Transcript Protein Synthesis - Quinnipiac University

Regulation of Gene Expression
In Prokaryotes
Regulation of Gene Expression
• Constituitive Gene Expression (promoters)
• Regulating Metabolism (promoters and operators)
• Regulating Development (sigma switches)
Constituitive Gene Expression (promoters)
promoter
coding sequence
Regulating Metabolism
(promoters and operators)
promoter operator
coding sequence
Major and minor grooves - protein binding
Major and minor grooves - protein binding
Recognition involves the major groove
Regulatory Proteins Bind DNA
negative control
positive control
Many regulatory proteins are dimers and bind to
palindromes
Repressors
& metabolic
pathways
repressor deactivated
genes ON
product
repressor activated
genes OFF
energy
Synthetic Pathway
precursor molecules
energy
macromolecule
Degredative Pathway
repressor activated
genes OFF
substrate
repressor deactivated
genes ON
Metabolic signals and repressor activity
metabolic
signal
molecule
gene off
gene on
gene on
gene off
repressor
protein
DNA
binding
site
the lac operon
Lactose Metabolism
the metabolic signal for repression
Negative Control
When activated by binding of the metabolic signal molecule,
the lac repressor binds to the operator, blocking RNA polymerase
Negative control in the lac operon
the lac operon
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
But alternatively, control regions can be
involved - a dominant operator mutation
But alternatively, control regions can be
involved - one inducer mutation
But alternatively, control regions can be
involved - another inducer mutation
the Lac control region
Cyclic AMP
Positive Control
cAMP is present when glucose is unavailable
cAMP binds to CAP protein, which then binds to the promoter
binding of the CAP-cAMP complex to the promoter, activates it
CAP-cAMP positioning of CTD
CAP-cAMP acts in formation of closed promoter
The Lactose Operon:
Control of a degredative pathway
Practice
Answers
Trp operon, control of a
a biosynthetic pathway
The Tryptophan Operon:
Control of a synthetic pathway
Tryptophan
Synthesis
allosteric
protein
Attenuation of trp
The leader sequence:
two trp codons and a stop codon
The mechanism of attenuation - termination
Region 2 can bind with 1 or 3, but affinity for 1
is higher
Over riding attentuation if shortage of trp
causes ribosome to stall, 2 binds with 3
…no terminator hairpin forms
Control of development:
Sigma switching
Different sigmas and their regions of homology
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
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
Sigma Switching
•
Early phase.
Early genes have promoters recognized by the host’s 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 CORE’s binding site than it’s own sigma, thus displacing the host’s sigma and turning off the
early genes and turning on the mid genes.
early
transcripts
early proteins,
including gp28
middle
transcripts
middle proteins,
including gp33, gp34
late
transcripts
late proteins
hostphase genes have promoters recognized by gp28. Gp33 and gp34 are
Middle
middle proteins that actsigma
as a sigma factor for the late genes.
•
Middle phase .
•
Late phase
gp28
sigma
gp33-34
sigma
Lambda
Lysogenic Life Cycles - Temperate Viruses
Genetic map of Lambda
3 phases again
N antitermination
Q antitermination
cI and cro
duke it out
Establishing Lysogeny
Maintaining Lysogeny
Induction
SOS
Prokaryote versus Eukaryote Comparison
Prokaryotes
Eukaryotes
Transcription Factor
(eukaryotic sigma)
sigma
Step 1
promoter
Step 1
promoter
promoter
Step 2
Step 2
Positive control in eukaryotes - gene enhancers
Gene activation in Eukaryotes: A different complicated initiation complex for each
different context in which a gene is expressed