HAU16008-04-3 - CHAPTER 9 Post-Transcriptional Gene...

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HAPTER 9 Noncoding RNAs: Molecular Biology and Molecular Medicine , edited by Jan Barciszewski and Volker A. Erdmann. ©2003 Eurekah.com and Kluwer Academic / Plenum Publishers. Post-Transcriptional Gene Silencing in Plants Matthew A. Escobar and Abhaya M. Dandekar Abstract A ccumulating genetic and biochemical evidence suggests that antisense-mediated gene silencing, cosuppression, RNA interference and virus-induced gene silencing are all unique inputs into a common RNA silencing pathway triggered by double stranded RNA. This pathway, termed post-transcriptional gene silencing (PTGS) is characterized by accumulation of 21-25 nt small-interfering RNAs, sequence-specific degradation of target mRNAs, and methylation of target gene sequences. PTGS appears to be ancient and highly conserved, as several groups of homologous genes required for silencing in plants, animals, and fungi have been identified. Though biochemical dissection of PTGS is still in its infancy, sev- eral key activities have been identified, such as Dicer, the endonuclease responsible for synthe- sis of short-interfering RNAs, and RISC, the nucleoprotein complex which mediates mRNA degradation. Several lines of evidence suggest that PTGS plays a key role in viral defense in plants, but further study is required to investigate the intriguing possibility that PTGS can act as an endogenous gene control mechanism. Introduction The characterization of antisense RNA-mediated controls of plasmid replication and maintenance in prokaryotes prompted several studies of mRNA silencing in eukaryotic genes in the mid-1980s. 1,2 Rothstein et al (1987) first demonstrated an “antisense effect” in intact plants by silencing an integrated nopaline synthase ( nos ) transgene through expression of antisense nos RNA in tobacco. 3 Interestingly, this eukaryotic antisense silencing appeared to result from increased turnover of the targeted nos mRNA, in contrast to the inhibitory effects on transla- tion 4 or DNA synthesis 5 commonly associated with antisense RNA in bacteria. A similar homology-dependent RNA degradation phenomenon, resulting from the over expression of an endogenous gene in transgenic plants, was discovered soon thereafter. 6,7 These phenomena, termed antisense-mediated gene silencing and cosuppression, became powerful techniques for plant improvement and functional gene analysis, but relatively little was understood about their underlying mechanisms. 8 The seminal discoveries that double stranded RNA (dsRNA) and plant viruses are potent initiators of gene silencing has recently invigorated basic research on silencing mechanisms. 9,10 It has become apparent that the diverse threads of antisense-mediated gene silencing, cosuppression, dsRNA-mediated gene silencing (RNA interference), and virus-induced gene silencing converge with the synthesis of dsRNA. 11
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HAU16008-04-3 - CHAPTER 9 Post-Transcriptional Gene...

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