Regulation of gene expression

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Chapter 8

• Regulation of gene expression

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Regulation of gene expression was observed on
many different levels Transcription regulation is
the most common no energy wasted on formation of
intermediate structures Figure 8-3
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The gene regulatory protein binds to the major
groove of DNA Regulatory DNA sequence is a DNA
sequence to which a gene regulatory protein binds
to determine when, where, and in what quantities
a gene is to be transcribed Gene regulatory
protein is a general name for any protein that
binds to a specific DNA sequence to alter the
expression of the gene The regulatory proteins
interact with the major groove of DNA (formation
of H-bonds, ionic bonds) Figure 8-4
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Examples of regulatory proteins
1.Homeodomain three alpha helices and the 3
interacts with DNA Aspargine residue in helix
3 binds to adenine 2.Zinc finger helix and a
sheet are held together by a molecule of
zinc 3.Leucine zipper two alpha helices binding
to DNA on both sides, like a cloths pin Figure 8-5
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Procaryots Operon is a set of genes transcribed
into a single mRNA Example 5 genes coding for
enzymes necessary to produce amino acid
tryptophan Gene Regulatory Proteins 1.Activator-
protein that binds to a specific region of DNA to
permit the transcription of an adjacent
gene 2.Repressor-protein that binds to specific
region of DNA and inhibits the transcription of a
gene Fig8-6
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A tryptophan repressor protein-a gene regulatory
protein regulated by tryptophan levels in a
cell -No tryptophan in a cell, the repressor is
inactive, production of mRNA transcript coding
for tryptophan biosynthetic enzymes, the gene is
turned on -Tryptophan is present in a cell,
tryptophan binds to the repressor and the gene is
turned off Figure 8-7, 8-8
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The lac operon-controls the levels of lactose and
glucose in a cell Figure 8-9 The lac operon
codes for genes and proteins required to import
and digest a disaccharide lactose
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Eucaryotic Transcription Promoter
region-nucleotide sequence in DNA to which RNA
polymerase binds in order to begin transcription
Transcription factor-protein required to
initiate or regulate transcription in eucaryotic
cell
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Transcription factors RNA Polymerase in
eucaryotic cells requires transcription
factors TFII D is a transcription factor which
recognizes and binds to TATA box (TBP-TATA
Binding Protein)
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Transcription factors TATA box is a consensus
sequence in the promoter region of many
eucaryotic genes -rich in adenines and
thymines -specifies the position of
transcription initiation -typically it is
located 25 nucleotides upstream of the
transcription start site TFII H-contains an
enzyme kinase which phosphorylates RNA Polymerase
II phosphorylated RNA Polymerase II initiates
transcription Figure 8-10, 8-11
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RNA polymerase II has functions other than mRNA
transcription It is involved in capping the
5 end of mRNA transcript polyadenylation of the
3 end of mRNA transcript mRNA splicing and
excising introns It is activated by
phosphorylation (by TFIIH) Figure 8-12
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Enhancers are regulatory DNA sequences (that can
be many thousands base pairs away) to which gene
regulatory proteins bind, influencing the rate of
transcription of a gene Enhancer binds
activator protein (broken DNA represents a very
long distance between enhancer and TATA box) TATA
box binds transcription factors Transcription
begins upon formation of a complex composed of
activator protein, mediator, numerous
transcription factors, and RNA polymerase Fig 8-13
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Activator proteins recruit histone modifying
proteins or chromatin remodeling complexes An
example of histone modifying protein is the
histone acetylase, an enzyme which attaches an
acetyl group to selected lysines on the histone
protein Altered structure of chromatin makes it
more accessible to transcription initiation
factors
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Repressor proteins use histone deacetylases
(remove acetyl group) and thus decrease the DNA
accessibility to transcription factors This
process is associated with turning a gene
off Figure 8-14