Plants as a modell system to analyze epigenetic phenomena

转录后基因沉默(PTGS)和转录后基因沉默(PTGS)可以遗传给后代。它们都是可逆的。因此，后代不会表现出遗传上的变化，而是表观遗传上的变化。在哺乳动物和植物中，这些变化通常与相应基因的超甲基化有关。在植物中，RNA引导的DNA甲基化(RdDM)参与了TGS和PTGS。有趣的是，不仅TGS和PTGS而且RdDM也是由双链RNA(DsRN)启动的。哺乳动物的基因印迹和X染色体失活似乎与正义/反义RNA的产生和从头DNA甲基化的发生有关。因此，有理由认为RdDM也存在于哺乳动物中，并参与表观遗传过程。为了找到支持这一假说的证据，应该在植物系统中研究RdDM，它仍然几乎没有被描述为特征。有了预期的数据，就有可能分析哺乳动物中的从头甲基化是否也可以由RNA指导。此外，还将检查dsRNA导入哺乳动物细胞后是否可以启动从头DNA甲基化。与以前的实验不同，dsRNA将通过反向重复(IR)结构的转录提供。此外，IR转录本是否能够激活RNA干扰(RNAi)机制将是有趣的。

Transcriptional (TGS) and post-transcriptional gene silencing (PTGS) can be transmitted to the progeny. They are both reversible. Thus, the progeny do not show genetic but epigenetic changes. In mammals and plants, these changes are generally associated with hypermethylation of the corresponding genes. In plants, RNA-directed DNA methylation (RdDM) is involved in both TGS and PTGS. Interestingly, not only TGS and PTGS but also RdDM are initiated by double stranded RNA (dsRN). Gene imprinting and X-chromosome inactivation of mammals seem to be correlated with production of sense/antisense RNA and with occurrence of de novo DNA methylation. Thus, it is reasonable to assume that RdDM also exists in mammals and that it is involved in epigenetic processes. In order to find evidence for the hypothesis, RdDM which is still barely characterised should be investigated in a plant system. With the expected data it could be possible to analyse whether de novo methylation can be directed by RNA in mammals, as well. In addition, it will be examined whether de novo DNA methylation can be initiated upon intruduction of dsRNA into mammalian cells. In contrast to previous experiments, the dsRNA will be provided by transcription of inverted repeat (IR) constructs. Moreover, it will be interesting to see whether the IR transcripts are capable to activate the RNA interference (RNAi) mechanism.