Chapter 11: Gene expression
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-1
Regulation of gene expression
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Some proteins are required continuously or
often—the genes encoding them are expressed
constitutively
Many other proteins are required for specific
purposes—the genes are inducible under
particular conditions
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-2
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
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Prokaryote gene expression
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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
(cont.)
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Prokaryote gene expression
(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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11-5
Induction
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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 (Fig. 11.2 (a))
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11-6
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 (Fig. 11.2 (b))
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11-7
Fig 11.2(b): Repression
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11-8
Bacterial operons (Fig 11.3)
•
An example is the lac operon, responsible for the
metabolism of lactose to glucose and galactose
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11-9
The lac operon
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The genes in the lac operon are induced in the
presence of lactose in the culture media
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
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-10
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, the free repressor protein
can bind to the promoter and prevent transcription
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
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11-11
Fig 11.4: Regulation of the lac operon
(a)
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Fig 11.4: Regulation of the lac operon
(b)
(c)
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11-13
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. 11.5)
allowing transcription from the operon in the
absence of repressor binding
(cont.)
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11-14
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
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11-15
Fig 11.5: Regulation of the lac operon in (a)
low and (b) high glucose conditions
(a)
(b)
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11-16
Fig 11.6: Expression of the lac operon
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The trp operon
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This 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
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
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11-18
The 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
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-19
Fig 11.7: Regulation of the trp operon
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11-20
Gene regulation 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
(cont.)
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Gene regulation in eukaryotes
(cont.)
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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
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-22
Fig 11.8: Regulation of transcription in eukaryotic cells
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11-23
Yeast GAL regulation
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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
(cont.)
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Yeast GAL regulation (cont.)
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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
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Fig 11.10: Regulation of galactose (GAL)-inducible genes in yeast
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Enhancers and silencers
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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 (up to 50kb), and
may be positioned either upstream or downstream
of the promoter they enhance
Each enhancer or silencer element acts
independently of others
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Regulatory cascades
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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 the
development of complex organisms
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11-28
DNA conformation
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Coiling of DNA into nucleosomes during
chromosome condensation inhibits transcription
Acetylation of histone proteins binding the
nucleosomes reduces the DNA binding strength
DNA is not coiled so tightly and RNA polymerase II
can bind to initiate transcription
Enhancer-binding complexes often include
histone-acetylating enzymes
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11-29
RNA-mediated regulation
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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
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
11-30
Fig 11.11: Control of gene expression by microRNAs
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11-31