Viroids, Viruses, and RNA Silencing

PATHOGENICITY MODEL:© National Academy of Sciences, USAViroid replication generates dsRNA intermediates, which are processed by Dicer into 21- to 25-nucleotide siRNAs. These siRNAs are then incorporated into siRNA – ribonuclease complexes (RISC). If the siRNA sequences significantly match host mRNAs, RISC may target them for degradation leading to disease symptoms. RISC can also target the viroid, forcing it to evolve and to adopt and maintain an RNA silencing-resistant structure. (Fr

Written byLeslie Pray

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Viroid replication generates dsRNA intermediates, which are processed by Dicer into 21- to 25-nucleotide siRNAs. These siRNAs are then incorporated into siRNA – ribonuclease complexes (RISC). If the siRNA sequences significantly match host mRNAs, RISC may target them for degradation leading to disease symptoms. RISC can also target the viroid, forcing it to evolve and to adopt and maintain an RNA silencing-resistant structure. (From M.-B. Wang et al., Proc Natl Acad Sci, 101:3275–80, 2004.)

There is growing evidence that small RNA's believed to play an antiviral defense role in many organisms, may be acting as doubleagents. In March 2004, an international team of scientists reported that viroids, small infectious particles of naked RNA, may be employing RNA-silencing machinery to work their damage.1 Several weeks later, another international team announced the discovery of microRNAs in the Epstein-Barr virus (EBV), suggesting that miRNA-mediated gene suppression might ...

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