summary: when and where do cells have control over which genes are expressed and which proteins are...
TRANSCRIPT
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Gene Regulation
… on / off
… fast / slow
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Summary:When and where do cells have control over which
genes are expressed and which proteins are active?
Proteins are activated through processing and signals can enlist cellular machinery to destroy proteins
Proteins determine when and how often RNA polymerase can bind
Modifications to pre-mRNA affect the life and use of RNA in the cytoplasm
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Newer Ideas to Look For• The study of epigenetics
investigates ways that whole sections of chromosomes are activated and deactivated.
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Looking at Individual Genes
• The simplest and most well understood model of the regulation of individual genes is found in prokaryotes.
• We will study two examples of gene control in E. coli ; a bacteria found in the human digestive tract
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FEED
ME!
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The Operon
Coding Region: Gene(s)
Control Region
Operon
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The Lac Operon in E. coli
GenesControl Region
Produce the enzymesnecessary for digesting lactose
Respond to the presence or absence of lactose and
glucose
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The Lac Operon in E. coli
lacZ lacY lacA
operator
promoter
activator
ON
OFF
Each lac gene codes for an enzyme only needed when lactose is present
FAST
SLOW
Binding site for RNAP
(CAP binding site)
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The Lac Operon in E. coli
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The Lac Operon in E. coli• With no proteins bound to either the
operator or activator regions, RNAP is free to bind to the promoter and transcribe the genes
lacZ lacY lacAOPA
pre-mRNA
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The Lac Operon in E. coli• If there is no lactose present, this
would be a waste of valuable resources like ATP and amino acids
lacZ lacY lacAOPA
pre-mRNA
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The Lac Operon in E. coli• To prevent transcription, the genes
can be “turned off” when a repressor protein is bound to the operator sequence
lacZ lacY lacAOPA
repressor
GET OUT OF THE WAY!
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The Lac Operon in E. coli• When lactose is present, it will bind
to the repressor, change its 3D shape and remove it from the operator.
lacZ lacY lacAOPA
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The Lac Operon in E. coli• When lactose is present, it will bind
to the repressor, change its 3D shape and remove it from the operator.
lacZ lacY lacAOPA
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The Lac Operon in E. coli• RNAP is now free to bind the
promoter and transcribe the sequence
lacZ lacY lacAOPA
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The Lac Operon in E. coli• Since the presence of lactose removes
the repressor and “turns on” the genes, lactose is called an inducer.
lacZ lacY lacAOPA
repressor
inducer
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What it “really” looks like
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The Lac Operon in E. coli
Operon is OFF
lacZ lacY lacAOPA
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The Lac Operon in E. coli
Operon is ON
lacZ lacY lacAOPA
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The Lac Operon in E. coli• Glucose is the preferred energy
source for cells
glucose
lactoseCell
OM NOM NOM NOM NOM
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The Lac Operon in E. coli• If glucose levels are LOW,
lactose metabolism is prioritized
• If glucose levels are HIGH, lactose metabolism is of lower priority
glucose
LOW
cyclic AMP (cAMP)
HIGH
How can glucose levels be detected in the nucleus?
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cAMP and CAP
• cAMP binds to the activator protein “CAP” (Catabolite activator protein)
• Together they will bind to DNA ahead of the promoter to help RNAP bind
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cAMP and CAP
• When glucose levels are low, cAMP activated CAP proteins will stimulate the transcription of over 100 genes including the lac operon
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The Lac Operon in E. coli• Without the cAMP-CAP complex
bound to the activator, RNAP is less likely to bind and produces fewer transcripts in a given time
lacZ lacY lacAOPA
“SLOW”
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The Lac Operon in E. coli• With the cAMP-CAP complex bound to
the activator, RNAP binds more readily, speeding up the production of RNA transcripts
lacZ lacY lacAOPA
“FAST”
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3D model of cAMP-CAP complex
bound to DNA
http://proteopedia.org/wiki/index.php/Catabolite_gene_activator_protein
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Summary
glu
cose
lact
ose
ON
/ O
FF
FAST
/ SLO
W
lacZ lacY lacAOPA
lacZ lacY lacAOPA
lacZ lacY lacAOPA
lacZ lacY lacAOPA
OFF
OFF
ON
ON
FAST
SLOW
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The trp Operon in E. coli
• If ingested levels of theamino acid tryptophanare low, e. coli is ableto synthesize it on its own.
• When tryptophan levels are low, the try operon is turned on to make the enzymes necessary to synthesize more.
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The trp Operon in E. coli
• Again, the cells should not waste resources making more tryptophan if it is already present
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The trp Operon in E. coli
• No tryptophan = operon on• Enzymes are synthesized to make trp
trpEOP trpD trpC trpB trpA
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The trp Operon in E. coli
• Tryptophan present = operon off• Trp is a co-repressor
(needed for repressor to function)
trpEOP trpD trpC trpB trpA
repressor
co-repressor
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What it “really” looks like
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Negative gene regulation
• Repressors are used to turn off genes
lacZ lacY lacAOPA
repressor
inducer
repressor
co-repressor
trpEOP trpD trpC trpB trpA
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Negative Gene RegulationThe lac Operon
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Positive Gene RegulationThe lac Operon
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• On your own, summarize the post transcriptional and post translational control cells have over their genes using your textbook
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Co-Repressor