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First published online September 29, 2004
doi: 10.1242/10.1242/jcs.01479

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RNA-directed DNA methylation

Department of Biochemistry and Molecular Biology, Johns Hopkins University Bloomberg School of Public Health, 615 N. Wolfe Street, Baltimore, MD 21205, USA

View larger version (24K): [in a new window]   Fig. 1. RNAi and RNA-directed DNA methylation (RdDM) are both triggered by dsRNA. dsRNAs are cleaved by enzymes of the Dicer family to generate small, 21-26 nucleotide siRNAs. These in turn are taken up by the RNA-induced silencing complex (RISC) to direct degradation of complementary RNA sequences. Such RNAs are also involved in RdDM, which is mediated by three classes of methyltransferase: DRM, CMT and MET1.

View larger version (29K): [in a new window]   Fig. 2. Mechanisms for generating dsRNA in plants. dsRNA can be generated through replication of ssRNA viruses (A) or transcription of inverted repeats (B). It can also be generated from endogenous genes or transposons whose transcripts become substrates for RNA-directed RNA polymerase (RdRP); the action of Dicer-like enzymes can then amplify the effect, generating primers for RdRP (C). Alternatively, transcription from promoters on opposite strands could yield complementary RNAs that undergo intramolecular pairing.