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22. Small RNAs

Learning objectives

  • What are small RNAs?
  • Describe the miRNA, siRNA, and the RNA interference pathway
  • Describe saRNA
  • Describe snRNA
  • Describe snoRNA
  • Describe piRNA
  • Describe tmRNA

There are many other types of RNA than the three big ones. So-called small RNAs are RNAs < 200 nucleotides in length. They have very different lengths and functions, but many are involved in RNA silencing, the inhibition of transcription. Some small RNAs are double-stranded.

miRNA and siRNA

miRNA (microRNA) are regulatory RNAs. They’re approx. 20 – 30 nucleotides long. They bind to a sequence on the 3’-end of mRNAs to downregulate their translation into proteins. siRNAs (small interfering RNAs) are regulatory RNAs. siRNAs are double-stranded.

RNA interference (RNAi) is a biological pathway by which miRNA and siRNA inhibit translation. In this pathway miRNA or siRNA combine with ribonucleoproteins to form the so-called RISC complex. The binding of RISC to the target mRNA either inhibits translation or induces degradation of the mRNA. RNAi is a valuable tool in biotechnology and medicine, and has potential in anti-cancer treatment.

saRNA

saRNAs (small activating RNAs) induce a long-lasting activation of genes. They’re 21 nucleotides long. Like siRNAs these RNAs are double-stranded.

snRNA

snRNAs (small nuclear RNAs) are a component of small nuclear ribonucleoprotein particles (snRNPs), which are major components of the spliceosome.

snoRNA

snoRNAs (small nucleolar RNAs) modify bases on tRNA, rRNA and snRNAs. They can catalyse methylation (transfer of a methyl group to a base) or pseudouridylylation (conversion of uridine to pseudouridine).

piRNA

piRNAs (Piwi-interacting RNA) are the largest class of small RNAs. They are 25-30 nucleotides long, and create complexes with the Piwi protein. Their function is to silence retrotransposons in germ line cells.

rasiRNA (repeat associated small interfering RNA) are a subclass of piRNAs. They perform the same functions as piRNAs, but also help establishing and maintaining the heterochromatin structure.

tmRNA

tmRNA (transfer-messenger RNA) is a bacterial RNA molecule. Like its name suggests it has both tRNA and mRNA-like properties.

tmRNA helps ribosomes that are stuck. If a ribosome tries to transcribe an mRNA that doesn’t contain a stop codon, the ribosome never gets the signal to release the polypeptide. The ribosome will do nothing, with the polypeptide still attached to it. tmRNAs recycles the ribosome, makes sure the mRNA is degraded, and adds a tag to the polypeptide that signals that it should be broken down.

qiRNA

qiRNAs are 20-21 nucleotides long. Its expression is induced by DNA damage. It then inhibits protein translation.

Summary

  • What are small RNAs?
    • small RNAs are <200 nucleotide non-coding RNAs with special functions
  • Describe miRNA, siRNA, and the RNA interference pathway
    • In the RNA interference pathway combine with ribonucleoproteins to form the RISC complex. The RISC complex binds to mRNA and prevents its translation
  • Describe saRNA
    • saRNAs are regulatory RNAs which induce long-lasting gene activation
  • Describe snRNA
    • snRNAs (small nuclear RNAs) are a component of small nuclear ribonucleoprotein particles (snRNPs), which are major components of the spliceosome
  • Describe snoRNA
    • snoRNAs modify bases on tRNA, rRNA and snRNAs. They can catalyse methylation (transfer of a methyl group to a base) or pseudouridylylation (conversion of uridine to pseudouridine).
  • Describe piRNA
    • piRNA create complexes with the Piwi protein and silence retrotransposons in germ line cells
  • Describe tmRNA
    • tmRNA has both tRNA and mRNA-like properties.
    • tmRNA helps ribosomes that are stuck due to lack of a stop codon on the mRNA