Table 1 The characteristics of different types of small RNAs

Micro RNA(miRNA)

 ~ 20–25

Hairpin loop-like precursor miRNA

transcribed by RNA pol II, cleaved by Drosha/DGCR8 and Dicer

Binds to Argonaute protein and forms a RISC complex that participates in transcriptional gene silencing, inhibition of mRNA translation and in mRNA decay

[16, 17]

PIWI-interacting RNA(piRNA)

 ~ 18–30

Long single-stranded RNA

transcribed by RNA pol II, processing by primary procession pathway and ping-pong cycle by Zuc, Aub, AGO3

Interacts with PIWI proteins to regulate gene silencing in a miRNA-like manner and regulates germ cell genome stability through recruitment of epigenetic regulators

[18, 19]

Small interfering RNA(siRNA)

 ~ 20–25

Long double-stranded RNA

transcribed by RNA pol II, processing by Dicer to cleavage

Binds to Argonaute protein and forms a RISC complex that participates in mediating mRNA decay

[10, 20]

Transfer RNA(tRNA)

 ~ 74–93

Precursor tRNA

transcribed by RNA pol III, processing by Ribonuclease P, Ribonuclease Z,endonuclease nucleotidyl transferase to cleavage and modify

Transports amino acids to the ribosome to synthesize polypeptide chains

[21, 22]

Small nuclear RNA (snRNA)

 ~ 90–200

\

Transcripted by RNA polymerase III and II

Combines with proteins to form small nuclear ribonucleoproteins (snRNPs) involved in pre-mRNA splicing

[23]

Small nucleolar RNA (snoRNA)

 ~ 60–400

\

Transcribed by RNA polymerase II, processing by ribonuclease

Binds to specific proteins to form small nucleolar ribonucleoprotein (snoRNPs) that regulate the modification of ribosomal RNA (rRNA)

[24]

5S RNA

120

pre-5S RNA

Transcribed by RNA polymerase III

Binds to ribosomal protein L5 to form a stable complex that is involved in the assembly of the larger ribosomal subunits

[25]

Vault RNA (vtRNA)

 ~ 88–140

\

Transcribed by RNA polymerase III

Binds to Vault proteins and form complexes; functions remain poorly characterized

[26, 27]

Y RNA

 ~ 80–110

\

Transcribed by RNA polymerase III, processing by ribonuclease

Binds to Ro60 protein and La protein to form Ro-RNP (Ro60 containing ribonucleoprotein) complex which regulates RNA stability, cellular stress responses, as well as initiation of chromosomal DNA replication and Ro60 protein activation

[28, 29]

Transfer RNA-derived small RNA (tsRNA)

 ~ 15–40

tRNA

Cleaved by Angiogenin, Dicer, RNase T2, RNase Z, ELAC2, Rnylp, etc

Functions by mimicry or displacement of tRNA; forms RNP complexes; binds to Argonaute

[30, 31]

Y RNA derived small RNA (ysRNA)

 ~ 22–25 and ~ 27–36

Y RNA

May cleaved by Caspase or RNsase1

Function is unclear; abundant in extracellular spaces, such as in serum, plasma and other biofluids

[32,33,34]

vtRNA derived small RNA (vtsRNA)

 ~ 23–32

vtRNA

Dicer

Function is unclear; predicted to function in a miRNA-like manner

[35, 36]

ribosomal RNA derived small RNA (rsRNA)

 ~ 21–44

rRNA

Unclear

Function is unclear; enriched in mature mouse sperm;

present in human peripheral blood serum; its presence is linked with inflammation in humans

[34, 37]

Small nucleolar RNA derived small RNA (snosRNA)

17–19; 20–24; > 26

snoRNA

Unclear

Function is unknown; likely resembles miRNA; can associate with argonaute proteins and influence translation; longer snosRNAs may form complexes with hnRNPs and influence gene expression

[38, 39]