RNAi-MEDIATED PATHWAYS IN THE NUCLEUS

1. RNAi-MEDIATED PATHWAYS INTHE NUCLEUS Marjori A.Matzke* and James A.Birchler

2. RNAi 分布：plant, animal and fungi 長度: 21~26 nt 來源: dsRNA 經由 Dicer (RNaseIII enzyme) 作用後製造出small interfering RNA(siRNA) 作用原理: 利用siRNA辨認特定mRNA序列並且利用 RNA-induced silencing complex (RISC)執行 target-mRNA cleavage 造成 mRNA degradation , 而依照target sequence 和effect complex的組成而有不同的silencing形式出現

3. 特別的形式： 利用RNA-dependent RNA polymerase (RdRP) 來藉由ssRNA製造dsRNA來起始或放大RNAi的作用 Example: Transposons 和 viruses在replication時製造dsRNA

4. MicroRNA (miRNA): 可藉由 targeting mRNA 誘導出silencing 來源: miRNAs are produced through processing by Dicer of imperfect RNA hairpins, which are typically encoded in non-protein-coding regions of plant and animal genomes.

5. Dicer Proteins of the Dicer family are ribonuclease III enzymes that process dsRNA to produce small RNAs. RNA-dependent RNA polymerases These synthesize dsRNA from an ssRNA template to initate or amplify the RNAi process. Argonaute Members of this family provide the small RNA-binding component of silencing-effector complexes.

6. Overview of nuclear pathways • RNA-directed DNA methylation (RdDM) • RNAi-mediated heterochromatin formation • DNA elimination • the silencing of unpaired DNA during meiosis

7. RNA-directed DNA methylation (RdDM) 利用DNA-dependent RNA polymerase 使RNA帶有訊號 利用RNA-dependent RNA polymerase2 大概是使DNA可容易接近RNA signal MET1:只針對CGs的cytosine甲基化 DRM2:只針對非CG的cytosine甲基化 在RNA消失後甲基化的維持 去甲基化,使gene再活化 可能和small RNA有關

8. Properties of heterochromatin. • heterochromatin is gene-poor relative to euchromatin and it surrounds the centromeric region in many species. • Heterochromatin is usually not conducive to high levels of gene expression

9. RNAi-mediated heterochromatin formation Heterochromatin中的Tandem repeat or multiple copies of transposable elements 藉由RdRP轉換成雙股形式 siRNA被認為引導histone methyltransfrease到chromatin進行H3k9的甲基化

10. DNA elimination that internal eliminated segment (IES) elimination and other genome alterations occur. Cells then separate and one micronucleus degenerates Early in conjugation, the micronuclear genome is bidirectionally transcribed The resulting dsRNA is processed by Dicer into small RNAs, termed scan RNAs (scnRNAs) The scnRNAs are exported into the old macronucleus, and those that can base pair to genomic DNA (red dotted lines) are degraded. The remaining scnRNAs, which correspond to sequences eliminated in the previous conjugation, migrate to the developing new macronucleus

11. the silencing of unpaired DNA during meiosis 內圈RNA以DNA-dependent RNA polymerase合成 已內圈當模板合成出收股RNA 可能被製造出siRNA,尚未被研究出來 Small RNA+RISC+AGO 辨認Homologous unpaired region

12. Conclusions Furthermore, the phylogenetic distribution of the four known RNAi-mediated nuclear pathways is not yet known. Does RdDM occur regularly in mammals? Are there examples of programmed DNA elimination mediated by RNAi in organisms other than ciliates? 還有許多疑問尚未被解答出來,期待科學家努力的成果,也期待有更進步的科學幫助我們探訪這個奇妙的世界!