SPT5-like, a new component in plant RdDM

Nature Publishing Group
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Journal Article
EMBO REPORTS, 2009, 10 (6), pp. 573 - 575
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Plant DNA is extensively methylated at cytosine residues; however, unlike in animals, the methylation occurs not only at CG sites but also at CNG and CHHin which H represents A, C or Tsites. A significant proportion (~30%) of this cytosine methylation in Arabidopsis is dependent on the RNA-silencing pathway known as RNA-directed DNA methylation (RdDM; Matzke et al, 2009). RdDM has a crucial role in silencing retrotransposons and endogenous repeats, thereby maintaining genome stability. It probably also has a role in regulating the transcriptional activities of plant genes that contain, or are adjacent to, transposons or repetitive elements, many of which might have a role in stress responses. In addition, RdDM is required for the maintenance and systemic transmission of post-transcriptional gene silencing in plants (Brosnan et al, 2007; Eamens et al, 2008), presumably by causing modification of the transgene DNA that is necessary for producing secondary small-interfering RNAs (siRNAs; Daxinger et al, 2009). The existence of RdDM was initially demonstrated using exogenous viral RNAs (Wassenegger et al, 1994; Jones et al, 1999), and was subsequently shown to be mediated by 24-nucleotide (nt) siRNAs, which form the predominant population of endogenous small RNAs (Kasschau et al, 2007; Mosher et al, 2008). siRNAs can also be generated by viruses and transgenes in plants to direct RdDM (Mette et al, 2000; Wang et al, 2001).
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