Transcript Gene expression - McGraw Hill Higher Education

Chapter 12: Gene expression
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12-1
Regulation of gene expression
• Some proteins are required continuously or
often—the genes encoding them are expressed
constitutively
• Many other proteins are required for specific
purposes—the genes encoding them are
differentially regulated
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-2
Control points for expression
• Transcriptional regulation
• Post-transcriptional regulation
– includes co-transcriptional control—RNA splicing
– mRNA stability and transport
• Translation
• Post-translational regulation
– protein stability
– activation or inactivation, e.g. cleavage or
phosphorylation
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-3
Regulation of gene expression in
prokaryotes
• Most gene regulation is transcriptional in
prokaryotes
• Prokaryotic genes are located in functional groups
• Genes encoding enzymes in the same pathway
are arranged in sequence under the control of a
single promoter
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-4
Regulation of gene expression in
prokaryotes (cont.)
• The genes and their regulatory sequences are
called an operon
• Transcription from the promoter produces multigene RNA transcripts
• Multiple polypeptides are translated from these
RNAs
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12-5
Induction
• Expression of inducible genes is either positive or
negative at the level of transcription
• Positive regulation (induction) is often used in the
catabolism of compounds for energy production—
the presence of the molecule stimulates
expression of genes in the pathway
Copyright  2010 McGraw-Hill Australia Pty Ltd
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Slides prepared by Karen Burke da Silva, Flinders University
12-6
Repression
• Negative regulation (repression) may be used in
anabolic or synthetic pathways, where the
presence of the synthesised compound represses
the expression of the genes in the pathway
• Thus the genes are only expressed in the absence
of the compound
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-7
Fig 12.12: A bacterial operon
The lac operon (in Escherichia coli), responsible for the
metabolism of lactose to glucose and galactose
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-8
Regulation of the lactose (lac) operon
• The genes in the lac operon are induced in the
presence of lactose
• The lac operon is an example of negative
regulation—the genes are repressed in the
absence of lactose
• The repressor protein binds to lacO, the operator
region of the lac promoter, and prevents
transcription by blocking RNA polymerase binding
to the promoter
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-9
Regulation of the lac operon (cont.)
• A metabolite of lactose called allolactose binds to
a repressor protein and changes its shape,
preventing binding to the lac promoter
• This is called an allosteric interaction and is a
common molecular regulatory mechanism
• As lactose is used up in a cell, the free repressor
protein can bind to the promoter and prevent
transcription
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-10
Fig 12.4a: Regulation of the lac operon
Copyright  2010 McGraw-Hill Australia Pty Ltd
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12-11
Fig 12.4b and c: Regulation of the lac
operon (cont.)
(b)
(c)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-12
Regulation of the lac operon (cont.)
• The lac operon is also positively regulated
• When glucose is present as well as lactose, the lac
operon is repressed to allow metabolism of
glucose
• This is mediated by the catabolite activator protein
(CAP)
• cAMP normally binds to CAP protein (Fig. 12.5)
allowing transcription from the operon in the
absence of repressor binding
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-13
Regulation of the lac operon (cont.)
• High levels of glucose lower cAMP concentration
• The CAP–cAMP complex cannot form and
transcription is reduced even in the presence of
lactose
• The double regulation of the lac operon ensures
that it is only expressed at low glucose
concentrations when lactose is present
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-14
Fig 12.5: Regulation of the lac operon in
(a) low glucose and (b) high glucose conditions
(a)
(b)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-15
Fig 12.6: Expression of the lac operon
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-16
Question 1:
Predict what the outcome of a mutant of the
repressor gene would be
a) The lac genes would be expressed efficiently only in the
absence of lactose
b) The lac genes would be expressed efficiently only in the
presence of lactose
c)The lac genes would be expressed continuously
d) The lac genes would never be expressed efficiently
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-17
Regulation of the tryptophan
biosynthesis (trp) operon
• When tryptophan is present in an environment, it
can be more efficient to import than to synthesise
• The trp operon contains five genes for tryptophan
biosynthesis under the control of a single
promoter
• The absence of tryptophan leads to trp operon
expression
• Regulatory events are similar to lac operon
• The trp repressor protein binds to the operator
sequence only in the presence of tryptophan
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-18
Regulation of the tryptophan
biosynthesis (trp) operon (cont.)
• Tryptophan binds to the trp repressor protein,
changing its conformation and allowing binding
• Transcription cannot proceed
• Low levels of tryptophan prevent repressor
binding
• Transcription and synthesis of tryptophan
proceed
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-19
Fig 12.7: Regulation of the trp operon
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-20
Regulation of gene expression in
eukaryotes
• Most eukaryotic genes are controlled at the level of
transcription
• The mechanisms involve trans-acting regulatory
proteins interacting with cis-acting control
sequences on the DNA
• DNA control sequences include promoters,
promoter-proximal elements, enhancers and
silencers
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-21
Regulation of gene expression in
eukaryotes (cont.)
• Eukaryotic promoters do not provide sufficient
recognition signals for RNA polymerase II to
initiate transcription in vivo
• Core promoter elements must each be recognised
by regulatory proteins (basal factors and
coactivators) which bind to these sites
• RNA polymerase II is bound and initiated properly
and transcription commences
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-22
Fig 12.8: Regulation of transcription in eukaryotic cells
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PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-23
Yeast GAL regulation
• The yeast Saccharomyces cerevisiae is a good
model of eukaryote gene expression
• In the presence of a medium containing galactose,
five genes are expressed
• Unlike the genes in an operon, these five genes
are separate and transcribed independently
• The expression of the genes is controlled by both
positive and negative protein regulators
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-24
Yeast GAL regulation (cont.)
• The GAL4 (Gal4p) protein binds to an upstream
DNA activator sequence (UAS) on all five genes
and induces their expression
• The GAL80 protein (Gal80p) acts as a
transcription repressor by binding to Gal4p,
preventing the induction of transcription
• When galactose is present, a metabolite
dissociates the Gal80p from the Gal4p, allowing
transcription to proceed
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-25
Fig 12.11: Regulation of galactose (GAL)-inducible genes in yeast
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-26
Combinations of enhancers and
silencers
• Series of DNA sequences that are recognised and
bound to by DNA-binding proteins called
transcription factors
• Enhancers and silencers can, however, act over
longer distances than promoters, and may be
positioned either upstream or downstream of the
promoter they enhance
• Each enhancer or silencer element acts
independently of others
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-27
Gene regulatory cascades in
eukaryotes
• Transcription factors bind to enhancer or silencer
sequences
• They are often tissue-specific, allowing a complex
pattern of gene expression
• Transcription factors are themselves gene
products
• Coordinated expression of many genes can
therefore be achieved by just a few genes
• Such regulatory cascades are important in
embryonic development of eukaryotes
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-28
RNA-mediated regulation of gene
expression
• Not all regulatory genes act by producing a protein
product
• Small RNA molecules have been discovered that
bind directly to mRNA and block their translation
• These ‘microRNAs’ provide another level of gene
regulation
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-29
Fig 12.12: Control of gene expression by microRNAs
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-30
Question 2:
A repressible operon (e.g. the trp operon) is turned
‘off’ when:
a) The repressor binds to the gene
b) The repressor–corepressor complex binds to the
operator
c) The repressor binds to DNA polymerase
d) The repressor does not bind to the gene
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-31
Summary
• Gene expression is controlled
• Genes may be expressed constantly or
differentially regulated
• Prokaryotic genes are regulated primarily at the
level of transcription
• Eukaryotic transcription requires activation by
transcription factors
• Non-protein products can regulate gene
expression – microRNAs
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
12-32