Upload
sandeep-thapa
View
104
Download
6
Tags:
Embed Size (px)
Citation preview
Basic Principles of Gene Expression
DNA encodes hereditary information (genotype) ->
decoded into RNA -> protein (phenotype)
DNA
RNA
Protein
Transcription
Translation
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Prokaryotic cell
Nuclearenvelope
TRANSCRIPTION
Eukaryotic cell
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSCRIPTION
RNA PROCESSING
Eukaryotic cell
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Ribosome
Polypeptide
Eukaryotic cell
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Ribosome
Polypeptide
Eukaryotic cell
The synthesis of RNA molecules using DNA strands as the templates so that the genetic information can be transferred from DNA to RNA.
Transcription
TYPES OF RNA
RNA molecules play a variety of roles in the cell. The major types of RNA are:
• Ribosomal RNA (rRNA), which is the most abundant type of RNA in the cell.
• Transfer RNA (tRNA), which is the second most abundant type of RNA.
• Messenger RNA (mRNA), which carries the information specifying the amino acid sequence of a protein to the ribosome. Messenger RNA is the only type of RNA that is translated. The mRNA population in a cell is very heterogeneous in size and base sequence, as the cell has essentially a different mRNA molecule for each of the thousands of different proteins made by that cell.
• Heterogeneous nuclear RNA (hnRNA or pre-mRNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of mRNA, formed during its posttranscriptional processing.
• Small nuclear RNA (snRNA), which is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) mRNA.
• Micro-RNA**: short, non-coding, ~ 22 nt long, at least some of which control the expression or repression of other genes during development.
• Ribozymes**, which are RNA molecules with enzymatic activity. They are found in both prokaryotes and eukaryotes.
• Both processes use DNA as the template.
• Phosphodiester bonds are formed in both cases.
• Both synthesis directions are from 5´ to 3´.
Similarity between replication and transcription
replication transcription
template double strands single strand
substrate dNTP NTP
primer yes no
Enzyme DNA polymerase RNA polymerase
product dsDNA ssRNA
base pair A-T, G-C A-U, T-A, G-C
Differences between replication and transcription
• The whole genome of DNA needs to
be replicated, but only small portion of genome is transcribed in response to the development requirement, physiological need and environmental changes.
• DNA regions that can be transcribed into RNA are called structural genes.
Template
The template strand is the strand from which the RNA is actually transcribed. It is also termed as antisense strand.
The coding strand is the strand whose base sequence specifies the amino acid sequence of the encoded protein. Therefore, it is also called as sense strand.
G C A G T A C A T G T C5' 3'
3' C G T C A T G T A C A G 5' template strand
coding strand
transcription
RNAG C A G U A C A U G U C5' 3'
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.
– Eukaryotic systems have three kinds of RNA polymerases, each of which is a multiple-subunit protein and responsible for transcription of different RNAs.
core enzymeholoenzyme
Holoenzyme
The holoenzyme of RNA-pol in E.coli consists of 5 different subunits: 2
.
subunit MW function
36512Determine the DNA to be transcribed
150618 Catalyze polymerization
155613 Bind & open DNA template
70263Recognize the promoter
for synthesis initiation
RNA-pol of E. Coli
• Rifampicin, a therapeutic drug for tuberculosis treatment, can bind specifically to the subunit of RNA-pol, and inhibit the RNA synthesis.
• RNA-pol of other prokaryotic systems is similar to that of E. coli in structure and functions.
RNA-pol I II III
products 45S rRNA hnRNA
5S rRNA
tRNA
snRNA
Sensitivity to Amanitin
No high moderate
RNA-pol of eukaryotes
Amanitin is a specific inhibitor of RNA-pol.
• Each transcriptable region is called
operon.
• One operon includes several structural genes and upstream regulatory sequences (or regulatory regions).
• The promoter is the DNA sequence that RNA-pol can bind. It is the key point for the transcription control.
Recognition of Origins
5'
3'
3'
5'-50 -40 -30 -20 -10 1 10
start -10 region
T A T A A T A T A T T A
(Pribnow box)
-35 region
T T G A C A A A C T G T
Prokaryotic promoter
Consensus sequence
• The -35 region of TTGACA sequence
is the recognition site and the binding site of RNA-pol.
• The -10 region of TATAAT is the region at which a stable complex of DNA and RNA-pol is formed.
General concepts
• Three phases: initiation, elongation, and termination.
• The prokaryotic RNA-pol can bind to the DNA template directly in the transcription process.
• The eukaryotic RNA-pol requires co-factors to bind to the DNA template together in the transcription process.
Transcription of Prokaryotes
• Initiation phase: RNA-pol recognizes the promoter and starts the transcription.
• Elongation phase: the RNA strand is continuously growing.
• Termination phase: the RNA-pol stops synthesis and the nascent RNA is separated from the DNA template.
a. Initiation
• RNA-pol recognizes the TTGACA region, and slides to the TATAAT region, then opens the DNA duplex.
• The unwound region is about 171 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-pol once the first 3,5 phosphodiester bond is formed.
• The core enzyme moves along the DNA template to enter the elongation phase.
b. 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.
• RNA-pol, 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.
c. Termination
• The RNA-pol stops moving on the DNA template. The RNA transcript falls off from the transcription complex.
• The termination occurs in either -dependent or -independent manner.
-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.
• The stem-loop structure alters the
conformation of RNA-pol, leading to the pause of the RNA-pol moving.
• Then the competition of the RNA-RNA hybrid and the DNA-DNA hybrid reduces the DNA-RNA hybrid stability, and causes the transcription complex dissociated.
• Among all the base pairings, the most unstable one is rU:dA.
Stem-loop disruption
Transcription of Eukaryotes
• Transcription initiation needs promoter and upstream regulatory regions.
• The cis-acting elements are the specific sequences on the DNA template that regulate the transcription of one or more genes.
a. Initiation
structural geneGCGC CAAT TATA
intronexon exon
start
CAAT box
GC box
enhancer
TATA box (Hogness box)
Cis-acting element
• RNA-pol does not bind the promoter directly.
• RNA-pol II associates with six transcription factors, TFII A - TFII H.
• The trans-acting factors are the proteins that recognize and bind directly or indirectly cis-acting elements and regulate its activity.
Transcription factors
• TBP of TFII D binds TATA
• 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)
Recognizes and binds to TATA box; TBP + 10 TBP associated factors
Binds and stabilizes the TFIID complex
Recruits RNA pol II + TFIIF to the location
• TF II H is of protein kinase activity to
phosphorylate CTD of RNA-pol. (CTD is the C-terminal domain of RNA-pol)
• Only the p-RNA-pol can move toward the downstream, starting the elongation phase.
• Most of the TFs fall off from PIC during the elongation phase.
Phosphorylation of RNA-pol
• The elongation is similar to that of
prokaryotes.
• The transcription and translation do not take place simultaneously since they are separated by nuclear membrane.
b. Elongation
• The termination sequence is AATAAA followed by GT repeats.
• The termination is closely related to the post-transcriptional modification.
c. Termination
• The nascent RNA, also known as primary transcript, needs to be modified to become functional tRNAs, rRNAs, and mRNAs.
• The 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 edition
Modification of hnRNA
CH3
O
O OH
CH2
PO
O
O
N
NHN
N
O
NH2
AAAAA-OH
O
Pi
5'
3'
O
OHOH
H2CN
HNN
N
O
H2N O P
O
O
O P
O
O
O P
O
O
5'
a. Capping at the 5- end
m7GpppGp----
ppp5'NpNp
pp5'NpNp
GTP
PPi
G5'ppp5'NpNp
methylating at G7
methylating at C2' of the first and second nucleotides after G
forming 5'-5' triphosphate group
removing phosphate group
m7GpppNpNp
m7Gpppm
2'Npm2'Np
Pi
• 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.
b. Poly-A tailing at 3 - end
• There is no poly(dT) sequence on the DNA template. The tailing process dose not depend on the template.
• The tailing process occurs prior to the splicing.
• The tailing process takes place in the nuclei.
A~G no-coding region 1~7 coding region
L 1 2 3 4 5 6 77 700 bp
The structural genes are composed of coding and non-coding regions that are alternatively separated.
Split gene
EA B C D F G
Exon and intron
Exons are the coding sequences that appear on split genes and primary transcripts, and will be expressed to matured mRNA.
Introns are the non-coding sequences that are transcripted into primary mRNAs, and will be cleaved out in the later splicing process.
U pA G pU5' 3'5'exon 3'exon
intron
pG-OH
pGpA
G pU 3'U5' OH
first transesterification
Twice transesterification
second transesterification
U5' pU 3'
pGpA
GOH
5'
3'
• Taking place at the transcription level
• One gene responsible for more than one proteins
• Significance: gene sequences, after post-transcriptional modification, can be multiple purpose differentiation.
d. mRNA editing
Different pathway of apo B
Human apo B gene
hnRNA (14 500 base)
liverapo B100
( 500 kD) intestineapo B48
( 240 kD)
CAA to UAAAt 6666
Base modification
( 1 )( 1 )
( 3 )
( 2 )
( 4 )
1. Methylation A→mA, G→mG
2. Reduction U→DHU
3. Transversion U→ψ
4. DeaminationA→I
§3.3 Modification of rRNA
• 45S 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.
• The rRNA precursor of tetrahymena has the activity of self-splicing (1982).
• The catalytic RNA is called ribozyme.
• Self-splicing happened often for intron I and intron II.
§3.4 Ribozyme
• Both the catalytic domain and the substrate locate on the same molecule, and form a hammer-head structure.
• At least 13 nucleotides are conserved.
• Be a supplement to the central
dogma
• Redefine the enzymology
• Provide a new insights for the origin of life
• Be useful in designing the artificial ribozymes as the therapeutical agents
Significance of ribozyme
• Promoter: DNA sequence where RNA polymerase binds to transcribe the gene
• Transcription start site: the nucleotide where RNA pol initiates transcription
• Transcription unit: the transcribed DNA
Transcription: DNA->RNA
Structure of a gene
Basic components for transcription
dsDNA with a promoter
RNA polymerase
rNTPs (ribonucleotides triphosphates)ATP, CTP, GTP, UTP
LE 17-7
ElongationNon-templatestrand of DNA
RNApolymerase
RNA nucleotides
3 end3
5
5
Newly madeRNA
Templatestrand of DNA
Direction of transcription(“downstream”)
Synthesis of an RNA Transcript
• The three stages of transcription:– Initiation– Elongation– Termination
LE 17-7Promoter
35
Transcription unit
DNA
InitiationRNA polymerase
Start point
Template strandof DNA
RNAtran-script
UnwoundDNA
Elongation
3
3
53
5
5
3 5
RewoundDNA
5 3
35 35
RNAtranscript Termination
35
5 3Completed RNA transcript
Termination of Transcription
Different in prokaryotes and eukaryotes• In prokaryotes• RNA pol stops transcription at the end of the
terminator (DNA sequence)
• In eukaryotes• pre-mRNA is cleaved from the growing RNA chain• RNA pol eventually falls off the DNA
RNA processing in eukaryotes, not prokaryotes
1. Addition of methylated cap to 5’ end of messenger RNA (mRNA)-> increases stability and translation of mRNA
2. Addition of poly(A) tail to 3’ end (polyadenylation) -> increases stability and translation of mRNA
3. Splicingremoval of introns and joining together of exons
All processing events occur in nucleus
before transport to cytoplasm
Draw
LE 17-10
5 Exon Intron Exon Intron Exon 3Pre-mRNA
1 30 31 104 105 146
Codingsegment
Introns cut out andexons spliced together
1 146
5Cap
5Cap
Poly-A tail
Poly-A tail
5 3UTR UTR
(mature) mRNA
• RNA splicing:
carried out by spliceosomes
• Spliceosomes complex of proteins and several small nuclear
ribonucleoproteins (snRNPs)
Recognize splice sites (specific RNA sequences)
cleave out introns and splice together exons (coding region)
LE 17-11
Exon 15
Intron Exon 2
Other proteinsProtein
snRNA
snRNPs
RNA transcript (pre-mRNA)
Spliceosome
5
Spliceosomecomponents
Cut-outintron
mRNA
Exon 1 Exon 25
Ribozymes
• Catalytic RNAs molecules that function as enzymes; involved in splicing
• Non-protein biological catalyst
Can you think of a ribozyme with a different function?
Telomerase
Functional and Evolutionary Importance of Introns
• Some genes can encode more than one kind of polypeptide
-different combinations of exons can be spliced together
• Called alternative RNA splicing
• Increases the potential number of different proteins (and thus functions) in an organism
• Increased adaptive potentialDraw Splice Variants
• In many cases, different exons code for the different domains in a protein
• Protein domains– Distinct conformational regions often with discrete
functions
Exons and protein domains
LE 17-12
Gene
Transcription
RNA processing
Translation
Domain 2
Domain 3
Domain 1
Polypeptide
Exon 1 Intron Exon 2 Intron Exon 3
DNA