RNA interference (RNAi) is the process of mRNA degradation that is induced by double-stranded RNA in a sequence-specific manner. RNAi has been observed in all eukaryotes, from yeast to mammals. The RNAi pathway is thought to be an ancient mechanism for protecting the host and its genome against viruses and rogue genetic elements that use double-stranded RNA (dsRNA) in their life cycles. They have also been shown to play a role not only in mRNA and dsRNA stability/degradation, but also in regulation of translation, transcription, chromatin structure, and genome integrity. In plants and animals, RNA silencing has been adapted to play a critical role in regulation of cell growth and differentiation using a class of small RNAs. In the RNA interference process, the dsRNAs get processed into 20-25 nucleotide (nt) small RNAs by an RNase III-like enzyme called Dicer. Then, the small RNAs assemble into endoribonuclease-containing complexes known as RNA-induced silencing complexes (RISCs), unwinding in the process. The small RNA strands subsequently guide the RISCs to complementary RNA molecules, where they cleave and destroy the cognate RNA (effecter step). Cleavage of cognate RNA takes place near the middle of the region bound by the siRNA strand. The small RNAs that provide target specificity to the silencing machinery includes short interfering RNAs (siRNAs), repeat-associated siRNAs (rasiRNAs), and microRNAs (miRNAs) and is distinguished by their origin. siRNAs are processed from dsRNA precursors made up of two distinct strands of perfectly base-paired RNA, while miRNAs originate from a single, long transcript that forms imperfectly base-paired hairpin structures. siRNAs were originally identified as intermediates in the RNAi pathway after induction by exogenous dsRNA; however, endogenous sources of siRNAs have now been recognized. The endogenous siRNAs are derived from repetitive sequences within the genome, and are termed repeat-associated siRNAs, or rasiRNAs. miRNAs were discovered through their critical roles in development and cellular regulation, and represent a large class of evolutionarily conserved RNAs. miRNAs have always been recognized as being of endogenous origin. RNA interference has emerged as a natural mechanism for silencing gene expression over the past decade. This ancient cellular antiviral response can be harnessed to allow specific inhibition of the function of any chosen target genes, including those involved in causing diseases such as cancer, AIDS, and hepatitis. It is already proving to be an invaluable research tool, allowing much more rapid characterization of the function of known genes. More importantly, the technology considerably bolsters functional genomics to aid in the identification of novel genes involved in disease processes. Last but not the least the technology can be harnessed as a novel therapeutic agent and is suitable for combating viral diseases, cancers and inflammatory diseases.

Imgenex (San Diego) recently launched the pSuppressorAdeno construction kit for adenovirus mediated gene knockdown. The kit provides the ability to infect a broad range of cell types, including many primary cell lines as well as dividing and nondividing cells, according to a company official. The kit also offers the flexibility to validate sequences using the nonviral expression plasmid prior to construction of adenoviruses, notes Sujay K. Singh, Ph.D., president and CEO of Imgenex, which markets plasmid-based RNA interference (RNAi) products. "One of the greatest advantages is the ability of recombinant adenovirus vectors to reduce gene expression both in vitro and in vivo," he adds. RNAi, initially considered a bizarre attribute of petunias and later a gene-silencing mechanism in worms, is creating a stir as one of the hottest new technologies in molecular biology. It is revolutionizing the field of functional genomics.

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