L13 - RNAi and viruses Flashcards
Intended Learning Outcomes • Describe the concept of RNA-interference. • Describe the roles of the endonucleases DICER and Argonaute-2. • Explain how the RISC (RNA-induced Silencing Complex) complex inhibits virus replication in plants and insects. • Explain the role of viral encoded suppressors of RNAi (VSRs) in viral infection. • Discuss the role of RNAi in the regulation and inhibition of virus replication in mammalian cells.
What are the key RNAi molecules involved in gene silencing?
dsRNA, shRNA, siRNA, and miRNA.
What is the main function of siRNA?
It guides the RNA-induced silencing complex (RISC) to degrade complementary mRNA.
What enzyme processes dsRNA into siRNA?
Dicer.
What is the function of the RISC complex?
It silences gene expression by degrading target mRNA.
How does R2D2 assist in RISC loading?
It binds the more stable end of siRNA, recruiting Dicer-2.
What happens to the passenger strand of siRNA?
It is destroyed during RISC activation.
What is the function of the PIWI domain in Ago-2?
It cleaves RNA in a sequence-specific manner.
How does Ago-2 contribute to RNAi?
It binds siRNA and guides mRNA degradation.
What type of RNA does the PIWI domain act on?
Single-stranded RNA (ssRNA) guided by siRNA.
What determines whether RISC degrades mRNA or suppresses translation?
The complementarity between siRNA and mRNA.
What happens if siRNA has a perfect match to its target?
The mRNA is cleaved and degraded.
What happens if siRNA has a bulged mismatch?
It results in translational suppression rather than cleavage.
How is RNAi amplified in plants and nematodes?
Through RNA-dependent RNA polymerase (RdRP).
What does RdRP do?
It synthesizes secondary siRNAs from target mRNAs, enhancing RNAi effects.
Why is this amplification significant?
It strengthens and prolongs the RNAi response against viruses.
What type of genome does Tombusvirus have?
A single-stranded RNA genome (~4800 bases).
How is Tombusvirus transmitted?
Through direct sap contact and contaminated soil.
What is a key feature of its replication strategy?
Its genome can be directly translated into proteins.
What is p19, and why is it important for Tombusvirus?
It is a viral suppressor of RNAi (VSR) that binds siRNA to prevent degradation.
How does p19 recognize siRNA?
It specifically binds 21-nucleotide dsRNA fragments.
What is the structural feature of p19?
It acts like a caliper, measuring and holding dsRNA.
What happens when p19 is deleted?
The virus is controlled by RNAi, leading to reduced spread and degradation.
What are the effects of a p19-deficient virus in tobacco plants?
No systemic symptoms and limited viral replication.
What does this indicate about p19’s role?
It is crucial for viral evasion of RNAi defenses.