MicroRNAs (miRNA) are a class of small non-coding RNAs that involve in regulation of gene expression.
lin-4 was the first discovered miRNA by Ambros and colleagues in C. elegans(Caenorhabditis elegans). After 7 years of the discovery of lin-4, Reinhart et al. reported lin-7( another miRNA in C. elegans).
A unique characteristic of miRNAs is that one miRNA can regulate the expression of several genes, while one gene can be targeted by different miRNAs.
Nomenclature guidelines for miRNA :
1. Respect species-specific gene nomenclature conventions.
- MIR125a and Mir125 are orthologous miRNA genes present in human and mouse.
- The Italicized form “MIR” (MiR, miR) with a capitalized “R”, is representative for the intermediate transcripts.
- The non-italicized form “MIR” (MIR, MiR) with a capitalized “R”, is representative for the mature microRNAs.
- The “mmu-” should be removed for the mouse gene “mmu-Mir-125a” and kept only if the database covering multiple species. For not getting confused, the reference to the organism should be clearly stated in the text.
- the gene “hsa-MIR-125a” becomes MIR125A,
- “mmu-Mir-125a” becomes “Mir125a”, and
- “dre-mir-125aa” becomes “mir125aa”.
- The gene “Mir125a” has “pri-MiR125a” as the primary miRNA transcript.
- The miRNA precursor is“pre-MiR125a”.
- The mature form, “MiR125a”.
Guide or mature strand and Passenger or star strand: Two mature product of Mir gene
Biogenesis and regulation of miRNA
- In the first step of biogenesis of miRNA the gene is transcribed into large primary transcript called pri-miRNA. The pri-miRNA is 5’ capped and 3’ polyadenylated in structure. The transcription of miRNA is typically mediated by RNA polymerase II, although some of the pre-miRNAs are also generated by the RNA polymerase III.
- Now, the pri-miRNAs are cleaved by a microprocessor complex. This microprocessor complex is composed of the RNA-Binding protein known as DGCR8 and type III RNase Drosha. The cleaved products are approx 85 nucleotide stem-loop structures known as precursor miRNA (Pre-miRNA).
- The Ran/FTP/Exportin 5 complex transport the pre-miRNA from the nucleus to the cytoplasm. In the cytoplasm, the pre-miRNAsare processeds by another RNase III enzyme Dicer into 20-22 nucleotide long miRNA/miRNA * duplex ( * represent the passenger strand and the other complementary strand is called as the guide or mature strand).
- When this duplex unwound, the mature miRNA is incorporated into a protein complex. This protein complex is termed as RNA-induced silencing complex (RISC). The mature miRNA guides RISC to target mRNA.
Commercial miRNA microarray platforms
Gene expression microarray technology is a very powerful high throughput tool capable of monitoring the expression of thousands of genes in an organism simultaneously.