MicroRNAs: biogenesis and molecular functions

Abstract

Small regulatory RNAs are essential and ubiquitous riboregulators that are the key mediators of RNA interference (RNAi). They include microRNAs (miRNAs) and short-interfering RNAs (siRNAs), classes of approximately 22 nucleotide RNAs. miRNAs and siRNAs bind to Argonaute proteins and form effector complexes that regulate gene expression; in animals, this regulation occurs primarily at the post-transcriptional level. In this review, we will discuss our current understanding of how miRNA and siRNAs are generated and how they function to silence gene expression, focusing on animal and, in particular, mammalian miRNAs.

Figure 1

Biogenesis and function of microRNAs (miRNAs) and short‐interfering RNAs (siRNAs). As is true for almost all RNAs, miRNAs and siRNAs are derived from larger precursor RNAs. The precursor for miRNAs and siRNAs is double‐stranded (ds) RNA. In the case of miRNAs, the immediate precursor RNA is termed pre‐miRNA, adopts a hairpin structure and has a 5′‐phosphate and a 2‐nucleotide, 3′ overhang. In the case of siRNAs, the precursor is long dsRNA. Both are processed by the Dicer nuclease into duplexes termed miRNA or siRNA duplexes. The mature miRNA or siRNA is then released from the miRNA or siRNA duplex and binds to an Ago protein, to form a core effector complex that is commonly known as miRNP or RISC. miRNAs or siRNAs then act as specificity determinants to deposit the Ago proteins that they are bound to (RISCs, RNA‐induced silencing complex; miRNPs) onto their RNA targets, which are typically mRNAs. The deposited miRNP/RISC silences gene expression by suppressing the translation of the targeted mRNA and/or by destabilizing the targeted mRNA. Ago, Proteins that bind directly to miRNAs and siRNAs and are the core protein components of mitNPs/RISCs; Dicer, dsRNA endonuclease; miRNP, RiboNucleoProtein containing miRNA and an Ago protein.

Figure 2

Overview of microRNA (miRNA) processing.

Figure 3

Precursor miRNA (Pre‐miRNA) processing: an RNA view.

Figure 4

RNA‐induced silencing complex (RISC) assembly in Drosophila melanogaster.

Figure 5

miRNP assembly in humans.

Figure 6

Target RNA cleavage by Ago2‐containing miRNP.

Figure 7

A. Principles of miRNA binding to target RNA. Binding of the miRNA to the target mRNA always occurs in the presence of an Ago protein; the protein was omitted to highlight the nucleotides that base‐pair. Base‐pairing between nucleotides 2 and 8 of an miRNA (an area known as “seed” or “proximal” or nucleus”) and its cognate mRNA target (MRE, miRNA recognition elements) is essential for binding of most miRNAs to their targets. Base‐pairing of other nucleotides of a miRNA and its target also occurs and may be required when complementarity in the proximal area is not perfect, or to enhance binding or function. B. Mechanisms of repression of targeted mRNA by miRNPs.