Transcription

Transcription
 Prokaryotic transcription
 The RNA polymerase
 The origin & prokaryotic promoters
 The initiation, elongation,& termination.
 Prokaryotic termination signals
 Prokaryotic transcription product
 Eukaryotic transcription
 Eukaryotic RNA polymerases
 Eukaryotic promoters

 Enhancers & transcriptional elements
 The processes of initiation, elongation &
 termination
 Eukaryotic termination signals
 RNA processing & modifications
The spliceosomes
Thalassemias & globin mRNA splicing
Modification to mRNA, tRNA

The Central
Dogma
• DNA codes for RNA
• RNA codes for protein

Transcription: synthesis of one RNA
molecule using one of the two DNA
strands as a template by the enzyme
RNA Polymerase.
Newly Synthesized RNA

The RNA polymerase-catalyzed synthesis of
RNA on a DNA template strand

Prokaryotes Eukaryotes
exons introns
nucleus
cytosol
translation
translation
transcription
DNA
transcription
nuclear export
splicing
pre-mRNA
mRNA
processing
Eukaryotic Transcription and Tanslation are separated
by space and time

RNA Polymerase
• The enzyme responsible for the RNA
synthesis is DNA-dependent RNA
polymerase.
–The prokaryotic RNA polymerase
is a multiple-subunit protein of
~480kD.

1. Requires no primer for polymerization.
2. Requires DNA for activity and is most
active with a double-stranded DNA as
template.
3. 5’  3’ synthesis.
4. Require Mg2+ for RNA synthesis activity.
5. lacks 3’  5’ exonuclease activity, and the
error rate of nucleotides incorporation is
10-4 to 10-5.
6. Usually are multisubunit enzyme.
RNA Polymerase
(NMP)n + NTP  (NMP)n+1 + PPi

The holoenzyme of RNA-polymerase in E.coli
consists of 5 different subunits: 2    
Subunit MW Function
 36.5 KD
Determines the DNA to be
transcribed
 150 KD Catalyzes polymerization
 155.5 KD Binds & open DNA template
 70 KD
Recognizes the promoter
for synthesis initiation
ω 11 KD Subunit packing
RNA Polymerase of E. Coli

Eukaryotic Promoter Sequences
Promoter
Enhancers
Activators

Structure of bacterial prokaryotic
promoter region
TATA-Box / Pribnow box
• This is a stretch of 6
nucleotides ( 5'- TATAAT-3')
centered about 8-10
nucleotides to the left of the
transcription start site.
-35 Sequence
• A second consensus
nucleotide sequence
( 5'- TTGACA-3'), is centered
about 35 bases to the left of
the transcription start site.
Biochemistry For Medics- Lecture Notes 17

Transcription process
1. Promoter binding
2. DNA unwinding
3. RNA chain initiation
4. RNA chain elongation
5. RNA chain termination

• Initiation phase: RNA-polymerase
recognizes the promoter and starts
the transcription.
• Elongation phase: the RNA strand
is continuously growing.
• Termination phase:
RNA-polymerase stops synthesis
and the nascent RNA is separated
from the DNA template.
Transcription of Prokaryotes

Initiation of Transcription at
Promoters
Transcription is divided into three steps for
both prokaryotes and eukaryotes.
Initiation, Elongation and Termination.
The process of elongation is highly
conserved between prokaryotes and
eukaryotes, but initiation and
termination are somewhat different.

Initiation
• RNA-polymerse recognizes the
TTGACA region (-35 sequence), and
slides to the TATAAT region
(-10 sequence), then opens the DNA
duplex.
• The unwound region is about 17 bp.

• The first nucleotide on RNA transcript
is always purine triphosphate. GTP is
more often than ATP.
• The pppGpN-OH structure remains on
the RNA transcript until the RNA
synthesis is completed.
• The three molecules form a
transcription initiation complex.
RNA-pol (2) - DNA - pppGpN- OH 3

• No primer is needed for RNA synthesis.
• The  subunit falls off from the
RNA-polymerase once the first
3,5-phosphodiester bond is formed.
• The core enzyme moves along the DNA
template to enter the elongation phase.

Elongation
• The release of the  subunit causes
the conformational change of the
core enzyme. The core enzyme slides
on the DNA template toward the 3
end.
• Free NTPs are added sequentially to
the 3 -OH of the nascent RNA strand.
(NMP)n + NTP (NMP)n+1 + PPi
RNA strand substrate
elongated
RNA strand

• RNA-polymerase, DNA segment of
~40nt and the nascent RNA form a
complex called the transcription
bubble.
• The 3 segment of the nascent RNA
hybridizes with the DNA template, and
its 5 end extends out the
transcription bubble as the synthesis
is processing.

Transcription bubble
Nascent RNA

Nascent RNA
Removal of
Sigma subunit

Termination
• The RNA Polymerase stops moving on
the DNA template. The RNA transcript
falls off from the transcription complex.
• The termination occurs in either
 -dependent or  -independent manner.

The  factor, a hexamer, is a ATPase
and a Helicase.
Termination function of  factor
-dependent termination

-independent termination
• The termination signal is a stretch of
30-40 nucleotides on the RNA
transcript, consisting of many GC
followed by a series of U.
• The sequence specificity of this
nascent RNA transcript will form
particular stem-loop structures to
terminate the transcription.

 -independent termination
Hairpin

Cleavage of this transcript produces 5S,
16S, and 23S rRNA molecules and a tRNA
molecule.
Spacer regions are shown in yellow.
Synthesis of rRNA and tRNA

It binds to the β-subunit of the RNA Polymerase
to block the initiation of transcription.

It binds with the β-subunit of prokaryotic
RNA Polymerase and thus inhibits the
Elongation phase of Transcription.

The tricyclic ring system (phenoxazone) of
Actinomycin D intercalates between adjacent
G-C base pairs, and the cyclic polypeptide
arms fill the nearby narrow groove and
inhibits Elongation phase of Transcription.
Actinomycin D

It inhibits the Elongation phase of Transcription
Cordycepin (3-deoxy Adenosine)

Eukaryotic RNA polymerases
RNA Polymerase-I: Transcribes / Synthesizes
• 28s rRNA
• 18s rRNA
• 5.8s rRNA
RNA Polymerase-II: Transcribes
• m-RNA
• Some sn-RNA
RNA Polymerase-III: Transcribes
• t-RNA
• 5s rRNA
• Some sn-RNA

RNA POLYMERASE-II
•RNA polymerase II is central to eukaryotic
gene expression and has been studied
extensively.
•RNA polymerase II is a multi subunit
enzyme with 12 subunits.
•RNA polymerase II requires an array of
other proteins, called transcription factors
(TF II) in order to form the active
transcription complex.

α-Amanitin ( Fungal toxin from Amanita
phalloides) - cyclic octapeptide with
unussual amino acids.
Inhibitor of eukaryotic RNA polymerase
(mainly of type II )

Eukaryotic Transcription
Promoters
 Much more complex than those found in bacteria.
 These are consensus sequences located at the
upstream regions of Coding strand.
 Mutation of this region usually significantly lowers
the rate of transcription.

1) TATA box ( Hogness Box)
Very similar to the prokaryotic TATA box,
except the sequence is slightly different
(TATAAA) and it is located in between
-25 to -30.
2) CAAT box
Located in between -70 to -80.
Always contains CCAAT.
3) GC box
Usually has the sequence GGGCGG
and is typically found at -110.

ENHANCERS :
 Enhancers elements are the
sequences located in a variety of
regions of a gene both upstream
and downstream of the
transcription start site and even
within the transcribed portions of
some genes.
 Enhancers increases the
transcription rate by several folds.

• RNA-pol II does not bind to the
promoter sequences directly.
• RNA-pol II associates with six
transcription factors.
• TFII A, TFII B, TFII D, TFII E, TFII F
and TFII H
Transcription factors

Pre-initiation complex (PIC)
RNA pol II
TF II F
TBP TF IID
TATA
DNA
TF II
A
TF II
B
TF II E
TF II H
• TBP of TFII D binds TATA–Box(-10 sequence)
• TFII A and TFII B bind TFII D
• TFII F- RNA-pol complex binds TFII B
• TFII F and TFII E open the dsDNA (helicase
and ATPase)
• TFII H: completion of PIC

Pre-initiation complex (PIC)
RNA pol II
TF II F
TBP TF II D
TATA
DNA
TF II
A
TF II
B
TF II E
TF II H
DNA + RNA Poly-II + TBP + Transcription Factors (TF)
5’ 3’
5’3’

• TF II H is of protein kinase activity to
phosphorylate CTD of RNA pol-II.
(CTD is the C-terminal domain of RNA pol-II)
• Only the RNA Polymerase can move toward
the downstream, starting the elongation
phase.
• Most of the Transcription Factors fall off
from PIC during the elongation phase.
Phosphorylation of RNA-Polymerase-II

• When the RNA Polymerase transcribes the
terminator region of the DNA, the polymerase
releases the mRNA
• The termination sequence is AATAAA followed
by GT repeats.
Termination

Elongation
TFIIF remains associated with RNA Pol-II
throughout elongation.
The activity of the RNA poly-II is greatly
enhanced by proteins called Elongation factors

INHIBITORS
OF
EUKARYOTIC TRANSCRIPTION

Mitomycin
• Mitomycin- Intercalates
with DNA strands
• Blocks transcription
• Used as anticancer drug

ADRIAMYCIN
Inhibits the Initiation phase by preventing the
interaction of TF-IID with RNA-Poly-II and
DNA complex.

CYCLOSPORIN -A
Immunosuppressant Drug inhibits Transcription
in T-Cells.

DRB
(5,6-dichlorobenzimidazone-1-β-D-ribofuranoside)
Inhibits the Elongation phase of Transcription
by selectively inhibiting RNA Poly-II.

Flavopyridol (Alvocidib)
Inhibits the Elongation phase of Transcription
by selectively inhibiting RNA Poly-II.

Tagetitoxin
Inhibits tRNA synthesis by binding to
RNA Poly-III.

Post-Transcriptional
Modifications

• The nascent RNA, also known as
Primary transcript, needs to be
modified to become functional, mRNAs,
tRNAs and rRNAs.
• These modification is critical to
eukaryotic systems.

• Primary transcripts of mRNA are called as
heteronuclear RNA (hnRNA).
• hnRNA are larger than matured mRNA by many
folds.
• Modification includes
– Capping at the 5- end
– Tailing at the 3- end
– mRNA splicing
– RNA editing
Modification of hnRNA

Post Transcriptional modifications of
Pre-mRNA (or) hnRNA
• Introns are removed from the primary
transcript in the nucleus, exons (coding
sequences) are ligated to form the mRNA
molecule.
Exons

CH3
O
O OH
CH2
PO
O
O
N
NH
N
N
O
NH2
AAAAA-OH
O
Pi
5'
3'
O
OHOH
H2C
N
HN
N
N
O
H2N O P
O
O
O P
O
O
O P
O
O
5'
Capping at the 5- end
m7GpppGp----

• The 5- cap structure is found on hnRNA
too.  The capping process occurs in
nuclei.
• The cap structure of mRNA will be
recognized by the cap-binding protein
required for translation.
• The capping occurs prior to the splicing.

The matured mRNAs are much shorter than
the DNA templates.
DNA
mRNA
mRNA splicing

A~G no-coding region 1~7 coding region
L 1 2 3 4 5 6 7
7 700 bp
The structural genes are composed of Coding
(Exons) and Non-coding (Introns)regions
that are alternatively separated.
Split gene
EA B C D F G

• Introns (or) intervening sequences are the RNA
sequences which do not code for the proteins.
• Introns usually start with 5´-GU.
• Introns usually end with 3´-AG.
• RNA splicing involves the removal introns
from pre-mRNA and is carried out by small
nuclear complexes Spliceosomes.
Splicing of hnRNA / pre-mRNA

Spliceosome
• The spliceosome is a large Protein-RNA
complex in which splicing of pre-mRNAs
occurs.
• The spliceosome is made up of specialized
RNA and Protein complexes called small
nuclear RiboNucleoProteins (snRNPs, often
pronounced “snurps”).
• Each snRNP contains RNAs with 100 to 200
nucleotides long, known as small nuclear
RNAs (snRNAs).

84
• Five snRNAs (U1, U2, U4, U5, and U6)
involved in splicing reactions are generally
found in abundance in eukaryotic nuclei.
• Splice sites of Introns are recognized by
snRNPs.

1
2
3
4
5
Exon 1 Exon 2
Intron

Self splicing Introns (Group –I Introns)
Intron
Exon 1 Exon 2

Self splicing Introns (Group –II Introns)

Eukaryotic m-RNA after modifications

89
Splice site mutations
• Mutation at splice sites can lead to improper
splicing and production of abberant proteins
• Eg:  - thalassemia
• β-subunit of hemoglobin is not formed in
sufficient amount.
• It results from point mutation in -globin
gene where the GA mutation occurs.
• This creates a new splice acceptor site
nineteen nucleotides upstream from the
normal splice acceptor
• A faulty beta-globin protein is made, leading
to severe anemia.

Distribution of β-globin gene mutations associated
with β-thalassemia.

World distribution of β-thalassemia

Clinical syndromes in
β-THALASSEMIAS
β-Thalassemia
major
Severe; requires blood
transfusions
β-Thalassemia
intermedia
Severe but does not
require regular blood
transfusions
β-Thalassemia
minor
Asymptomatic with mild
or absent anemia; red cell
abnormalities seen

95
Endo- and exonucleases to generate
ends of tRNA
• Endonuclease RNase P cleaves to generate the
5´ end.
• Exonuclease RNase D trims 3′ to 5′, leaving the
mature 3´ end.

tRNA nucleotidyl
transferase
ATP ADP
Addition of -CCA-OH at 3′ region
At 3´-CCA region of tRNA an activated Amino acid
will be attached during Protein Synthesis.

Base modifications
（1）
（1）
（3）
（2）
（4）
1. Methylation
A→mA, G→mG
2. Reduction
U→DHU
3. Transversion
U→ψ
4. Deamination
A→I

98
methylation
uridine
ribosyl thymine
5
pseudouridine ()
transformation of
the linkage to ribosyl
Modification of some bases
processing
transcript
of intron
mature tRNAprecursor tRNA
anticodon
a leader sequence
3´-terminal UU replaced by amino
acid attachment site CCA-3´-OH

Modification of rRNA
• 45S Pre-rRNA transcript in nucleus is the precursor of
3 kinds of rRNAs.
• The matured rRNA will be assembled with ribosomal
proteins to form ribosomes that are exported to
cytosolic space.

Transcription

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