Transcript Chapt16_lecture

Control of Gene Expression
Chapter 16
Control of Gene Expression
• Initiation of Transcription is controlled by controlling
gene expression.
• Regulatory proteins bind to DNA to either block or
stimulate transcription, depending on how they
interact with RNA polymerase
• Prokaryotic organisms are able to respond to changes
in their environment by regulating gene expression.
• Eukaryotic cells are able to maintain homeostasis by
regulating gene regulation
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Regulatory Proteins
• Binding of regulatory proteins to specific DNA
sequences results in controlling gene expression.
– regulatory proteins gain access to the bases of
DNA at the major groove via their DNA-binding
motifs
• DNA-binding motifs are regions of regulatory
proteins which bind to DNA
– helix-turn-helix motif
– homeodomain motif
– zinc finger motif
– leucine zipper motif
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Prokaryotic Regulation
• Control of transcription initiation can be:
– positive control – increases transcription
when activators bind DNA
– negative control – reduces transcription
when repressors bind to DNA regulatory
regions called operators
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Prokaryotic Regulation
• Prokaryotic cells often respond to their environment
by changes in gene expression.
• Genes involved in the same metabolic pathway are
organized in operons.
• Some operons are induced when the metabolic
pathway is needed. Example: Lactose utilization
• Some operons are repressed when the metabolic
pathway is no longer needed. Example: Tryptophan
utilization
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Prokaryotic regulation
• The lac operon contains
genes for the use of lactose
as an energy source.
• Regulatory regions of the
operon include the CAP
binding site, promoter, and
the operator.
• The coding region contains
genes for 3 enzymes:
– b-galactosidase,
permease, and
transacetylase
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Respression of lac operon
• lac repressor
binds to the
operator to
block
transcription
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How lac operon is induced?
• In the presence of
lactose, an
inducer molecule
binds to the
repressor protein
• repressor can no
longer bind to
operator
• transcription
proceeds
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How the presence of glucose prevents
induction of lac operon?
• In the presence of both glucose
and lactose, bacterial cells prefer
to use glucose.
• Glucose prevents induction of the
lac operon.
– binding of CAP – cAMP
complex to the CAP binding
site is required for induction
of the lac operon
– high glucose levels cause low
cAMP levels
– high glucose  low cAMP 
no induction
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Prokaryotic Regulation
• The trp operon encodes genes for the
biosynthesis of tryptophan.
• The operon is not expressed when the cell
contains sufficient amounts of tryptophan.
• The operon is expressed when levels of
tryptophan are low.
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Trp operon is controlled by trp
respressor
• The trp operon is negatively regulated by the
trp repressor protein
– trp repressor binds to the operator to block
transcription
– binding of repressor to the operator requires
a corepressor which is tryptophan
– low levels of tryptophan prevent the
repressor from binding to the operator
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Eukaryotic Regulation
• Controlling the expression of eukaryotic genes
requires transcription factors.
– general transcription factors are required
for transcription initiation
• required for proper binding of RNA
polymerase to the DNA
– specific transcription factors increase
transcription in certain cells or in response
to signals
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Eukaryotic Transcription
• General transcription factors
bind to the promoter region of
the gene.
• RNA polymerase II then binds
to the promoter to begin
transcription at the start site
(+1).
• Enhancers are DNA sequences
to which specific transcription
factors (activators) bind to
increase the rate of
transcription.
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Eukaryotic Transcription
• Coactivators and mediators are also
required for the function of transcription
factors.
– coactivators and mediators bind to
transcription factors and bind to other
parts of the transcription apparatus
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Eukaryotic Chromosome Structure
• Eukaryotic DNA is packaged into
chromatin.
• Chromatin structure is directly related to
the control of gene expression.
• Chromatin structure begins with the
organization of the DNA into nucleosomes.
• Nucleosomes may block RNA polymerase
II from gaining access to promoters.
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Eukaryotic Chromosome Structure
• Methylation (the addition of –CH3) of DNA or
histone proteins is associated with the control of
gene expression.
• Clusters of methylated cytosine nucleotides bind
to a protein that prevents activators from
binding to DNA.
• Methylated histone proteins are associated with
inactive regions of chromatin.
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Posttranscriptional Regulation
• Control of gene expression usually involves the
control of transcription initiation.
• But gene expression can be controlled after
transcription, with mechanisms such as:
– RNA interference
– alternative splicing
– RNA editing
– mRNA degradation
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Posttranscriptional Regulation
• RNA interference involves the use of small
RNA molecules
• The enzyme Dicer chops double stranded
RNA into small pieces of RNA
– micro-RNAs bind to complementary RNA
to prevent translation
– small interfering RNAs degrade particular
mRNAs before translation
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Posttranscriptional Regulation
• Introns are spliced out of
pre-mRNAs to produce the
mature mRNA that is
translated.
• Alternative splicing
recognizes different splice
sites in different tissue
types.
• The mature mRNAs in each
tissue possess different
exons, resulting in different
polypeptide products from
the same gene.
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Posttranscriptional Regulation
• RNA editing creates mature mRNA that are
not truly encoded by the genome.
• For example –
– apolipoprotein B exists in 2 isoforms
– one isoform is produced by editing the
mRNA to create a stop codon
– this RNA editing is tissue-specific
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Posttranscriptional Regulation
• Mature mRNA molecules have various halflives depending on the gene and the location
(tissue) of expression.
• The amount of polypeptide produced from a
particular gene can be influenced by the halflife of the mRNA molecules.
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Protein Degradation
• Proteins are produced
and degraded
continually in the cell.
• Proteins to be degraded
are tagged with
ubiquitin.
• Degradation of proteins
marked with ubiquitin
occurs at the
proteasome
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This project is funded by a grant awarded under the President’s Community Based Job Training Grant as
implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC
is an equal opportunity employer and does not discriminate on the following basis:
against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability,
political affiliation or belief; and
against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA),
on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United
States, or his or her participation in any WIA Title I-financially assisted program or activity.
This product was funded by a grant awarded under the President’s High Growth Job Training Initiative, as
implemented by the U.S. Department of Labor’s Employment & Training Administration. The information contained
in this product was created by a grantee organization and does not necessarily reflect the official position of the U.S.
Department of Labor. All references to non-governmental companies or organizations, their services, products, or
resources are offered for informational purposes and should not be construed as an endorsement by the
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Helix-Turn-Helix Motif
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Homeodomain Motif
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Zinc Finger Motif
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Leucine Zipper Motif
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This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S. Department
of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and does not discriminate on
the following basis:
against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political affiliation or belief; and
against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on the basis of the
beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or his or her participation in any WIA
Title I-financially assisted program or activity.
“This workforce solution was funded by a grant awarded under the President’s
Community-Based Job Training Grants as implemented by the U.S.
Department of Labor’s Employment and Training Administration. The solution
was created by the grantee and does not necessarily reflect the official position
of the U.S. Department of Labor. The Department of Labor makes no
guarantees, warranties, or assurances of any kind, express or implied, with
respect to such information, including any information on linked sites and
including, but not limited to, accuracy of the information or its completeness,
timeliness, usefulness, adequacy, continued availability, or ownership. This
solution is copyrighted by the institution that created it. Internal use by an
organization and/or personal use by an individual for non-commercial purposes
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owner.”
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