Control of Gene Activity

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Control of Gene Activity

• Chapter 18 Regulation of Gene Expression

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Gene Regulation

• Prokaryotes and eukaryotes alter gene expression
  in response to their changing environment
• In multicellular eukaryotes, gene expression
  regulates development and is responsible for
  differences in cell types
• RNA molecules play many roles in regulating gene
  expression in eukaryotes

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Prokaryotic regulation

• Gene expression in bacteria is controlled by the
  operon model
• An operon is a cluster of functionally related
  genes can be under coordinated control by a
  single on-off switch
• Bacteria do not require same enzymes all the time
• They produce just enzymes needed at the moment
• The regulatory switch is a segment of DNA
  called an operator (positioned within the
  promoter)

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Prokaryotic regulation

• The operon can be switched off by a protein
  repressor
• The repressor prevents gene transcription by
  binding to the operator and blocking RNA
  polymerase
• The repressor is the product of a separate
  regulatory gene
• The repressor can be in an active or inactive
  form, depending on the presence of other
  molecules
• A corepressor is a molecule that cooperates with
  a repressor protein to switch an operon off

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Operon Components

• The operon includes the following
• 1) Regulatory gene
• Located outside the operon
• Codes for a repressor protein molecule?
• 2) Promoter
• Sequence of DNA where RNA polymerase attaches
• 3) Operator
• A short sequence of DNA where repressor binds,
  preventing RNA polymerase from binding
• 4) Structural genes
• Code for enzymes of a metabolic pathway
• Transcribed as a unit

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E. coli Tryptophan

• E. coli is a bacteria that lives in your colon
• It has a metabolic pathway that allows for the
  synthesis of the amino acid tryptophan (Trp)
• This pathway starts with a precursor molecule and
  proceeds through five enzyme catalyzed steps
  before reaching the final product tryptophan
• It is important that E. coli be able to control
  the rate of Trp synthesis because the amount of
  Trp available from the environment varies
  considerably

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E. coli Tryptophan

• If you eat a meal with little or no Trp, the E.
  coli in your gut must compensate by making more
• If you eat a meal rich in Trp, E. coli doesn't
  want to waste valuable resources or energy to
  produce the amino acid because it is readily
  available for use
• Therefore, E. coli uses the amount of Trp present
  to regulate the pathway
• If levels are not adequate, the rate of Trp
  synthesis is increased
• If levels are adequate, the rate of Trp synthesis
  is inhibited

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trp Operon

• The Trp operon has three components
• Five Structural Genes
• These genes contain the genetic code for the five
  enzymes in the Trp synthesis pathway
• One Promoter
• DNA segment where RNA polymerase binds and starts
  transcription
• One Operator
• DNA segment found between the promoter and
  structural genes
• It determines if transcription will take place

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trp Operon
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trp operon

• When nothing is bonded to the operator, the
  operon is "on"
• RNA polymerase binds to the promoter and
  transcription is initiated
• The 5 structural genes are transcribed to one
  mRNA strand
• The mRNA will then be translated into the 5
  enzymes that control the Trp synthesis pathway

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trp operon

• The operon is turned "off" by a specific protein
  called the repressor
• The repressor is inactive in this form and can
  not bind properly to the operator
• To become active and bind properly, a corepressor
  must associate with the repressor
• The corepressor for this operon is Tryptophan
• This makes sense because E. coli does not want to
  synthesize Trp if it is available from the
  environment

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trp operon
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trp operon

• An active repressor binds to the operator
  blocking the attachment of RNA polymerase to the
  promoter
• Without RNA polymerase, transcription and
  translation of the structural genes can't occur
  and the enzymes needed for Tryptophan synthesis
  are not made
• By default the trp operon is on and the genes for
  tryptophan synthesis are transcribed
• When tryptophan is present, it binds to the trp
  repressor protein, which turns the operon off
• The repressor is active only in the presence of
  its corepressor tryptophan thus the trp operon
  is repressed if tryptophan levels are high

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Repressible vs Inducible

• The trp operon is a repressible operon
• This type is one that is usually on
• Binding of a repressor to the operator shuts off
  transcription
• The end product, Trp, decreases or stops the
  transcription of the enzymes necessary for its
  production
• The opposite is called an inducible operon
• This type is one that is usually off
• A molecule called an inducer inactivates the
  repressor and turns on transcription
• An example of an inducible system is lac
  (lactose) operon

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lac operon

• The Lac operon has 3 components
• Three Structural Genes
• Contain the genetic code for the 3 enzymes in the
  lactose catabolic pathway
• One Promoter
• DNA segment where RNA polymerase binds and starts
  transcription
• One Operator
• DNA segment found between the promoter and
  structural genes
• It determines if transcription will take place
• If the operator in turned "on", transcription
  will occur

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lac operon

• The lac operon is an inducible operon whose genes
  code for enzymes used in the hydrolysis and
  metabolism of lactose
• By itself, the lac repressor is active and
  switches the lac operon off
• The active repressor binds to the operator
• This blocks RNA polymerase from transcribing the
  genes
• Basically-- the lac operon is in the off
  position and needs to be turned on

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lac operon OFF
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lac operon

• A molecule called an inducer inactivates the
  repressor to turn the lac operon on
• What inducer is used in the lac operon? Lactose
• This makes sense because the cell only needs to
  make enzymes to catabolize lactose if lactose is
  present
• When lactose enters the cell it binds to the
  repressor and changes its shape so that it can't
  bind to the operator
• RNA polymerase can now start transcription of the
  3 structural genes that will control lactose
  catabolism

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lac operon ON
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Eukaryotic Regulation

• Eukaryotes lack a universal regulatory mechanism
  to control expression of genes coding proteins
• In multicellular organisms gene expression is
  essential for cell specialization
• DNA in eukaryotes is packaged as chromatin within
  a nucleus

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Chromatin in Eukaryotes

• Most chromatin is loosely packed in the nucleus
  during interphase and condenses prior to mitosis
• Loosely packed chromatin is called euchromatin
• The genes within this area are easily accessed,
  thus easily transcribed
• During interphase a few regions of chromatin are
  highly condensed into heterochromatin
• Genes within this area are difficult to access,
  thus they are usually not transcribed

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Eukaryotic regulation

• There are four primary levels of control of gene
  activity
• 1. Transcriptional Control
• 2. Posttranscriptional Control
• 3. Translational Control
• 4. Posttranslational Control

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Transcriptional Control

• Takes place in nucleus, the site of transcription
• Determines which structural genes are transcribed
  and the rate of transcription
• Includes organization of chromatin
• Includes the action of regulatory proteins that
  may activate or inhibit transcription (such as
  transcription factors)

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Regulatory Proteins

• General transcription factors are essential for
  the transcription of all protein-coding genes
• Some transcription factors function as activators
• An activator is a protein that binds to an
  enhancer and stimulates transcription of a gene
• Enhancers are DNA sequences that may be far away
  from a gene or even located in an intron
• Some transcription factors function as repressors
• A repressor is a protein that prevents the
  expression of a particular gene
• Some activators and repressors act indirectly by
  influencing chromatin structure to promote or
  silence transcription

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Posttranscriptional control

• Takes place in the nucleus or cytoplasm
• Involves differential processing of pre-mRNA
• Differential excision of introns and splicing of
  mRNA can vary type of mRNA
• In alternative RNA splicing, different mRNA
  molecules are produced from the same primary
  transcript, depending on which RNA segments are
  treated as exons and which as introns
• Involves regulation of transport of mature mRNA
• The life span of mRNA molecules in the cytoplasm
  is a key to determining protein synthesis
• The mRNA life span is determined in part by
  sequences in the leader and trailer regions (cap
  and tail)

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Translational control

• Takes place in the cytoplasm, the site of
  translation
• Life expectancy of mRNA molecules can vary, as
  well as their ability to bind ribosomes
• Some mRNA's may need additional changes before
  they are translated
• The initiation of translation of selected mRNAs
  can be blocked by regulatory proteins that bind
  to sequences or structures of the mRNA

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Posttranslational control

• Takes place in the cytoplasm after protein
  synthesis
• May involve activation or degradation of the
  protein
• After translation, various types of protein
  processing, including cleavage and the addition
  of functional groups, are subject to control
• Proteasomes are giant protein complexes that bind
  protein molecules and degrade them
• Eukaryotic Gene Control Animation

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Review Questions

1. Differentiate between prokaryotic and eukaryotic
   gene regulation.
2. Explain the use of an operon as a prokaryotic
   form of gene regulation.
3. Name and describe the four main parts of an
   operon.
4. Define the following terms operator, repressor,
   inducer, regulatory gene, and corepressor.
5. Describe the functioning of the trp operon as a
   repressible operon and state its overall
   significance to E. coli.
6. Differentiate between repressible and inducible
   operons.
7. Describe the functioning of the lac operon as an
   inducible operon and state its overall
   significance to E. coli.
8. Differentiate between euchromatin and
   heterochromatin in eukaryotes.
9. Name and describe the important traits of the 4
   primary levels of control of gene activity in
   eukaryotes.
10. Differentiate between activators, enhancers, and
    repressors.
11. Describe alternative RNA splicing and its
    significance to gene control.
12. Define proteasome.