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

GeneralIn prokaryotes, control of gene expression

occurs at 3 levels:

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)

GeneralIn eukaryotes, control of gene expression

occurs at these same 3 levels + 2 other levels:

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 –

Figure 18-4

Condensed chromatin

Decondensed chromatin

Acetyl group on histone

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

Post Transcription ControlProkaryotes – noneEukaryotes – alternative RNA splicing, fig

18.12.

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.

p53 Acts as a Tumor Suppressor

Cancer-causing amino acidmutations occur in regionsinvolved in DNA binding