RNAi Suppression and Its Application

Abstract

In eukaryotes, RNA interference (RNAi) is a gene silencing mechanism mediated by small RNAs (sRNAs), currently classified as small interfering RNA (siRNA), microRNA (miRNA), and piwi-interacting RNA (piRNA). These small RNAs are produced in Dicer (a ribonuclease III enzyme)-dependent (siRNA and miRNA) or Dicer-independent (piRNA) manner and are effected by a group of Argonaut (AGO) proteins. These small RNAs mediate silencing of target genes with complementary sequence at transcriptional or posttranscriptional level thereby to control a wide variety of biological functions. In worms and plants, RNA-dependent RNA polymerases (RdRPs) amplify RNAi by converting AGO cleavage products into dsRNAs for the generation of secondary siRNAs. One of the well characterized functions of RNAi is antiviral, which has been shown to serve as major viral innate immunity in fungi, plants, and invertebrates. Typically, RNAi-directed viral immunity (RDVI) is initiated with Dicer processing of viral dsRNAs, usually the replication intermediates, into siRNAs. These virus-derived siRNAs (viRNA) will then be used as sequence guide for target viral RNA destruction. Host-encoded miRNAs also contribute to viral control in mammal or bacterial control in plant. As a counterdefensive mechanism, many viruses and some bacteria are found to encode RNAi suppressors, previously known as pathogenicity factors. These RNAi antagonists target key components of RNAi for suppression, which eventually leads to defects in viRNA biogenesis or function. Since transgene expression in plants and invertebrates is often targeted by RNAi for suppression but can be reversed by various RNAi suppressors, codelivery of a VSR has been used to facilitate the isolation and biochemical characterization of a broad range of proteins of interests. RNAi suppressors can also be used as genetic tool for the study of biological functions controlled by certain class of endogenous sRNA (siRNA or miRNA). This is because, when expressed as transgenes, some RNAi suppressors can specifically target and interfere with the biogenesis or function of certain class of endogenous sRNA but not the other.