Small RNAs

Small RNAs：micro RNA、small interfering RNA（So si R has no silencing through inhibiting translation?）

Small RNAs

• micro RNA
  • RNA source
    • pre-miRNA is a 60-70nt ss RNA
    • single mature miR generated from a pre-miRNA
    • processed mature miRNA ~21nt
  • discovery
    • lin-4 mutant in C.elegans is heterochronc mutant
    • lin-4 encodes only 1 22nt and 1 60nt small RNAs
    • lin-4 RNAs complement to lin-14
    • lin-4 is a translational repressor
    • seven years ago let-7 is found have similar effect
    • let-7 has homologous in both human and Drosophila
    • now >1000 miR found in human
      • regulate 60% protein encode genes
  • natural sources
    • cellular genome
    • viral genome
      • block apoptosis, facilitate immune escape, prevent cell cycle arrest, promote latency
  • function
    • repress gene expression
  • encoding on genome
    • can in introns of other genes and other transcriptionally silent regions
    • often multiple miRs in single transcription unit
  • miR maturation
    • RNApol II or pol III transcribes pri-miR
    • pri-miR folds into stem loop
    • Drosha cleaves out all pri-miR regions exept stemloop region and form pre-miR stem loop
    • exportin 5 bond ends of pre-miR and export it through nuclear pore to cytoplasm
    • Dicer in cytoplasm cleaves pre-miR 's loop region into mature miR; a short double strand RNA
  • miR silencing
    • RISC loading
      • RISC associates mature miR using Ago binding to it
    • removal of mature-miR's complementary strand
      • by nuclease
      • by RISC loading process
    • Silencing
      • RISC-ss miR complement to targeted mRNA
        • usally in 3'UTR
      • stop translation
        • RISC cleaves the mRNA
          • more common in animals
          • more complex in plants
        • block translation
          • elongation block
            • slow elongation or ribosome drop off
          • initiation block
            • repressed cap recognation or 60S joining
  • proteins involved
    • pri-miR processing
      • Drosha
        • endonuclease cleaves pri miR non-stem loop regions
      • exportin 5
        • exports pre-miR to cytoplasm
      • Dicer
        • cleaves loop region of pre-miR
    • miR silencing
      • RISC(miRNPs)
        • RNA induced silencing complex
        • contain Argonaute (Ago)
          • a RNA binding protein binds to miR
        • contain Dicer and TRBP
        • have nuclease activity to cleave mRNA
  • mRNA targets
    • usually different from miR gene
  • evolution
    • evolutionary conserved
• small interfering RNA
  • So si R has no silencing through inhibiting translation?
  • other names
    • RNA interference, cosuppression, and posttranscriptional gene silencing
  • RNA source
    • long ds RNA
      • can be a self complementarity formed very long stemed loop
      • can be intermolecular-complementarity of 2 ss RNA
    • multiple siRS generated from a long ds RNA
  • encoding on genome
    • transposable elements
  • natural sources
    • endogenouse
      • genome encoding
        • may antiviral porpose
    • exogenouse
      • viruses
  • siR maturation
    • dicer cleaves one pre-siR dsRNA into multiple mature siRs
  • siR silencing
    • siR first complement to target mRNAs
      • usually near perfect complementarity
    • RISK incorporates into siR-mRNA hybridization
    • siR containing RISK cleaves target mRNA
  • mRNA targets
    • usually genes which siR derived from
  • evolution
    • evolutinary unconserved