Regulation of gene expression is extremely important for development and maintenance of cellular function. RNA-mediated genetic regulation utilizes several types of small RNAs, such as small interfering RNA (siRNA) and micro-RNA (miRNA) for targeted genetic knockdown (1). Organisms ranging from fungi to humans rely on this type of genetic regulation. Thus, understanding how this form of regulation works provides insights into a highly conserved mechanism across evolutionary history (2).
The mode of action of these small RNAs are quite similar. Both single stranded miRNA and double-stranded siRNA utilize an RNA-induced silencing complex (RISC) to regulate target mRNA sequences (3). miRNA competitively inhibits through complementary sequence binding in the 3’ upstream element of their target messenger-RNA (mRNA) (4). This binding to mRNA typically leads to target degradation or suppression upon shortening of the mRNA poly(A) tail and the removal of the 5’ cap end structure. Distinct from miRNA, siRNA strands are separated and the more stable 5’-end strand is integrated into RISC (3). This remaining siRNA strand aligns the RISC complex with the target messenger RNA (mRNA) where Ago-2, a protein found in RISC, mediates cleavage of the mRNA and thus target inhibition (1). siRNA can facilitate very efficient gene knockdown through very low concentrations (picomolar range) as the siRNA-loaded RISC utilizes a catalytic mechanism of binding, dissociation, followed by binding to facilitate the degradation of multiple mRNA with one RISC complex (1). siRNA also inhibits only the expression of one specific target mRNA. In contrast, miRNA tends to target several different mRNA sequences due to the relatively short ‘seed sequence’ found on the miRNA, thus providing a more expansive knockdown effect.
The introduction of either siRNA or miRNA directly into cells results in knockdown of target genes. This provides an opportunity to fine-tune genetic expression and study the requirement of specific mRNA. This has broader implications for identifying the function of various genes in larger cellular processes such as cellular signaling, cancer progression, and cellular apoptosis.