©2001 timothy g. standish initiation of transcription: in eukaryotes timothy g. standish, ph. d

©2001 Timothy G. Standish

Initiation ofInitiation of Transcription:Transcription:

In EukaryotesIn EukaryotesTimothy G. Standish, Ph. D.


Expression Control In EukaryotesExpression Control In Eukaryotes Some of the general methods used to control expression

in prokaryotes are used in eukaryotes, but nothing resembling operons is known

Eukaryotic genes are controlled individually and each gene has specific control sequences preceding the transcription start site

In addition to controlling transcription, there are additional ways in which expression can be controlled in eukaryotes


Eukaryotes Have Large Eukaryotes Have Large Complex GeneomesComplex Geneomes

The human genome is about 3 x 109 base pairs or ≈ 1 m of DNA

Because humans are diploid, each nucleus contains 6 x 109 base pairs or ≈ 2 m of DNA

Some gene families are located close to one another on the same chromosome

Genes with related functions appear to be distributed almost at random throughout the the genome


Highly Packaged DNA Cannot Highly Packaged DNA Cannot be Expressedbe Expressed

Because of its size, eukaryotic DNA must be packaged

Heterochromatin, the most highly packaged form of DNA, cannot be transcribed, therefore expression of genes is prevented

Chromosome puffs on some insect chomosomes illustrate areas of active gene expression


Only a Subset of Genes is Only a Subset of Genes is Expressed at any Given TimeExpressed at any Given Time

It takes lots of energy to express genes Thus it would be wasteful to express all genes all the time By differential expression of genes, cells can respond to

changes in the environment Differential expression, allows cells to specialize in

multicelled organisms. Differential expression also allows organisms to develop

over time.


DNA

Cytoplasm

Nucleus

G AAAAAA

Export

Degradation etc.G AAAAAA

Control of Gene ExpressionControl of Gene Expression

G AAAAAA

RNAProcessing

mRNA

RNA

Transcription

Nuclear pores

Ribosom

e

Translation

Packaging

Modification

Transportation

Degradation


Logical Expression Control PointsLogical Expression Control Points DNA packaging Transcription RNA processing mRNA Export mRNA masking/unmasking and/or

modification mRNA degradation Translation Protein modification Protein transport Protein degradation

Increasing costIncreasing cost

The logical place to control

expression is before the

gene is transcribed

The logical place to control

expression is before the

gene is transcribed


Three Eukaryotic Three Eukaryotic RNA PolymerasesRNA Polymerases

1 RNA Polymerase I - Produces rRNA in the nucleolus, accounts for 50 - 70 % of transcription

2 RNA Polymerase II - Produces mRNA in the nucleoplasm - 20 - 40 % of transcription

3 RNA Polymerase III - Produces tRNA in the nucleoplasm - 10 % of transcription


A “Simple” Eukaryotic GeneA “Simple” Eukaryotic Gene

Terminator Sequence

Promoter/Control Region

Transcription Start Site 5’ Untranslated Region 3’ Untranslated Region

Exons

Introns

3’5’ Exon 2 Exon 3Int. 2Exon 1 Int. 1

RNA Transcript


5’DNA

3’

EnhancersEnhancers

Enhancer Transcribed Region

3’5’ TF TFTF

3’5’ TF TFTF

5’RNA

RNAPol.

RNAPol.

Many bases

Promoter


Eukaryotic RNA Polymerase IIEukaryotic RNA Polymerase II RNA polymerase is a very fancy enzyme that

does many tasks in conjunction with other proteins

RNA polymerase II is a protein complex of over 500 kD with more than 10 subunits:


Eukaryotic RNA Polymerase II Eukaryotic RNA Polymerase II PromotersPromoters

Several sequence elements spread over about 200 bp upstream from the transcription start site make up RNA Pol II promoters

Enhancers, in addition to promoters, influence the expression of genes

Eukaryotic expression control involves many more factors than control in prokaryotes

This allows much finer control of gene expression


RNA Pol. II

InitiationInitiation

T. F.

RNA Pol. II

5’mRNA

Promoter

T. F.

T. F.


Eukaryotic PromotersEukaryotic Promoters

5’ Exon 1Promoter

Sequence elements

~200 bp

TATA

~-25

InitiatorInitiator“TATA Box”“TATA Box”

Transcription start site

(Template strand)-1+1

SSTATAAAASSSSSNNNNNNNNNNNNNNNNNYYCAYYYYYNN

S = C or G Y = C or T N = A, T, G or C


InitiationInitiationTFIID BindingTFIID Binding

-1+1


TFIID

“TATA Box”

TBP Associated Factors (TAFs)

TATA Binding Protein (TBP)


InitiationInitiationTFIID BindingTFIID Binding

TFIID

80o Bend

-1+1



InitiationInitiationTFIIA and B BindingTFIIA and B Binding

TFIID

TFIIA

-1+1


TFIIB


InitiationInitiationTFIIF and RNA Polymerase BindingTFIIF and RNA Polymerase Binding

TFIID

TFIIA

-1+1


TFIIB

RNA PolymeraseTFIIF


InitiationInitiationTFIIE BindingTFIIE Binding

TFIID

TFIIA

-1+1


RNA PolymeraseTFIIBTFIIF

TF

IIE

TFIIE has some helicase activity and may by involved in unwinding DNA so that transcription can start


InitiationInitiationTFIIH and TFIIJ BindingTFIIH and TFIIJ Binding

TFIID

TFIIA

-1+1



TF

IIE

TFIIH has some helicase activity and may by involved in unwinding DNA so that transcription can start

TFIIH

P PP

TFIIJ



TFIID

TFIIA

-1+1



TF

IIETFIIH

P PP

TFIIJ



-1+1


RNA PolymeraseP P

P

©2001 timothy g. standish initiation of transcription: in eukaryotes timothy g. standish, ph. d

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