Chapter 18 CONTROL OF GENE EXPRESSION
SummaryGeneral concepts – controlControls
Chromosome remodelingTranscriptionPost-transcriptionTranslationPost translation
GeneralIn prokaryotes, gene expression is
controlled by the external environment. Ex. fuel availability
In eukaryotes, gene expression is controlled by the internal environment. Ex. hormones
Figure 17-1
RNA polymerase
DNA
mRNA
Transcriptional control Translational control Post-translational control
Protein
Ribosome
RNA polymerase
Onset oftranscription
Life span (stability)of mRNA
Translationrate
Protein activationor inhibition (bychemicalmodification)
Figure 18-1
Nucleus Chromatin (DNA-protein complex)
1. Chromatin remodeling
2. Transcription
“Open” DNA (Some DNA not closely bound to proteins)
3. RNA processing
Primary transcript(pre-mRNA)
Cap TailMature mRNA
Cytoplasm
4. mRNA stability
5. Translation
Degraded mRNA (mRNA lifespan varies)
mRNA
Polypeptide
Active protein
6. Post-translationalmodification (folding, transport, activation, degradation of protein)
Chromatin RemodelingChromatin structure, fig 18.2
Sections of DNA are wound around 8 histone proteins = nucleosome. Nucleosomes are connected by a linker, H1 histone protein, and a short section of DNA.
Nucleosomes are further condensed into a 30nm fiber.
Figure 18-2Nucleosomes in chromatin
Nucleosomes
DNA
Nucleosome structureLinkerDNA
H1 protein attachedto linker DNA andnucleosome
DNA
Group of8 histoneproteins
Nucleosome
In some cases, nucleosomes may be grouped into30-nanometer fibers.
30 nm
Chromatin RemodelingChromosome Structure cont’d
DNA nucleosomes must be “unwound” or remodeled to allow transcription.
Mechanisms - 2Chromatin remodeling complexes –
Methylation or acetylation –
Regulation of TranscriptionProkaryotes
Primary type of control of gene expression.Negative control
Most genes are “normally” inhibited by repressor proteins that bind to DNA preventing transcription. Release of the repressor allows transcription.
Repressor protein is released by a metabolite that requires expression of genes to be used by prokaryote, fig 17.7.
Figure 17-7
Repressor present, lactose absent:
Repressor present, lactose present:
No repressor present, lactose presentor absent:Transcription occurs.
Repressorsynthesized
DNA
lacl+
RNA polymerasebound to promoter
(blue DNA)
lacZ lacY
TRANSCRIPTION BEGINS-Galactosidase Permease
mRNA
lacZ lacY
RNA polymerasebound to promoter
(blue DNA)
Lactose-repressorcomplex
Repressorsynthesized
No functionalrepressor synthesized
mRNATRANSCRIPTION BEGINS
-Galactosidase Permease
lacZ lacY
RNA polymerasebound to promoter
(blue DNA)
Lacl –
Repressor binds to DNA.No transcription occurs.
Lactose binds to repressor,causing it to release fromDNA. Transcription occurs(lactose acts as inducer).
Normallacl gene
Normallacl gene
lacl+
Mutantlacl gene
The repressor blocks transcription
Regulation of TranscriptionProkaryotes cont’d
Positive control: CAP (catabolite activator protein) binds to promoter
when prokaryote must use an alternate fuel, fig 17.15; activated by cAMP.Ex. cAMP dependent on glucose concentration.
Transcription occurs more frequently.
Figure 17-15lac operon
Promoter Repressor
INFREQUENT TRANSCRIPTION
INFREQUENT TRANSCRIPTION
CAPsite
CAPsite
CAPsite
FREQUENT TRANSCRIPTION
Operator
Operator
Operator
RNA polymerase boundloosely to promoter
RNA polymerase boundloosely to promoter
RNA polymerase boundtightly to promoter
Glucose HIGH
Glucose HIGH
Glucose LOW
Lactose LOW
Lactose HIGH
Lactose HIGH
lacZ
lacZ
lacZ
lacY
lacY
lacY
lacA
lacA
lacA
Inducer-repressor complex
Regulation of TranscriptionEukaryotes
Transcription factors must be present to allow RNA polymerase to bind to promoter, fig 18.11.Basal transcription factors – common to all
promoters. No control
Figure 18-11
Basal transcription complex
Basal transcription factorsassociated with TBP
Other basal transcription factors
TATA
TBP
RNApolymerase II
Promoter Start site
Regulation of TranscriptionEukaryotes
Regulatory transcription factors – proteins specific for certain genes. Bind to…
Regulatory sequences on DNA. Protein binding may inhibit (silencers) or increase (enhancers) transcription, figs 18.7, 18.10.
Coactivators – help regulatory transcription factors bind.
Figure 18-7
Start site
Exon Intron
Enhancer Promoter Enhancer
IntronExon ExonPromoter-proximal element
Enhancer
Figure 18-9
EXTRACELLULAR SIGNALS TRIGGER CELL-SPECIFIC GENE EXPRESSION.
Extracellularsignals
Receptor protein in membrane
Intracellularsignals
Regulatoryproteins
1. Signal arrives at cell with message: “Become a muscle cell.”
Promoter-proximalelement
Promoter
RNA polymeraseExon Intron Exon Intron Exon
TRANSCRIPTION
Gene for muscle-specific protein
EnhancerEnhancer
Nuclear envelope
Cytoplasm
Plasma membrane
3. Regulatory proteins are produced or activated in response to intracellular signal.
2. Signal transduction results in production of intracellular signal.
4. Regulatory proteins bind to regulatory sites in DNA, triggering expression of muscle-cell-specific genes.
Figure 18-10THE ELEMENTS OF TRANSCRIPTIONAL CONTROL: A MODEL
Regulatorytranscriptionfactor
Chromatin remodelingcomplex (or HATs)
1. Regulatory transcription factors recruit chromatin-remodeling complex, or HATs. Chromatin decondenses.
ExposedDNA
Promoter-proximalelement
Promoter Exon Exon Exon
Exon
Intron Intron
Intron
Intron
Intron
Intron
ExonExon
Exon
Exon
Exon
Promoter
Transcribed portion of gene formuscle-specific protein
Co-activators
Regulatorytranscriptionfactors
Promoter-proximal element
Basal transcription complex
TRANSCRIPTION
RNA polymerase II
Basal transcription complex
2. When chromatin decondenses, a region of DNA is exposed, including the promoter.
3. Regulatory transcription factors recruit proteins of the basal transcription complex to promoter. Note looping DNA.
4. RNA polymerase II completes the basal transcription complex; transcription begins.
EnhancerEnhancer
Figure 18-12
Tropomyosin gene
Intron Intron Intron
Exon Exon Exon Exon
Processed mRNAs
Skeletal muscle
Smooth muscle
Some exons are specific to tropomyosin in skeletal or smooth muscle; some exons are common to both muscle types
Translational ControlProkaryotes
mRNAs are degraded by ribonucleases (RNAses). The time that mRNA’s are intact varies according to need for the protein encoded on the mRNA.
Initiation or elongation may be inhibited.
Translational ControlEukaryotes
mRNA degradation controlled by need, RISC proteins activated by short pieces of RNA (miRNAs).
Translation dependent on production of other proteins, ex. egg fertilization.
Translation inhibited/enhanced by temperature, etc.
Modification of 5’cap or poly A tail
Post Translational ControlProkaryotes – proteins are produced as
inactive forms and must be chemically modified to become active.
EukaryotesInactive forms must be activated by chemical
modifications.STATs = signal transducers and activators of
transcription.
Post Translational ControlEukaryotes
STAT (signal transducers and activators of transcription) proteinsMust be phosphorylated to be active. Activate transcription of genes that trigger cell
growth and division.Important in activating immune cells.STAT mutations – mutant STATs are activated
without phosphorylation and permanently activate transcription.
Figure 18-14
Cytoplasm
Signalingmolecule
Cell-surface receptor
Inactive STAT protein(two single polypeptide chains)
Activated STAT protein (dimer of two polypeptide chains)
Nuclear envelope
Enhancer
TRANSCRIPTION
Transcription activatedNucleus
Cancer and Gene RegulationTumor supressor genes (p53) – mutation of
gene produces mutant protein that does not stop cell cycle.
Proto-oncogenes – normal genes that activate each phase in the cell cycle and growth when other “growth factors” are present.Mutation produces oncogenes that permanently
activate the cell cycle in the absence of growth factors.