Regulation of Gene Expression In Prokaryotes

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Regulation of Gene ExpressionIn Prokaryotes
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Regulation of Gene Expression

• Constituitive Gene Expression (promoters)
• Regulating Metabolism (promoters and operators)
• Regulating Development (sigma switches)

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Constituitive Gene Expression (promoters)
promoter
coding sequence
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Regulating Metabolism (promoters and operators)
promoter
coding sequence
operator
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Major and minor grooves - protein binding
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Major and minor grooves - protein binding
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Recognition involves the major groove
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Regulatory Proteins Bind DNA
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Many regulatory proteins are dimers and bind to
palindromes
negative control
positive control
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Repressors metabolic pathways
repressor deactivated genes ON
product
repressor activated genes OFF
energy
Synthetic Pathway
precursor molecules
macromolecule
Degredative Pathway
energy
repressor activated genes OFF
substrate
repressor deactivated genes ON
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Metabolic signals and repressor activity
gene on
gene off
gene off
gene on
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the lac operon
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Lactose Metabolism
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the metabolic signal for repression
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Negative ControlWhen activated by binding of the
metabolic signal molecule,the lac repressor
binds to the operator, blocking RNA polymerase
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Negative control in the lac operon
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the lac operon
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Conventional interpretation of dominance -
focusing on enzyme function
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Conventional interpretation of codominance -
focusing on enzyme function
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But alternatively, control regions can be
involved - a recessive operator mutation
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But alternatively, control regions can be
involved - a dominant operator mutation
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But alternatively, control regions can be
involved - one inducer mutation
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But alternatively, control regions can be
involved - another inducer mutation
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the Lac control region
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Cyclic AMP
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Positive ControlcAMP is present when glucose is
unavailable cAMP binds to CAP protein, which
then binds to the promoterbinding of the
CAP-cAMP complex to the promoter, activates it
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CAP-cAMP positioning of CTD
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CAP-cAMP acts in formation of closed promoter
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The Lactose OperonControl of a degredative
pathway
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Practice
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Answers
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(No Transcript)
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Trp operon, control of aa biosynthetic pathway
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The Tryptophan OperonControl of a synthetic
pathway
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TryptophanSynthesis
allosteric protein
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Attenuation of trp
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The leader sequencetwo trp codons and a stop
codon
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The mechanism of attenuation - termination
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Region 2 can bind with 1 or 3, but affinity for 1
is higher
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Over riding attentuation if shortage of trp
causes ribosome to stall, 2 binds with 3 no
terminator hairpin forms
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Control of developmentSigma switching
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Different sigmas and their regions of homology
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RNA polymerase in bacteria
core enzyme
RNA polymerase
sigma
Sigma factors recognize promoters and
disassociate when the RNA polymerase binds to the
promoter, leaving the core enzyme to make the
transcript
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Phage SPOI (in B. subtilis)

• 3 phases of gene expression
• Early phase
• Mid phase
• Late phase
• Each phase uses a different sigma, each
  recognizing a different promoter
• The genes of each phase all have the same kind of
  promoter, recognized by one of the sigma factors

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Sigma Switching

• Early phase. Early genes have promoters
  recognized by the hosts RNA polymerase. gp28 is
  an early protein that acts as a sigma factor for
  the middle phase genes. gp28 has a higher
  affinity for the COREs binding site than its
  own sigma, thus displacing the hosts sigma and
  turning off the early genes and turning on the
  mid genes.
• Middle phase . Middle phase genes have promoters
  recognized by gp28. Gp33 and gp34 are middle
  proteins that act as a sigma factor for the late
  genes.
• Late phase

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Lambda
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Lysogenic Life Cycles - Temperate Viruses
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Genetic map of Lambda
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3 phases again
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N antitermination
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Q antitermination
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cI and cro duke it out
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Establishing Lysogeny
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Maintaining Lysogeny
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InductionSOS
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Prokaryote versus Eukaryote Comparison
Prokaryotes
Eukaryotes
Transcription Factor (eukaryotic sigma)
sigma
promoter
Step 1
Step 1
promoter
promoter
Step 2
Step 2
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Positive control in eukaryotes - gene enhancers
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Gene activation in Eukaryotes A different
complicated initiation complex for each different
context in which a gene is expressed