Table 1 Common molecules in microRNA biogenesis and function

Primary microRNA (pri-miR)‘

Initial transcription from independent (not intronic) miR gene results in a pri-miR, A pri-miR is often multiple kilobases in length and may encode more then one mature miRNA.

[12]

Pre-microRNA (pre-miR)

Processing of pri-miR by microprocessor results in pre-miR. PremiRs are 60–70 bp double stranded RNAs with intramolecular sequence complementarity, so that they form stem-loop structures.

[20]

microRNA (miR)

Mature single stranded RNA 20–22 bp in length. Exists in complex w ith Argonaute and accessory proteins as part of the RISC complex, providing target specificity.

[20]

Mirtron

miRs located within introns of protein coding genes, Transcribed as part of their host genes primary transcript, but are cleaved from this during mRNA splicing, resulting in a pre-miR.

[15]

Microprocessor complex

Multi-protein complex, containing the obligate members Drosha and DGCR8. Responsible for cleaving stem-loop structures from pn-miR, resulting in pre-miRNA.

[52]

Drosha

RNase III protein. One of two obligate members of the microprocessor complex.

[73]

DGCR8

Double stranded RNA binding domain protein. One of two obligate members of the microprocessor complex.

[73]

Dicer

RNase Ill-like protein. Cleaves ‘loops of stem-loop from pre-miR, resulting in a 20–22 bp miRNA duplex. Assists in loading one strand of this duplex, the guide strand, into RISC complex.

[12]

RISC

RNA induced silencing complex (RISC) is a multi-protein complex containing Argo bound to a single stranded miR. Association of the RISC complex with a mRNA target leads to mRNAdestabilization/degradation, either through direct RNase actions of Argo II or the activity of other recruited accessory proteins.

[74]

Argonaute (Argo)

Component of the RISC complex, it acts at the interface between a miR and mRNA target. There are four mammalian Argonautes, though only Argo II has RNase activity.

[16] [74]