Transcript Regulation of Gene Expression

Regulation of Gene Expression
Part 2:
Gene Regulation
in Prokaryotes and
Eukaryotes
Gene Regulation in Prokaryotes
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The lac operon is also subject to
positive regulation
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What happens if both glucose and lactose
are present?
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Involves catabolite repressor
Represses genes for catabolism of other sugars
if glucose is present
Mediated by cAMP and CAP
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CAP: catabolite gene activator protein
also CRP: cAMP receptor protein
Effects of glucose and lactose levels on the
expression of the lac operon
Gene Regulation in Prokaryotes
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The lac operon is also subject to positive
regulation (cont):
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Mechanism:
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[increase] cAMP and [decreased] glucose: allows CAP
binding to DNA
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[increased] glucose depresses [cAMP]
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Stimulates transcription of lac operon
Lactose-metabolizing enzymes produced
Restricts expression of lac
Supresses use of secondary sugars
Regulon: coordinates regulated operons (CAP and cAMP)
cAMP receptor protein (CRP)
Activation of lac operon by CAP
Gene Regulation in Prokaryotes
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The ara operon is (+) and (-) regulated
by a single protein
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E. coli arabinose operon
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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
Ara operon
Regulation of Ara operon
Regulation of Ara operon
Regulation of Ara operon
Gene Regulation in Prokaryotes
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The ara operon is (+) and (-) regulated
by a single protein
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Ara C : regulates its own synthesis
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Represses transcription of its gene
Called Autoregulation
Effects of some regulatory DNA sequences
can be exerted from a distance via DNA
looping
Gene Regulation in Prokaryotes
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Transcription Attenuation: regulates genes for
a.a. biosynthesis
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Genes for amino acid synthesizing enzymes are
clustered in operons
Operons expressed when [a.a.] are inadequate
Trp operon of E. coli:
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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
The trp operon
The trp operon
Trp receptor
Gene Regulation in Prokaryotes
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Transcription Attenuation: regulates genes for
a.a. biosynthesis (cont)
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Transcription attenuation is a second trp
regulating mechnism
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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]
Transcriptional attenuation in the trp operon
Transcriptional attenuation in the trp operon
Gene Regulation in Prokaryotes
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Transcription Attenuation: regulates genes
for a.a. biosynthesis (cont):
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Each a.a. biosynthetic operon uses a similar
strategy
Induction of SOS response requires
destruction of repressor
Gene Regulation in Prokaryotes
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Induction of SOS response requires
destruction of repressor
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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
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Triggers lysis of repressor
Mediated by Rec A protein
Rec A only binds to single stranded DNA
SOS response in E. coli
Gene Regulation in Prokaryotes
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Regulated Developmental Switch: bacteriophage
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Objective is assembly of new viruses without cell
destruction
Choices are lysis or lysogeny
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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
Gene Regulation in Prokaryotes
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Regulated Developmental Switch: bacteriophage
lamda (cont):
 Bacteriophage lambda has a complex regulatory
circuit
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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
Gene Regulation in Prokaryotes
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Some genes are regulated by genetic
recombination
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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
Salmonella typhimurium
Regulation of flagellin genes in Salmonella:
Phase variation
Regulation of flagellin genes in Salmonella:
Phase variation
Gene Regulation in Eukaryotes
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Mechanisms resemble those in
prokaryotes
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Positive regulation more common
Involves selective binding of proteins to
control sequences
Effect is modulation of rate of transcription
initiation
Gene Regulation in Eukaryotes
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Mechanisms resemble those in
prokaryotes
Gene Regulation in Eukaryotes
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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
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Lampbrush chromosomes
Chromosome “puffs”
Gene Regulation in Eukaryotes
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Transcription activator proteins required
for binding RNA polymerase
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Some have general function
Others are specific:
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TATA-binding protein at “TATA-box”
Activators required because eukaryotic
RNA-polymerase lacks ability to bind
promoters
Gene Regulation in Eukaryotes
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Complex regulatory problems seen in
development of multicellular animals
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Genes function temporally and spatially
Must act in succession
Must be highly coordinated
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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
Regulation of Gene Expression
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End of part two