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Advantages of C. elegans : 1. rapid life cycle 2. hermaphrodite 3. prolific reproduction 4. transparent 5. only ~1000 cells 6. laser ablation 7. complete cell lineage map 8. genetics. RNA interference: A type of gene regulation Involving small RNA molecules - PowerPoint PPT Presentation
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Advantages of C. elegans:
1. rapid life cycle
2. hermaphrodite
3. prolific reproduction
4. transparent
5. only ~1000 cells
6. laser ablation
7. complete cell lineage map
8. genetics
RNA interference:
A type of gene regulation Involving small RNA molecules and induced by double stranded RNA
Adding a Chalcone synthase gene from Petunia fused to a strong viral promoterto transgenic Petunia interfered with expressionof the native homologous’gene.
Gene Silencing
Gene Silencing
1.More common from strong promoters. 2. More common from inverted repeat inserts
(could make a double stranded RNA).
3. Induced by RNA viruses (have double stranded RNA replication intermediates).
Tobacco plant expressing GFP proteinInfected with RNA virus with GFP geneVirus infection travels through veinsGFP expression inhibited starting at veins.
RNA viruses can block expression of a transgene if a copy of the transgene has been added
Gene silencing and RNA virusesshare potential to produce dsRNA
Fire and Mello used C. elegans to prove that dsRNA inhibits expression of endogenous genes homologous to that dsRNA
Double stranded GFP RNA interferes with expression of GFP in transgenic C. elegans
GFP expressed in nuclei After adding dsRNA for GFP
Features of RNAi
Induced by dsRNA with homology to exons
Catalytic: very small amounts of ds RNA are sufficient
Spreads: injection into gut silences genes in embryos
Small RNAs produced
Mechanism of RNAi
Dicer binds dsRNAAnd cleaves makingsiRNA
siRNAs direct Risc to copy homologous mRNA
Dicer binds shortstretch of dsRNA and cleaves it.
Catalysis: RdRP copies mRNA making more ds RNA. Dicer cuts that generating more siRNA
More RdRP is activated and more dsRNA is made.
Spread: dsRNA transported to other cells.Not in Drosophila or mammals
Double stranded GFP RNA interferes with expression of GFP in transgenic C. elegans
GFP expressed in nuclei After adding dsRNA for GFP
How would you design a genetic screen to identify the genes involved in the RNAi mechanism?
The real genetic screen for C. elegans genes essential for RNAi
What is the function of RNAi mechanism in non-transgenic organisms?
Protection against viruses
Keep Transposable elements inactive
Gene regulation
Gene regulation by small RNAs
SiRNAs degrade mRNAto stop gene expression quickly
StRNAs prevent translation to stop gene expression quickly
MIRNA gene
HASTY
5’
3’
CC UU C C AUGAGAGAGU CU GAUAUUGGC UGGUUCA UCAGAU |:|||||||| || ||||||||| :||:||| |||||| 44nt loop UGCUCUCUCA GA CUAUAACCG GCCGAGU AGUCUA U CU C U
C C 5' UAUUGGC UGGUUCA UCACU 3' ||||||| ||||||| |||3' CUAUAACCG GCCGAGU AGU 5' C U
5’
3’
CC UU C C AUGAGAGAGU CU GAUAUUGGC UGGUUCA UCAGAU |:|||||||| || ||||||||| :||:||| |||||| 44nt loop UGCUCUCUCA GA CUAUAACCG GCCGAGU AGUCUA U CU C U
AAA
Pol II
DCL1
HEN1, HYL1
C C 5' UAUUGGC UGGUUCA UCACU 3' ||||||| ||||||| |||3' CUAUAACCG GCCGAGU AGU 5' C U
AGO1
AGO1
CUAUAACCGCGCCGAGUUAGU
UAUUGGCCUGGUUCACUCCACU
AGO1
CUAUAACCGCGCCGAGUUAGU
AAA ..... .............
AAA
DCL1
miRNA*
RISCcomponents
RISCmiRNA
Target
Cleaved target
57 unique miRNAs from 26 families
~200 MIRNA genes**
**Cumulative data from Bartels, Carrington, Chen, Weigel, Zhu, others
m7GpppN AAAAA
5’ Cap
Introns
Poly(A) tailmiRNA Precursor
Transcript
MIRNA genes are PolII genes that encode a hairpin shaped mRNA.Dicer cleaves the double stranded portion to makea short dsRNA.That combines with the RISC complex and directs cleavage of a specific target mRNA
HYL1
miRNAs and Targets in Animals
C. elegans 114Drosophila 78Zebrafish 362Mouse 245Human 321
Registered miRNAs
Fig. 3 from Lewis, Burge and Bartel (2005) Cell 120, 15-20
May target 1/3 of all genes
Development - e.g. timing, stem cell function, differentiationCell and organ identityCancer - mis-regulation, deletion, duplication of MIRNA genes
HerpesvirusesSV40
Transcriptional gene silencing is initiated by RNA directed methylation of promoter regions
dsRNA homologous to promoters leads to methylation and inactivation by recruitment of chromatin remodeling enzymes.
Expression of double stranded RNA leads to chromatin remodeling
Centromeres are transcriptionally inactive. They have condensed chromatinRepeated sequences lead to production of double stranded RNAsThe dsRNAs maintain the condensed state of the centromeres
RNA is a gene regulator as well as a carrier of information