Control of Gene Expression

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Control of Gene Expression

• Dr. Jason Linville
• University of Alabama at Birmingham
• jglinvil_at_uab.edu

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Summary

• Gene Regulation General
• Strategy for Controlling Genes
• Gene Regulation in Bacteria
• Lac operon
• Gene Regulation in Eukaryotes
• Human metallothionein gene

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Gene Control

• Why Control?

• Some genes are on (being transcribed) almost
  all the time.
• Called housekeeping genes
• Examples ribosome components, enzymes used in
  basic metabolic pathways

Many genes are only turned on when they are
needed.
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Gene Control

• Why Control?

Transcribing genes that are not needed is a waste
of energy and may interfere with the status of
the cell.
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Gene Control

• Regulation in Eukaryotes

• Respond to a Range of Stimuli
• Prokaryotes respond to external stimuli (food
  detected, metabolizing enzymes turned on)
• Eukaryotes also respond to internal stimuli (some
  types of cells release hormones, growth factors,
  etc., that affect other types of cells)

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Gene Control

• Regulation in Eukaryotes

• Developmentally Regulated
• Multicellular organisms progress through
  developmental stages
• Different genes expressed at different times
  during development

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Gene Control

• Regulation in Eukaryotes

• Cell Specialization in Multicellular Organisms
• Cell differentiation is a result of differences
  in gene expression
• Different genes expressed in different cells

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Gene Control gt Strategy

• Gene control is control over amount of gene
  product (RNA or protein) in cell

• Multiple ways to control the amount of gene
  product in a cell

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Gene Control gt Strategy

• Controlling Gene Product Amount

• Rate of transcription rate mRNA is produced
  faster produced more product
• mRNA degradation rate mRNA is broken down
  faster broken down less product
• mRNA processing capping, polyadenylation,
  splicing slower processing less product
• Translation rate of translation or number of
  ribosomes translating fast/more more product

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Gene Control gt Strategy

• Although control probably involves all of these,
  the most understood are changes in the rate of
  transcription.

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Gene Control gt Lac Operon

• Gene Expression in Bacteria

• Regulation of lac operon is a classic example

• Bacteria have 3 genes in a row (operon) that
  involve breaking down lactose for energy

• In order to be efficient, these genes should not
  be expressed unless lactose is present.

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Gene Control gt Lac Operon

• Goal
• Transcription low when lactose is absent

• Lac I (gene upstream from operon) produces a
  repressor which binds to promoter region.

• Binding of repressor prevents RNA polymerase from
  binding and transcribing genes

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Gene Control gt Lac Operon

• Goal
• Increase transcription when lactose is present

• Allolactose will bind to the repressor, changing
  its conformation and causing it to fall off the
  promoter site

• Promoter site now available for RNA polymerase to
  bind transcription of lac genes begins

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Gene Control gt Lac Operon

• Lactose vs. Allolactose

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Gene Control gt Lac Operon

• Goal
• Turn off transcription when lactose is used up

• Allolactose binding is an equilibrium event. As
  it dissociates, it is metabolized by
  ß-galactosidase

• The free repressor is available to bind the
  promoter site and stop transcription

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Gene Control gt Lac Operon
If both glucose and lactose are present, it is
more efficient for the bacteria to utilize
glucose, and not worry about lactose.
A system is in place where the presence of
glucose can prevent the metabolization of lactose.
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Gene Control gt Lac Operon

• Goal
• Decrease transcription if lactose and glucose are
  present.

• Glucose inhibits adenylate cyclase, which
  produces cAMP.

• High glucose low cAMP

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Gene Control gt Lac Operon

• Goal
• Decrease transcription if lactose and glucose are
  present.

• cAMP binds catabolite activator protein (CAP),
  which is then able to bind the cap site upstream
  of the promoter.

• CAP binding needed to stimulate binding of RNA
  polymerase and transcription.

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Gene Control gt Lac Operon

• Goal
• Decrease transcription if lactose and glucose are
  present.

• High glucose low cAMP low cAMP-CAP complex
  low transcription lactose not used

• Low glucose High cAMP high cAMP-CAP complex
  high transcription possible (lactose used)

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Gene Control gt Lac Operon

• Lesson from lac Operon

• Binding sites upstream of transcription origin.
  Proteins binding to these sites can repress or
  activate transcription

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Gene Control

• Human metallothionein gene

• Protects from toxic effects of metals
• Heavy metals stimulate transcription

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Gene Control gt Metallothionein

• Human metallothionein gene

9 Upstream Sites (4 types)

• TATA box where RNA polymerase attaches to gene

• Upstream Promoter Elements (UPEs)
• GC box or CAAT box
• Bind proteins that activate transcription

• Enhancers activators of transcription
• Longer than UPEs farther upstream

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Gene Control gt Metallothionein

• Human metallothionein gene

9 Upstream Sites (4 types)

• Transient Response Element activates
  transcription in a temporary fashion
• In metallothionein, heavy metals bind to proteins
  enabling them to bind metal response elements
• This binding promotes transcription of
  metallothionein gene

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Gene Control

• DNA binding proteins

There are common polypeptide conformations found
in most binding proteins.

• Helixturn-helix
• Zinc Fingers
• Leucine Zippers