RNAi

• Brief description of what this is what it does in the cell :

  • RNAi (RNA interference) is a conserved biological response to double-stranded RNA that mediated resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes 
  • The main components are small regulatory RNAs at 21-30 nucleotides long, including small interfering RNAs (siRNAs) and microRNAs (miRNAs)
• what is RNAi

  • They can suppress gene expression at the transcriptional level by modulating chromatin structure, and in certain cases small RNAs can activate gene expression. 
  • Most commonly act post-transcriptionally to destabilise and/or inhibit the translation or complementary target mTNA in a sequence-specific manner. 
  • Crucial component of the hosts defence system against exogenous and endogenous nucleic acids in the form of viral genome replication intermediates and transposing transcripts. 

   
• RNAi

  • Plants and insects lack an adaptive immune system, making RNAi a primary means of defence against viral infections. This mechanism allows them to rapidly respond to viral threats without the need for prior exposure to the virus.
• RNAi in plants and insects

  • Since RNAi can be quickly induced upon viral infection and can target a variety of RNA viruses, it offers plants and insects an affordable and adaptable defence mechanism. Because of its effectiveness and versatility, RNAi serves as plants' and insects' primary antiviral mechanism, enabling them to thrive despite the face of viral challenges.
• plants and insects RNAi

  • Additionally in plants as they are interconnected through plasmodesmata, which allows for the rapid spread of RNAi signals throughout the plant, cell-to-cell communication enables the systemic induction of RNAi, effectively containing viral infections across the entire plant.
• What is it that activates the protein/Pathway?  (what is the trigger and what is the sensor?)

  • The trigger for RNAi is viral double-stranded RNA (dsRNA)  
  • The sensor for RNAi is an enzyme called 'Dicer'. Which cuts dsRNA into small fragments (21 nucleotides) called small interfering RNAs. 
• What happens when the pathway is activated? (what is the signal?)

  • Once activated, the dsRNA is cut/diced into small fragments by enzyme 'Dicer'. 
  • siRNA binds to a Argonaute-containing effector complex called RISC (RNA-induced silencing complex)  and one strand of RNA is removed, leaving the remaining strand to bind to mRNA target sequences. The RISC can then cleave the mRNA to destroy it.

   
• Why does this contribute to antiviral immunity?  (what is the effector?)

  • By contributing through inhibition of the expression of crucial viral proteins by targeting viral mRNA for degradation through cellular enzymes.
  • The effector is RNAi is RISC (RNA induced silencing complex) which is a multiprotein complex which scans the mRNA for a complementary sequence and then can initiate gene silencing.
• What is the evidence that viruses evade this response (use specific virus examples)? 

  • Not having dsRNA as their viral genome
  • Viruses can use the RNAi system to manipulate gene expression of the host. 

   

  • Viruses express viral suppressors of RNAi (VSRs). Several viruses encode multiple VSRs. Many VSRs are dsRNA-binding proteins. The primary mechanism for some viruses is to hide siRNA molecules away from the RNAi pathway which is seen in P19 of tombusviruses. Once siRNA is hidden, p19 suppresses RNA silencing and promotes viral proliferation. 

   
• • Viruses are able to evade this response in multiple ways 
  • Mutation of RNAi target sites – Hepatitis C constantly mutates its genome which makes it difficult for RNAi to target and degrade the RNA 
  • Production of Viral Suppressors - TYLCV encodes VSR protein p19 which prevents viral RNA being targeted by binding to small interfering RNAs
• • Manipulation of host RNAi components – certain viruses like Epstein-Barr virus can use the RNAi machinery to their advantage by mimicking host microRNA and allowing them to evade detection
  • Packaging viral RNA – some viruses encapsulate their RNA to make it generally inaccessible for RNAi
  • These evasion techniques have been evidenced through natural and experimental observations