Recently, nearly 100 additional ~22 nt RNA molecules, termed microRNAs (miRNAs), were identified in Drosophila, C. elegans, and HeLa cells (35-38). Much like lin-4 and let-7, these miRNAs are formed from precursor RNA molecules that fold into a stem-loop secondary structure. The newly discovered ~22 nt miRNAs are believed to play a role in regulation of gene expression, and at least two of them are known to require Dicer for their production (37). It appears that the use of small RNAs for both gene regulation and RNAi is a common theme throughout evolution.
Inducing RNAi in Mammalian Cells From Mechanism to Application
Non-specific Gene Silencing by Long
While the natural presence of RNAi had been observed in a variety of organisms (plants, protozoa, insects, and nematodes), evidence for the existence of RNAi in mammalian cells took longer to establish. Transfection of long dsRNA molecules (>30 nt) into most mammalian cells causes nonspecific suppression of gene expression, as opposed to the gene-specific suppression seen in other organisms. This suppression has been attributed to an antiviral response, which takes place through one of two pathways.
In one pathway, long dsRNAs activate a protein kinase,
PKR. Activated PKR, in turn phoshorylates and inactivates the translation