In molecular biology, small nucleolar RNA derived microRNAs are microRNAs (miRNA) derived from small nucleolar RNA (snoRNA). MicroRNAs are usually derived from precursors known as pre-miRNAs, these pre-miRNAs are recognised and cleaved from a pri-miRNA precursor by the Pasha and Drosha proteins. However some microRNAs, mirtrons, are known to be derived from introns via a different pathway which bypasses Pasha and Drosha. Some microRNAs are also known to be derived from small nucleolar RNA.
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Discovery
Small nucleolar RNA derived microRNA was first described in 2008. The protozoal intestinal parasite Giardia lamblia lacks the Drosha protein involved in pre-miRNA cleavage from pri-miRNA. However the miRNA processing protein dicer is found in Giardia. A 26 nucleotide RNA, miR2, is processed from the Giardia lamblia snoRNA GlsR17 by dicer. MiR2 is a microRNA which may regulate the expression of variant surface proteins.
Also in 2008, a small RNA was found to be cleaved from SCARNA15, a Human small Cajal body-specific RNA, by Dicer. This small RNA was found to function as a microRNA. Potential microRNAs were also identified in several other Human snoRNAs.
Giardia snoRNA-derived miRNAs
Giardia lamblia produces at least 20 snoRNAs. SnoRNAs guide the modification of rRNA, and contain a region of 10 - 21 nucleotides which is complementary to the target RNA. Five of the Giardia lamblia snoRNAs have no complementarities to rRNA, implying that they may have a different function. MicroRNAs are derived from five of the box C/D snoRNAs from Giardia lamblia: