Methylation as a Crucial Step in Plant microRNA Biogenesis

Abstract

Methylation on the base or the ribose is prevalent in eukaryotic ribosomal RNAs (rRNAs) and is thought to be crucial for ribosome biogenesis and function. Artificially introduced 2'-O-methyl groups in small interfering RNAs (siRNAs) can stabilize siRNAs in serum without affecting their activities in RNA interference in mammalian cells. Here, we show that plant microRNAs (miRNAs) have a naturally occurring methyl group on the ribose of the last nucleotide. Whereas methylation of rRNAs depends on guide RNAs, the methyltransferase protein HEN1 is sufficient to methylate miRNA/miRNA* duplexes. Our studies uncover a new and crucial step in plant miRNA biogenesis and have profound implications in the function of miRNAs.

On the origin and functions of RNA-mediated silencing: from protists to man

“…Plants lack a Drosha homolog and the production of miRNAs from precursor RNAs appears to be carried out by a single Dicer-like protein Kurihara and Watanabe 2004;Millar and Waterhouse 2005). Plant miRNAs are also methylated on the ribose of the last nucleotide, a modification presumably involved in protecting miRNAs from 3′ end uridylation and degradation, whereas animal miRNAs do not appear to be modified Yu et al 2005). Mechanistically, most animal miRNAs are only partly complementary to their targets and mediate silencing primarily by translational repression, although localization to the processing bodies may also affect RNA stability (Humphreys et al 2005;Liu et al 2005b;Pillai et al 2005;Wienholds and Plasterk 2005;Zamore and Haley 2005).…”

tRNA modifications: Necessary for correct tRNA‐derived fragments during the recovery from stress?

“…This would create bona fide single-stranded RNAs with open ends that are accessible to exonucleases. Importantly, small RNAs can be stabilized by the activity of a conserved enzyme, Hen1, which adds 2 0 -O-methyl groups to terminal nucleotides of siRNAs [19]. In this respect, it is interesting that specific 2 0 -O-methyl-modified nucleosides (Cm, Gm, Um) exist in loop structures of almost all tRNAs (including bacterial tRNAs), which suggests the existence of stops to exonucleolytic degradation (Fig.…”

Principles of micro-RNA production and maturation

“…In Arabidopsis, the protein HEN1 methylates the ribose of the most 3 0 distal nt of an miRNA, at the duplex intermediate stage (Figure 2b; Yu et al, 2005). In hen1 mutants, miRNAs become uridylated at their 3 0 ends and destabilized .…”