regulation of gene expression in eukaryotes bit 220 chapter 24
TRANSCRIPT
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Regulation of Gene Expression in Eukaryotes
BIT 220
Chapter 24
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Opportunities for the control of gene expression in the eukaryotic cell
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Gene Expression
• Spatial – not every gene product needed in every cell type
• Temporal – Different genes expressed at different times– Environmental stimuli– Hormones– Especially seen in development- formation of
tissues and organs
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Spatial and Temporal examples
• Spatial– Tubulin in plants– Microtubules found in many places– TUA1- pollen grains; TUB1-roots
• Temporal– Figure 24.2, globin genes– Tetramer (then add heme group)– Some in embryo, fetus and after birth– Pseudogenes – duplicated gene w/termination signal
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Regulation
• Chap 12 – RNA processing– 5’ cap– Poly A tail– Intron removal
• In eukaryotes, more level of regulation than prokaryotes due to complex organelles
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Alternate splicing
• Figures 24.4 and 24.5
• Troponin T gene
• Sex lethal genes in Drosophila – helps define sexual differentiation in flies
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Cytoplasmic control
• mRNA stability:– Long vs. short lived mRNAs– Long- many rounds protein synthesis from
one mRNA– Short – rapidly degraded, needs more
transcription to replenish (half-life)
• Rapid mRNA degradation may be desirable
• Half-life problem with making a drug, too
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mRNA stability
• Poly A tails – can add stability
• Longer tails stabilize message more
• E.g., histone mRNAs no poly A tails; message very short lived
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Induction of transcription
• Not found as often in eukaryotes as in prokaryotes
• Induction can work by:– Temperature– Light– hormones
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Induction of transcription
• Temperature– Synthesize heat shock proteins (HSPs)– Transcriptional regulation – stress of high
heat signals HSPs to be transcribed– Studied in Drosophila- but occurs in humans
also
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Induction of transcription
• Light– Figure 24.7– RBC (ribulose 1,5 bisphosphate carboxylase)– Plants must absorb light energy– RBC produced when plants are exposed to
light (see Northern blot in figure)- remember what is a Northern blot?)
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Induction of transcription
• Hormones– Secreted, circulate through body, make
contact with target cell and regulate transcription
– Called signal molecules– 2 classes of hormones that activate
transcription• Steroid hormones• Peptide hormones
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Steroid hormones
• Small, lipid molecules derived from cholesterol
• Easily pass through cell membranes
• Examples– Estrogen– Progesterone– Testosterone– Glucocorticoids
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Steroid hormones
• Figure 24.8
• HRE’s- hormone response elements – mediate hormone induced gene expression
• Number of HRE’s dictate strength of response (work cooperatively)
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Peptide hormones
• Linear chain of amino acids
• Examples
– Insulin
– Growth hormone
– prolactin
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Peptide hormones
• Cannot pass through cell membrane easily, so convey signals through membrane bound receptors
• Signal transduction – hormone binds receptor on cell surface, signal gets internalized, then cascade of events begin
• Figure 24.9
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Molecular control
• Transcription factors – accessory proteins for eukaryotic gene expression
• Basal transcription factors– Each binds to a sequence near
promotor– Facilitates alignment of RNA
polymerase
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Special transcription factors
• Bind to enhancers
• Promotor specific (HRE’s for e.g.)
• Properties of enhancers:– Can act over several thousand bp– Function independent of orientation– Function independent of position – upstream,
downstream, etc. (different than promotors- close to gene and only one orientation)
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Figure 24.10
• Yellow gene in Drosophila
• Tissue specific enhancers for pigmentation for each body part
• Mosaic patterns- alterations in yellow gene transcription in some body parts but not others
• Also see SV40 enhancer (simian virus 40) – Figure 24.11
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How do enhancers work?
• Influence activity of proteins that bind promoters
• RNA pol and basal transcription factors
• Physical contact with other proteins?– Enhancer and promotor regions brought
together by DNA folding
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Transcription factors
• 2 chemical domains– DNA binding– Transcriptional activation
• Can be separate or overlapping
• Physical interaction also quite possible
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Transcription factor motifs
• Figure 24.12
• Zinc finger – DNA binding
• Helix-turn-helix - DNA binding (COOH required)
• Leucine zipper - binding
• Helix-loop-helix – helical regions allow for dimerization– Homo and hetero dimers
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Gene expression and chromosomes
• DNA needs to be accessible to RNA pol for transcription initiation
• Place on chromosome may affect this
• So, gene exp influenced by chromosomal structure
• E.g., lampbrush chromosomes, Figure 24.13
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Is transcribed DNA more “open”?
• Used DNAse I treatments
• Groudine and Weintraub – showed transcriptionally active DNA more easily degraded by DNAse I than untranscribed DNA (more “open” to nuclease digestion)
• Have DNAse I hypersensitivity sites – near promotors and enhancers
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Whole chromosomes:activation and inactivation
• Skip pages 615 to 1st column, page 622
• Equalizing activity of X chromosomes in XX versus XY organisms
• Recall mechanisms:– Humans, inactivate one X chromosomes in
females– In Drosophila, male X makes double the gene
product
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X compensation
• Inactivation, hyperactivation, hypoactivation
• What is molecular mechanism of dosage compensation?– Specific factor(s) bind to X- regulate its gene
expression above all other regulatory elements
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Dosage Compensation – example of X in humans
• XIC- X inactivation center – makes XIST (X inactive specific transcript) - 17kb mRNA with no ORF- so likely does not encode a protein
• Figure 24.22
• RNA is the functional product of the gene
• Found only in nucleus and not associated with active
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Opportunities for the control of gene expression in the eukaryotic cell