西亚试剂：A Dicer-Independent Route for Biogenesis of siRNAs that Dir

DNA methylation directed by 24-nucleotide (nt) small interfering RNAs (siRNAs) plays critical roles in gene regulation and transposon silencing in Arabidopsis. 24-nt siRNAs are known to be processed from double-stranded RNAs by Dicer-like 3 (DCL3) and loaded into the effector Argonaute 4 (AGO4). Here we report a distinct class of siRNAs independent of DCLs (sidRNAs). sidRNAs are present as ladders of ∼20–60 nt in length, often having the same 5′ ends but differing in 3′ ends by 1-nt steps. We further show that sidRNAs are associated with AGO4 and capable of directing DNA methylation. Finally we show that sidRNA production depends on distributive 3′-5′ exonucleases. Our findings suggest an alternative route for siRNA biogenesis. Precursor transcripts are bound by AGO4 and subsequently subjected to 3′-5′ exonucleolytic trimming for maturation. We propose that sidRNAs generated through this route are the initial triggers of de novo DNA methylation.

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