Epigenetics

RNA interference (RNAi)

In almost all eukaryotes, double stranded RNA (dsRNA) induces RNAi. RNAi comprises gene silencing mechanisms that act on the transcriptional, post-transcriptional and on the translational level. Transcriptional gene silencing (TGS) is mediated through epigenetic modifications of promoter elements. Post-transcriptional gene silencing (PTGS) is based on the specific mRNA degradation. It is initiated whensoever the dsRNA shares regions of full complementarity with the mRNA. In the case of high but not full complementarity between the mRNA and the dsRNA, breakdown of translation takes place. Due to the fact that RNAi has that many diverse effects on gene regulation in numerous eukaryotes, the characterization of RNAi processes ranks among the most outstanding activities in biological and medical research (see also Nobel prize in Medicine 2006). All RNAi mechanisms share common components. However, in most cases, TGS, PTGS and translational gene silencing require specific members of enzyme families. These classes of enzymes mainly comprise variants of the Dicer (Dcr) or Dicer-like (DCL), Argonaute (AGO) and RNA-directed RNA polymerase (RDR) proteins. Plants possess a complete set of these enzymes. This explains why most of our knowledge about RNAi was gained by studying plant systems. In addition, plants accumulate all types of small regulatory RNAs, e.g. short interfering RNAs (siRNAs), micro RNAs (miRNAs) and trans-acting short interfering RNAs (tasiRNAs). Another advantage of using plants as model systems is based on the knowledge of numerous plant viruses. Most of them are coding for proteins that inactivate RNAi. These silencing suppressor proteins are useful tools to analyze defined RNAi processes. In addition, they can help to develop new strategies for the production of virus resistant plants.

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RNA interference (RNAi)



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