Regulation of Gene Expression in Prokaryotes

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Chapter 16

• Regulation of Gene Expression in Prokaryotes

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Gene Regulation

• The growth and division of bacteria are regulated
  by genes
• Constitutive Genes-genes that are always active
  in growing cells
• They are essential to the normal functioning of a
  growing and dividing cells
• Regulated Genes-genes whose activity is
  controlled in response to the needs of a cell or
  organism
• Several mechanisms have evolved to turn genes on
  and off depending on the cells metabolic need for
  the gene products

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Gene Regulation

• Inducible (Adaptive) Systems-enzymes are produced
  when specific chemical substrates are present
• Inducer-the regulatory substance that brings
  about gene induction help control the expression
  of regulated genes
• Transcription of an inducible gene occur when a
  regulatory protein is present.
• When a regulatory protein is present the gene is
  turned on, mRNA is produced, and the gene is
  produced
• Ex inducible lac genes-gene activity is induced
  when lactose is added to the medium

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lac Operon

• Operon- a cluster of genes, the expressions of
  which are regulated together by
  operator-repressor protein interactions plus the
  operator region itself and the promoter
• Three structural genes in the lac operon
• lacZ (Beta- galactosidase)-breaks down lactose
  into glucose and galactose it catalyzes the
  isomerization of lactose to allolactose
• Allolactose not lactose is the inducer molecule
  responsible for the increased production of the
  three enzymes
• lacY (Lactose permease)-actively transports
  lactose into the cell
• lacA (Transacetylasae)-the function of this
  enzyme is poorly understood, but it may be
  involved in the removal of toxic by-products of
  lactose digestion from the cell.
• All three genes are transcribed as a single
  polycistronic mRNA
• The order of the controlling elements and genes
  in the lac operon is lacI-promoter-operator-lacZ-l
  acY-lacA

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lac Operon
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lac Operon

• Negative control of the lac operon occur when the
  repressor is bound to the operator, RNA
  polymerase can bind to the operons promoter but
  is blocked from initiating transcription
• Even in the absence of the inducer a low level of
  transcription still occur because when the
  repressor unbinds and before another binds, an
  RNA polymerase could initiate transcription of
  the operon

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lac Operon
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Jacob and Monod

• Jacob and Monod isolated mutants in which all the
  enzymes of the operon were synthesized
  constitutively (regardless of the presence or
  absence of the inducer)
• Two types of constitutive mutations
• Operator (lacO)
• Repressor gene (lacI)

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Operon Mutations

• Operator constitutive mutations (lacOc)-base pair
  alterations of the operator DNA sequence make it
  unrecognizable to the repressor protein
• The repressor cannot bind and the genes are
  constitutively expressed
• Cis-dominant
• The lacI gene mutations regulate transcription on
  the structural genes by producing a repressor
  molecule
• The repressor reversibly interacts with another
  molecule causing a conformational change
  (allosteric shift)
• As a result, the repressor loses its affinity for
  the lac operator and dissociates from the site
• With no repressor bound to the operator, RNA
  polymerase initiates synthesis of the genes
• Trans-dominant
• Superrepressor (Is)
• No production of lac enzymes in the presence or
  absence of lactose transcription of the genes
  never occur
• The superrepressor protein can bind to the
  operator, but cannot recognize the inducer
  allolactose
• Trans-dominant

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Constitutive Mutations
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Superrepressor
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Positive Control

• The positive control system turn on the
  expression of the operon ensuring that the lac
  operon will be expressed at high levels ONLY if
  lactose is the sole carbon source and NOT if
  glucose is present as well.
• If only lactose is present the positive
  regulation of the lac operon will occur
• Catabolite activator protein (CAP) binds with
  cAMP to form a CAP-cAMP complex
• CAP-cAMP complex binds to the CAP site, which is
  upstream of the site at which RNA polymerase
  binds to the promoter.
• This binding facilitates binding of the RNA
  polymerase and the initiation of transcription

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Positive Control

• When glucose is in the medium along with lactose,
  the glucose is used because catabolite repression
  occurs
• The lac operon is expressed at low levels even
  though lactose is present in the medium
• Glucose causes the amount of cAMP in the cell to
  be greatly reduced
• As a result, insufficient CAP-cAMP complex is
  available to facilitate RNA polymerase binding
  the lac promoter even though repressors are
  removed from the operator by the presence of
  allolactose

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Positive Regulation
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Gene Regulation

• Repressible Systems-the presence of a specific
  molecule inhibits genetic expression
• Gene activity is repressed when a chemical is
  added
• Ex trp operon-gene activity is repressed when
  tryptophan is present
• If a sufficient amount of tryptophan is present
  it is energetically inefficient for the organism
  to synthesize the enzymes necessary for
  tryptophan production

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trp operon

• Five structural genes A-E
• The promoter and the operator regions are
  upstream from trpE
• Between the promoter-operator and trpE is a short
  leader region, trpL
• With in trpL is an attenuator site that plays an
  important role in the regulation of the trp
  operon

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Regulation of the trp operon

• The regulatory gene for the trp operon is trpR
• The product of trpR is an corepressor protein,
  which is an inactive repressor that alone cannot
  bind to the operator
• When tryptophan is abundant within the cell, it
  interacts with the corepressor and converts it to
  an active repressor
• The active repressor binds to the operator and
  prevents the initiation of transcription of the
  trp operon
• In the presence of excess tryptophan a hairpin
  loop is formed that behaves as a terminator
  structure
• As a result the tryptophan enzymes are not
  produced

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Terminator Hairpin
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Regulation of the trp operon

• The second regulatory mechanism occur when
  tryptophan is limited
• Under severe tryptophan starvation the trp genes
  are expressed
• If tryptophan is scarce an antiterminator hairpin
  loop is formed
• As a result transcription is allowed to proceed
  past the involved DNA sequence and the entire
  mRNA is produced

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Antiterminator Hairpin
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trp Operon
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