RNA-directed DNA methylation

RNA-directed DNA methylation (RdDM) is an epigenetic process first discovered in plants.[1] During RdDM, double-stranded RNAs (dsRNAs) are processed to 21-24 nucleotide small interfering RNAs (siRNAs) and guide methylation of homologous DNA loci. In plants dsRNAs may be generated from four sources:

Viral replication intermediates
Products of the endogenous RNA-directed RNA polymerase
Transcribed inverted repeats
Transposable elements

Besides RNA molecules, a plethora of proteins are involved in the establishment of RdDM, like Argonautes, DNA methyltransferases, chromatin remodelling complexes. and the plant-specific Polymerase IV and Polymerase V. All these act in concert to add a methyl-group at the 5' position of cytosines. In contrast to animals, cytosines at all sequence context (CG, CHG, CHH) may get de novo methylated in plants.

References

Wassenegger, Michael; Heimes, Sabine; Riedel, Leonhard; Sänger, Heinz L. (1994-02-11). "RNA-directed de novo methylation of genomic sequences in plants". Cell. 76 (3): 567–576. doi:10.1016/0092-8674(94)90119-8. ISSN 0092-8674. PMID 8313476.

Sources

Wassenegger, Michael; Heimes, Sabine; Riedel, Leonhard; Sänger, Heinz L. (1994). "RNA-directed de novo methylation of genomic sequences in plants". Cell. 76 (3): 567–76. doi:10.1016/0092-8674(94)90119-8. PMID 8313476.
Law, Julie A.; Jacobsen, Steven E. (2010). "Establishing, maintaining and modifying DNA methylation patterns in plants and animals". Nature Reviews Genetics. 11 (3): 204–20. doi:10.1038/nrg2719. PMC 3034103. PMID 20142834.
Haag, Jeremy R.; Pikaard, Craig S. (2011). "Multisubunit RNA polymerases IV and V: Purveyors of non-coding RNA for plant gene silencing". Nature Reviews Molecular Cell Biology. 12 (8): 483–92. doi:10.1038/nrm3152. PMID 21779025.
Bender, Judith (2012). "RNA-Directed DNA Methylation: Getting a Grip on Mechanism". Current Biology. 22 (10): R400–1. doi:10.1016/j.cub.2012.04.010. PMID 22625855.
Dalakouras, Athanasios; Wassenegger, Michael (2013). "Revisiting RNA-directed DNA methylation". RNA Biology. 10 (3): 453–5. doi:10.4161/rna.23542. PMC 3672289. PMID 23324611.

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[1] Wassenegger, Michael; Heimes, Sabine; Riedel, Leonhard; Sänger, Heinz L. (1994-02-11). "RNA-directed de novo methylation of genomic sequences in plants". Cell. 76 (3): 567–576. doi:10.1016/0092-8674(94)90119-8. ISSN 0092-8674. PMID 8313476.