exon selection factor
DESCRIPTION
Overview of mRNA Splicing. Factors such as U1 and U2 snRNP identify splice sites. Exon selection factor. Exon selection factor. U1 snRNP. U2 snRNP. Exon 1. AGGU. Intron 1. A AGG. Exon 2. - PowerPoint PPT PresentationTRANSCRIPT
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Exon selection factor
Exon selection factor
U2 snRNPU1 snRNP
Intron 1
Overview of mRNA Splicing
Exon 1 AGGU Exon 2A AGG
Factors such as U1 and U2 snRNP identify splice sites
Exons are identified by RNA sequences within the exons that are recognized by exon selection factors.
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Beta globin splice mutations are one cause of beta thalassemia
EXON1 INTRON1 PHENOTYPE
AG GT AGT CONSENSUS
GCCAG GTTGGTAT NORMAL
GCCAG ATTGGTAT 0 (no beta chains)
GCCAG TTTGGTAT 0 (no beta chains)
GCCAG GTTGTTAT + (some beta chains)
GCCAG GTTGCTAT + (some beta chains)
GCCAG GTTGGCAT + (some beta chains)
AG
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Beta globin splice mutations:creation of a new acceptor site
NORMAL: INTRON 1 EXON 2TATTGGTCTATTTTCCCACCCTTAG GCTG
MUTATION:
TATTAGTCTATTTTCCCACCCTTAGGCTG
TATTAG TCTATTTTCCCACCCTTAGGCTG
Normal site used 10% of the time:normal protein from these RNAs
New site used 90% of the time: no protein from these RNAs (note the shift in reading frame).
10%
10%
90%
100%
Net result: this allele shows a 90% reduction in β-globin production
19 nucleotides
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Nonsense Mediated Decay
Exon/Exon junction
Normal stop codon is downstream or <50 bases upstream from splice junction
Premature stop codon >50 bases upstream from splice junction
>50 bases
mRNA Decay
Translation
A.
B.
Last exon
Last exon
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Nonsense Mediated DecayOrigin of premature stop codons
- Improper splicing- intron retained- frameshift
- Mutation
Possible consequences of premature stop codons:
- non functional protein- formation of amyloid- loss of a regulatory region from a protein that regulates growth cancer
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Nonsense Mediated Decay
Exon/Exon junctions
Normal Stop Codon
1st round of Translation
Spliceosomes
Protein complexes (Exon-junction-complexes; EJC)
Nucleus
Cytoplasm
More translation
Complexes removed by ribosome transit
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Nonsense Mediated Decay
Exon/Exon junctions
Normal stop codonPremature stop codon
mRNA Decay
1st round of Translation
Spliceosomes
Protein complexes (Exon-junction-complexes; EJC)
Nucleus
Cytoplasm
Stopped ribosome
Complex not removed
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• Some drugs that affect the accuracy of codon recognition by ribosomes (such as gentamicin) decrease Nonsense mediated decay.
• Treatment with these drugs allows a low level of expression from genes with premature stop codons.
• Possible treatment for several disorders including some alleles of cystic fibrosis.
Reference: Holbrook et al Nature Genetics 36:801-808 (2004)
Nonsense mediated DecayClinical applications (in trials)
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For most (but not all) promoters, a complex of proteins is assembled around the TATA box, located about 25-30 b.p. upstream from the start site. The consensus sequence of the TATA box is TATAAA
The core promoter
-30 start of transcription +30
TATA box
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DNA with TATA box binding protein
DNA
Protein
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The TATA binding protein binds to the TATA box
The core promoter
-30 start of transcription +30
TATA binding protein
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-30 start of transcription +30
The TATA binding protein is one subunit of a large complex: TFII-D.
The core promoter
TFII-D
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-30 start of transcription +30
Several other complexes bind to TFII-D.
The core promoter
TFII-DTFII-B
TFII-ATFII-F
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-30 start of transcription +30
RNA polymerase is recruited to the promoter.
The core promoter
TFII-DTFII-B
TFII-ATFII-F
RNA pol II
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-30 start of transcription +30
The factor TFII-H plays a key role in initiating transcription by phosphorylating the C-terminal domain of the large subunit of RNA pol II.
The core promoter
TFII-DTFII-B
TFII-ATFII-F
RNA pol IITFII-H
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…...(Tyr-Ser-Pro-Thr-Ser-Pro-Ser)52COOH
CTD: a pol II switchCTD: The COOH Terminal Domain of the RNA pol II large subunit
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…...(Tyr-Ser-Pro-Thr-Ser-Pro-Ser)52COOH
TFIIH
…...(Tyr-Ser-Pro-Thr-Ser-Pro-Ser)52COOH
PO3 phosphorylated Ser 5 of the repeats
Other kinases
More phosphorylation of the CTD
CTD: a pol II switchCTD: The COOH Terminal Domain of the RNA pol II large subunit
ATP
ADP
ATP
ADP
TFIIH controls the start of transcription
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Unphosphorylated CTD:Involved in initiation:
Binding of initiation factors
Phosphorylated CTD:Involved in elongation & RNA processing
Binds components involved in RNA cappingBinds components involved in RNA splicingBinds components involved in 3’ end formation
CTD: a pol II switchCTD: The COOH Terminal Domain of the RNA pol II large subunit
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From Orphanadies & Reinberg (2002) Cell 108:439-51
The CTD ties elongation to capping, splicing and 3’-end formation
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A model promoter
TGACTCA GACGTC GGGCGG
SP-1
CREBFos Jun CREB
AP-1 (Fos-Jun binding site)Regulation bygrowth factors,stress, and varioustransmembranesignals
CRE (Cyclic AMPResponse Element)Regulation by cAMP,and by Ca+. Interacts withcore promoter (through CREB Binding Protein)and modifies chromatin structure (through HAT).
SP-1 SiteProvides basalunregulatedtranscriptionalactivity. Manygenes have multiple SP-1 sites
Core PromoterBinds generaltranscriptionalmachinery
TATAAA
Positions of these elementsare relatively unimportant
HAT Histone Acetyl Transferase
HRENR NR
HRE (HormoneResponse Element)Regulation by hormonessuch as estrogen which enter the cell
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Binding of a leucine zipper protein to DNA
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HAT
Serine 133
Phosphorylation of CREB and the CREB binding protein (CBP)
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Plasma membrane
Nuclear membranes
Hormone receptor
Adenylate cyclase
ATP
Active Protein Kinase A
PO4
G
cAMP Inctive Protein Kinase A
PO4
Active pKA enters the nucleus and phosphorylates CREB on Serine 133
Hormone
G-protein
Core promoter
Phosphorylation of CREB: - stimulates interactions with several core promoter proteins - induces binding of HAT and acetylation of histones
Signaling mediated by cAMP and protein kinase A