tiny RNAs (20-to-25 nt RNAs) • in eukaryotic cells, tiny RNAs function as transcriptional regulators of gene expression in (at least) three distinct pathways, – (1) micro (miRNAs) regulate RNA translation. – (2) small interfering RNAs (siRNAs) direct RNA destruction via the RNA interference (RNAi) pathway, • (3) and transcriptional regulation via epigenetic processes,
Co-suppression Modes ...Transcriptional Gene Silencing (TGS), – RNA functions in the methylation of promoters and structural elements of genes, ...Post-Transcritional Gene Silencing (PTGS), – involves the specific degradation of mRNA via a double-stranded RNA intermediate, dsRNA.
RNAi RNA interference ...while attempting to do anti-sense KO of gene expression in C. elegans, Guo and Kemphues, Cell 81, 611 (1995) observed that sense and anti-sense strands worked equally, – in an anti-sense experiment, a gene is constructed so that it produces a complementary strand to an expressed transcript, • the goal is to complement, thus inactivate the mRNA. ...following up, other researchers found that dsRNA worked at least an order of magnitude better that either sense or anti-sense strands.
RNAi ...siRNA control of gene expression by RNA processing is now considered a common element in eukaryotic cells, – defense against viruses, – control of transposable elements, – regulate gene expression? …useful for doing Reverse Genetic studies, – dsRNA triggers sequence specific degradation of complementary mRNAs.