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Fig. 3. Biogenesis of siRNAs and hcRNAs. (A) Biogenesis of different
classes of siRNA (see text for more details). Endogenous siRNAs are
transcribed in the nucleus, whereas exogenous siRNAs are either chemically
synthesized or virally derived. siRNAs are further processed by RNase III
enzymes, such as Dicer. tasiRNAs are specific to plants and, after initial
cleavage by specific miRNAs (red) and complementary strand synthesis by the
RNA-dependent RNA polymerase (RdRp) RDR6, are processed by the Dicer DCL4.
They are then phosphorylated (P) and subsequently methylated (me) by the RNA
methyltransferase HEN1. In C. elegans and plants, secondary siRNAs
participate in a signal amplification loop. In plants, the cleaved mRNA is
converted into dsRNA by a RdRP, and is further processed by Dicer (the RNase
III in green). In C. elegans, secondary siRNAs have a 5' di- or
triphosphate group and associate with Class 3 Ago/Piwi protein members,
Sago-1/Sago-2, leading to target mRNA (black) cleavage. (B) Biogenesis
of hcRNAs (see also text for more details). In the yeast S. pombe and
in plants, hcRNAs are processed by RNase III enzymes. In S. pombe,
hcRNAs associate with Ago1 and form the RNA-induced transcriptional gene
silencing (RITS) complex, which participates in RNA-directed RNA polymerase
complex (RDRC) formation and histone (gray circles) methylation. In plants,
hcRNAs form a complex with AGO4, which participates in DNA methylation. Chp1,
chromodomain protein 1; Clr4, cryptic loci regulator 4; Cid12, caffeine
induced death protein; DRD1, defective in RNA-directed DNA methylation 1 (an
SNF2-like chromatin remodeling protein); DRM2, domain rearranged
methyltransferase 2; Hrr1, a helicase required for RNAi-mediated
heterochromatin assembly; Rdp1, RNA-directed RNA polymerase 1; Tas3, targeting
complex subunit 3.
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