II. Post-transcription | 21. Regulation of eukaryotic gene expression by RNA interference (miRNA, siRNA) Flashcards
3 Roles of miRNA
- Cleavage of the mRNA strand into two pieces
- Destabilization of the mRNA through shortening of its polyA tail
- Less efficient translation of the mRNA into proteins by ribosomes.
What is RNA interference (RNAi)?
a mechanism for gene silencing that is induced by double- stranded RNA (dsRNA).
-> It is sequence specific and involves the degradation of both dsRNA and of single-stranded RNA (ssRNA) molecules – usually mRNA – that are homologous in sequence to the dsRNA that triggered the response.
The function of RNAi?
1/ to inhibit protein synthesis by targeting a specific mRNA strand and cleave it up into short (23 nucleotides) double stranded-segments
(RNAi -> target mRNA -> breakdown).
2/ dsRNA inside a cell can lead to the production of siRNA (small interfering RNA) and miRNA (micro RNA).
(dsRNA is considered as ‘’alien’’ and normally destroyed by living cells of all organisms. RNAi destroys mRNA that has the same sequence as dsRNA detected in the cell.)
3 steps of RNA mediated gene silencing
1) Dicer (endonuclease) gets activated
2) Dicer breaks the dsRNA on the middle, producing siRNA and miRNA
3) The RISC complex (RNA induced silencing complex) is a
protein complex that together with siRNA + miRNA will degrade the mRNA, resulting in no protein production
What are pri-miRNA?
The miRNA precursors (called pri-miRNA, found in the dsRNA) are synthesized by RNA polymerase II and are capped and polyadenylated (post-transcriptional modification).
Summary 3 steps of pri-miRNA become miRNA?
1) Enzymes inside the nucleus act on pri-miRNA and make it shorter, producing the pre-miRNA
- Pri-miRNAs are processed to pre-miRNA by microprocessor (a heterotrimer of Drosha and 2 DGCR8 proteins)
2) The pre-miRNA will go through the nuclear pore
complex and out to the cytoplasm, where the dicer enzyme will activate it
3) The dicer binds to pre-miRNA: (1) cuts it in specific places, giving it a 3’-overhang of 2 nucleotide and (2) attachment of a phosphate to the 5’-end
How is RCSI complex formed?
The RISC complex (RNA induced silencing complex) is a
protein complex that together with siRNA + miRNA will degrade the mRNA
How does miRNA bind to target mRNA?
1 single strand (passenger strand) of miRNA will then bind to the target mRNA
Once a mRNA has been bound by a miRNA, several outcomes are possible
-> Give 2 possible outcomes
1/ If base-pairing between mRNA + miRNA is extensive (not so common), the mRNA is cleaved (sliced) by the argonaut protein
-> removes the mRNA’s poly-A-tail
-> degradation of mRNA
2/ If base-pairing between mRNA + miRNA is less extensive (more common), argonaut does not slice mRNA, rather the translation of mRNA is repressed and mRNA is shuttled to P-bodies, where it eventually undergoes poly- A-tail degradation
-> miRNA -> translation inhibition
Pri-miRNA: ______ is processed by Drosha/DGCR8
hairpin structure
Pri-miRNA: hairpin structure is processed by ____
Drosha/DGCR8
pre-miRNA is processed by ___
Dicer
pri-miRNA is frequently localized in _____
introns
Is the The canonical transcript a pri-miRNA or pre-miRNA?
pri-miRNA
Structure of pri-miRNA
1/ A 35-bp stem with an unstructured loop and unstructured flanking regions
2/ Every position within the stem (except for 1) has perfect pairing
3/ Many positions contain G-U wobble Paris
5/ Has few mismatches or small bulges
Pri-miRNAs are processed to pre-miRNA by ____
microprocessor
How is pri-miRNA processed pre-miRNA? (4 steps)
1/ Pri-miRNAs are processed to pre-miRNA by microprocessor (a heterotrimer of Drosha and 2 DGCR8 proteins)
2/ Drosha cuts both strands as RNAse III
3/ 5’-phosphate, 3’-OH, 2 overhang
4/ Export by exportin5, small GTPase
How is pre-miRNA become miRNA?
- Dicer processes further the pre-miRNA in the cytoplasm to miRNA
- In complex with TRBP
- It has a RNAse III activity
- 5’-phosphate, 3’-OH, 2 overhang
- The resulting dsRNA has a strand called 5p and another called 3p
(* Dicer ko is lethal)
How does RISC (RNA induced silencing complex) work? (5)
- Ago binds the guide strand of miRNA
- Passenger strand is degraded
- Preference for the strand with A or U at 5’
- Preference for less stable 5’ binding
- Forms complex with TNRC6
What is RNA editing?
1/ Discrete changes to specific nucleotide sequences within a RNA molecule after it has been generated by RNA polymerase.
-> Editing events may include the insertion, deletion and base substitution of nucleotides within the edited RNA molecule
- Mostly neuronal genes
- miRNA
- Transcribed repeats
What are the 4 Non-Canonical Pathways?
1/ Canonical hairpins
2/ Mirtrons
3/ Endogenous shRNAs
4/ Chimeric hairpins
5/ miR-451
How is siRNA produced?
1) Dicer enzyme catalyzes the production of siRNAs from long dsRNA and small hairpin RNAs into small fragments (about 23 nucleotide pairs)
2) These ds siRNAs then bind to the RISC complex, where argonaut cleaves one strand
of the siRNA (if argonaut cannot do this, helicase will show up and help)
How does siRNA work?
1) The newly formed single strand (passenger strand) of the complex will identify and bind to the target mRNA
2) mRNA is cleaved out and not translated
-> degraded
Biological roles of poly-A sites
Transcripts with longer 3’UTR have typically shorter half-life
can 1 miRNA regulate mRNAs?
YES!
What are the 4 Non-Canonical Pathways?
1/ Canonical hairpins
2/ Mirtrons
3/ Endogenous shRNAs
4/ Chimeric hairpins
5/ miR-451
