (a) Binding of a small effector molecule such as a hormone Transcription factor Transcription factor Homodimer (b) Proteinprotein interaction Transcription factor PO42 PO42 Inactive (c) Covalent modification such as phosphorylation Brooker, Fig 17.6 Active Glucocorticoid Example of hormone signal and homodimerization Cytoplasm Heat shock proteins leave when
hormone binds to receptor HSP 90 HSP 90 HSP 90 HSP 90 + Glucocorticoid receptor NLS Nuclear localization signal is exposed Nucleus Formation of homodimer Core promoter Nuclear pore 5 3 AG R AC A T CY T G T
TG TY C T ACARGA 3 Glucocorticoid Response Elements Brooker, Fig 17.7 Target gene 5 GRE Transcription activated or repressed Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Level of packaging for DNA replication or expression Level of packaging for Diffuse DNA Gene expression can be repressed by chromosome packaging Condensed chromosome for Peter J. Russell, iGenetics: Copyright Pearson Education, Inc., publishing as Benjamin Cummings. mitosis Experiment showing that the areas of unpackaging
correspond with gene expression Condensed area does not take up radioactive RNA precursor Chromosome pufftakes up radioactive uridine (mRNA nt precursor) Gene-specific DNA Packaging (for tissue-specific expression) DNA from Red Blood Cell Precursors B-globin Probe (11p15.5) DNA from Red Blood Cell Precursors Albumin Probe (4q11) Southern Blot of DNA from Red Blood cell precursors. Transcribed regions of chromosome (B-globin gene) are sensitive to DNAse enzyme digestion, while inactive regions (albumin gene) are protected. ATP-dependent chromatin-remodeling complex or (a) Change in nucleosome position Change in the relative positions
of a few nucleosomes ATP-Dependent Chromatin Modeling Change in the spacing of nucleosomes over a long distance ATP-dependent chromatin-remodeling complex (b) Histone eviction Histone octamers are removed. ATP-dependent chromatin-remodeling Variant histones complex Brooker, fig 17.8 (c) Replacement with variant histones Histone Variants Play Roles in Chromatin Structure DNA Repair (H2A.X) Centromerebinding of kinetochore proteins (cenH3) Inactivated X (macroH2A)
Telomere(spH2B) ac Amino-terminal tail p Ser ac 5 Histone Modifications and their Effect on Nucleosome Structure 5 Lys p ac Lys ac ac Ser 15 Lys 20 Lys 10 ac Lys Lys 10 15
20 H2A Globular domain ac p Ser m Arg m Lys 5 m ac m ac p Lys Ser 10 H2B m ac ac Lys10 Lys
ac 15 Lys ArgLys 5 ac 20 Arg Lys Lys 15 m Lys H4 20 H3 (a) Examples of histone modifications Core histone protein Histone acetyltransferase COCH3 Acetyl groups COCH3
Histone deacetylase COCH3 Brooker, fig 17.9 DNA is less tightly bound to the histone proteins (b) Effect of acetylation Example of Histone Code From: The language of covalent histone modifications Brian D. Strahl and C. David Allis Nature 403, 41-45(6 January 2000) Genes that can be activated are flanked by nucleosome-free regions (NFR) and wellpositioned nucleosomes. nucleosome-free regions (NFRs) -2 -1 Enhancer +1 +2 Transcriptional termination site Transcriptional
start site Activator -2 -1 +1 +2 BINDING OF ACTIVATORS: Activator proteins bind to enhancer sequences. Enhancer Model for histone displacement for transcription activation CHROMATIN REMODELING AND HISTONE MODIFICATION: Activator proteins recruit chromatin remodeling complexes (e.g.SWI/SNF) and histone modifying enzymes (e.g. histone acetyltransferase). Nucleosomes may be moved or evicted. Some histones are covalently modified ( e.g. acetylation). Histone acetyltransferase -2 SWI/ SNF +2 ac ac
ac ac ac +2 ac -2 ac FORMATION OF PRE-INITIATION COMPLEX: General transcription factors and RNA polymerase II are able to bind to the core promoter and form a preinitiation complex. ac ac ac Pre-initiation complex Deacetylated histones -2 -1 +1 +2 ac Pre-mRNA
ac ac ac ELONGATION: During elongation, histones ahead of the open complex are covalently modified by acetylation and evicted or partially displaced. Behind the open complex, histones are deacetylated and become tightly bound to the DNA. Open complex Brooker, fig 17.11 Evicted histone proteins Histone Chaperone Inheritance of Methylation patterns is not limited to X chromosome Methylation of DNA used to reduce gene expression (e.g. Barr Bodies [inactivated Xchromosomes] are heavily methylated). Methylation occurs on cytosine (changing it to 5methylcytosine) Methylation most commonly occurs at CpG islands De-methylation associated with acetylation of histones CH3 GC.. CG.. CH3 Prader-Willi Syndrome
Mild to moderate retardation Hypotonia Poor feeding in infancy Short stature, small hands and feet Small genitalia Compulsive overeating and massive obesity Caused by inheritance of 15q11-13 deletions from father (forced expression from maternal csome). fig from Thompson and Thompson, Genetics in Medicine, 6th ed. Angelmans Syndrome caused by inheritance of 15q11q13 deletions from mother (forced paternal expression) Severe retardation No speech Seizures jerky gait
inappropriate laughter, protruding tongue enlarged jaw The same deletion cause different syndromes, depending on whether it is inherited from the mother or the father, due to different methylation patterns during oogenesis and spermatogenesis. Methylation inhibits the binding of an activator protein Enhancer CpG island Core promoter Coding sequence Activator protein Methylation CH3 CH3 CH3 CH3 CH3 CH3 Methyl groups block the binding of an activator protein to an enhancer element. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Brooker, Fig 17- 13a CH3 CpG island CH3 CH3 CH3 CH3 Core promoter CH3 CH3 CH3 CH3 Chromatin in an open conformation A methyl-CpG-binding protein binds to the methylated CpG island. CH3 Methyl-CpG-binding protein recruits other proteins that change the chromatin to a closed conformation CH3 CH3
CH3 CH3 CH3 CH3 CH3 CH3 Methyl-CpG binding protein Chromatin in a closed conformation CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 The methyl-CpG-binding protein recruits other proteins, such as histone deacetylase, that convert the chromatin to a closed conformation. CH3 Histone deacetylase
Brooker, Fig 17- 13b Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 5 3 C G G C 3 5 de novo methylation 5 An infrequent and highly regulated event 3 CH3 C G G C CH3 3 5
DNA replication Hemimethylated DNA 5 3 5 CH3 C G 3 C G G C 3 CH3 5 3 5 Maintenance methylation 5 Fully methylated DNA 3 5 CH3
3 G C C G G C 3 CH3 CH3 5 3 DNA methylase converts hemimethylated to fully- methylated DNA. A frequent event. C G G C CH3 5 Brooker, Fig 17- 14 Levels at which eukaryotic gene expression is controlled Initiating or inhibiting Transcription [majority] Transcriptional activators, coactivators, repressors Promoters
DNA response elements, enhancers, silencers DNA packaging and chromatin structure Histone acetylation, histone variation DNA methylation Initiating or inhibiting Translation mRNA processing, alternative splicing RNA stability RNA silencing Initiating or inhibiting protein activity (Posttranslational modification)
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