Regulation of Gene Expression

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Regulation of Gene Expression

• Part 2
• Gene Regulation in Prokaryotes and Eukaryotes

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Gene Regulation in Prokaryotes

• The lac operon is also subject to positive
  regulation
• What happens if both glucose and lactose are
  present?
• Involves catabolite repressor
• Represses genes for catabolism of other sugars if
  glucose is present
• Mediated by cAMP and CAP
• CAP catabolite gene activator protein
• also CRP cAMP receptor protein

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Effects of glucose and lactose levels on the
expression of the lac operon
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Gene Regulation in Prokaryotes

• The lac operon is also subject to positive
  regulation (cont)
• Mechanism
• increase cAMP and decreased glucose allows
  CAP binding to DNA
• Stimulates transcription of lac operon
• Lactose-metabolizing enzymes produced
• increased glucose depresses cAMP
• Restricts expression of lac
• Supresses use of secondary sugars
• Regulon coordinates regulated operons (CAP and
  cAMP)

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cAMP receptor protein (CRP)
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Activation of lac operon by CAP
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Gene Regulation in Prokaryotes

• The ara operon is () and (-) regulated by a
  single protein
• E. coli arabinose operon
• One protein exerts both and regulation
• Binding a signal molecule alters conformation
  from repressor form
• Repressor binds one DNA regulatory site
• Activator, without signal molecules, binds to
  another DNA sequence

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Ara operon
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Regulation of Ara operon
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Regulation of Ara operon
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Regulation of Ara operon
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Gene Regulation in Prokaryotes

• The ara operon is () and (-) regulated by a
  single protein
• Ara C regulates its own synthesis
• Represses transcription of its gene
• Called Autoregulation
• Effects of some regulatory DNA sequences can be
  exerted from a distance via DNA looping

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Gene Regulation in Prokaryotes

• Transcription Attenuation regulates genes for
  a.a. biosynthesis
• Genes for amino acid synthesizing enzymes are
  clustered in operons
• Operons expressed when a.a. are inadequate
• Trp operon of E. coli
• 5 genes for conversion of chorismate to
  tryptophan
• mRNA from trp operon has 3 min half-life
• When trp increases, trp binds to trp repressor
• Causes conformational change in repressor protein
  that permits binding to the operator
• Trp operator overlaps promoter, binding repressor
  blocks RNA polymerase
• A self-regulation mechanism

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The trp operon
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The trp operon
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Trp receptor
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Gene Regulation in Prokaryotes

• Transcription Attenuation regulates genes for
  a.a. biosynthesis (cont)
• Transcription attenuation is a second trp
  regulating mechnism
• Uses translation termination site
• leader blocks transcription
• Halts transcription before operon halts RNA
  polymerase
• Couples transcription to translation via leader
  peptide
• Attenuation of transcripts increases as trp
  increases due to sensitivity of leader peptide to
  trp

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Transcriptional attenuation in the trp operon
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Transcriptional attenuation in the trp operon
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Gene Regulation in Prokaryotes

• Transcription Attenuation regulates genes for
  a.a. biosynthesis (cont)
• Each a.a. biosynthetic operon uses a similar
  strategy
• Induction of SOS response requires destruction of
  repressor

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Gene Regulation in Prokaryotes

• Induction of SOS response requires destruction of
  repressor
• SOS response is induced if chromosome is damaged
• An example of coordinated regulation of unlinked
  genes
• Multiple unlinked genes repressed by Lex A
  protein
• All genes induced simultaneously when DNA is
  damaged
• Triggers lysis of repressor
• Mediated by Rec A protein
• Rec A only binds to single stranded DNA

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SOS response in E. coli
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Gene Regulation in Prokaryotes

• Regulated Developmental Switch bacteriophage
• Objective is assembly of new viruses without cell
  destruction
• Choices are lysis or lysogeny
• Lysis results in destruction of infected cell
• Lysogenic cycle
• Virus may inhabit host cell for generations
• Viral DNA inserts into host, replicates passively
• Phage in this state Prophage
• Some trigger induction
• Virus enters lytic phase

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Gene Regulation in Prokaryotes

• Regulated Developmental Switch bacteriophage
  lamda (cont)
• Bacteriophage lambda has a complex regulatory
  circuit
• Oversees choice between pathways
• Involves many lambda proteins
• Two (N and Q) act as anti-terminators
• Modify host RNA polymerase to by-pass termination
  sites
• Other proteins serve as promoters or activators

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Gene Regulation in Prokaryotes

• Some genes are regulated by genetic recombination
• Occurs spontaneously in prokaryotes
• Called Phase Variation
• Physically moves promoters relative to genes
  regulated
• Mechanism used by some pathogens as defense
  against host immune system
• E.g.Salmonella

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Salmonella typhimurium
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Regulation of flagellin genes in Salmonella
Phase variation
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Regulation of flagellin genes in Salmonella
Phase variation
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Gene Regulation in Eukaryotes

• Mechanisms resemble those in prokaryotes
• Positive regulation more common
• Involves selective binding of proteins to control
  sequences
• Effect is modulation of rate of transcription
  initiation

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Gene Regulation in Eukaryotes

• Mechanisms resemble those in prokaryotes

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Gene Regulation in Eukaryotes

• Eukaryotic promoter and enhancer elements mediate
  expression of cell-specific genes
• Cells contain factors that recognize promoters
  and enhancers in the genes they transcribe
• Transcription is accompanied by changes in
  chromosomal structure
• Lampbrush chromosomes
• Chromosome puffs

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Gene Regulation in Eukaryotes

• Transcription activator proteins required for
  binding RNA polymerase
• Some have general function
• Others are specific
• TATA-binding protein at TATA-box
• Activators required because eukaryotic
  RNA-polymerase lacks ability to bind promoters

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Gene Regulation in Eukaryotes

• Complex regulatory problems seen in development
  of multicellular animals
• Genes function temporally and spatially
• Must act in succession
• Must be highly coordinated
• Most genes expressed early in development
• Genes must be turned off, on in cell to
  facilitate function
• Regulation involves expression and location of
  genes and their products in developing organisms

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Regulation of Gene Expression

• End of part two