Table 1.
Types of small silencing RNAs.
| Name | Organism | Length (nt) |
Proteins | Source of Trigger | Function | References |
|---|---|---|---|---|---|---|
| miRNA | Plants, algae, animals, viruses, protists | 20–25 | Drosha (animals only) + Dicer | Pol II transcription (pri-miRNAs | Regulation of mRNA stability, translation | 92–94,199,200,225 |
| casiRNA | Plants | 24 | DCL3 | Transposons, repeats | Chromatin modification | 38,44,51,52,61–63 |
| tasiRNA | Plants | 21 | DCL4 | miRNA-cleaved TAS RNAs | Post transcriptional regulation | 64–68 |
| natsiRNA | Plants primary secondary |
24 21 |
DCL2 DCL1 |
Bidirectional transcripts induced by stress | Regulate stress response genes | 71,72 |
| Exo-siRNA | Animals, fungi, protists Plants |
~21 21 & 24 |
Dicer | Transgenic, viral or other exogenous dsRNA | Post transcriptional regulation, antiviral defense | 4,5,8,226 |
| EndosiRNA | Plants, algae, animals, fungi, protists, | ~21 | Dicer (Except secondary siRNAs in C. elegans, which are products of RdRP transcription, and are therefore not technically siRNAs.) | Structured loci, convergent and bidirectional transcription, mRNAs paired to antisense pseudogene transcripts | Post transcriptional regulation of transcripts and transposons Transcriptional gene silencing |
75–79,82,83,86,87,199,200,227 |
| piRNA germ line | Drosophila melanogaster, mammals, zebrafish | 24–30 | Dicer-independent | Long, primary transcripts? | Transposon regulation, unknown functions | 156,162–168,176 |
| piRNA-like (soma | Drosophila melanogaster | 24–30 | Dicer-independen | In ago2 mutants in Drosophila | Unknown | 76 |
| 21U-RNA piRNAs | Caenorhabditis elegans | 21 | Dicer-independent | Individual transcription of each piRNA? | Transposon regulation ,unknown functions | 113,172–174 |
| 26G RNA | Caenorhabditis elegans | 26 | RdRP? | Enriched in sperm | Unknown | 113 |