1. RNnomics Advanced MCB Presentation Series I Group Members: 孙群, 刘军, 段文霞，刘少英

2. RNA Silencing: The Genome’s Immune System Science, 2002, Vol. 296:1263-1265 Ronald H. A. Plasterk Hubrecht Laboratory, Center for Biomedical Genetics, 3584 CT Utrecht, Netherlands

3. Genome ’ s Immune System Genomes are database sensitive to invasion by viruses: 45% of the human genome consists of remnants of previous transposon/virus invasions and elements A defense mechanism conserved among eukaryotes with two characteristics: 1. Specificity against foreign elements 2. Ability to amplify and raise a massive response against an invading nucleic acid.

4. Vertebrate Immune System Use a two-Step strategy: A large repertoire of antibody-encoding genes is generated from a limited set of gene segments by combinatorial gene rearrangements This repertoire is stored in a distributed fashion over large numbers of cells Clonal selection and expansion of specific cells results in an immune response specifically directed to the immunogen.

5. How to Protect the integrity of the genome How to recognize self from nonself How to amplify an initial response in a specific fashion Animal models: Nematodes, Flies, C. elegans

6. Ways of Genome ’ s Immune Response Selective methylation of transposon sequences in the genome An RNA-based silencing mechanism: ancient core mechanism, conserved among species from different kingdoms (fungi, animals, and plants) – likely acts as the "immune system" of the genome.

7. Function of RNA Silencing Protect against viruses and transposons Posttranscriptional gene silencing (PTGS) and virus-induced gene silencing dsRNA: not normally encountered in high concentration, exogenous dsRNA can trigger gene silencing RNAi: genome ’ s “ immune system ” with the systemic effect, can be induced by feeding

8. Fig. 1. A model for the molecular steps in RNA silencing.

9. Self and Nonself Transponsons or viruses induce dsRNA: corresponds to their own sequence: Both strands are transcripted dsRNA Terminal inverted repeats of transposon can result in snap-back dsRNA Some other sensor of the foreign transposons

10. Amplification Small amounts of dsRNA are able to silence a vast excess of target mRNA in C. elegans The Dicer enzyme cuts long dsRNA molecules into short "primary" siRNAs. Each siRNA can potentially target a homologous mRNA, this provides a level of amplification A catalytic mechanism, in which siRNAs are used multiple times, can provide further amplification.

11. Target-Dependent Amplification Short RNAs can serve as primers on target mRNA and subsequent generation of "secondary siRNAs" initiate an RNA-directed RNA polymerization reaction 1st step: mRNA is recognized by primary siRNAs. dsRNA is cut into short siRNAs, presumably these are converted from dsRNA into ssRNA, 2nd step: after the antisense siRNA has base paired to the target mRNA, target-directed amplification can occur – polarity in some species

12. Conclusion The gene immune system recognizes molecular parasites, raises an initial response, stabilizes and amplifies this response. Parts of the RNAi-silencing machinery is of conservation RNAi silencing refers to a family of mechanisms that are quite different in context and detail. Almost certainly be the case for more specific aspects of the biology: systemic RNAi in C.elegans, spreading of silencing in plants, and suppression of silencing induced by several plant viruses.

13. Thank You!