Molecular Biology

Me Instead of U

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Science  09 Sep 2005:
Vol. 309, Issue 5741, pp. 1651
DOI: 10.1126/science.309.5741.1651b

Small interfering (si)RNAs and micro (mi)RNAs play a central role in RNA silencing phenomena in which RNAs complementary to the si/miRNA are targeted for cleavage and destruction, translational silencing, or are (presumably) used as markers for directing heterochromatin formation. Much is known about the biogenesis of si/miRNAs, but what happens to them afterward?

In Arabidopsis, the HEN1 protein, implicated in RNA silencing, has been shown to add a methyl group to the 3' ends of miRNAs. Results from Li et al. show that Hen1 has a much wider scope, methylating many Arabidopsis siRNAs at their 3' ends, too. In the absence of HEN1, the unmethylated miRNAs and siRNAs have several uridine (U) residues added to their 3' ends, indicating that methylation interferes with uridylation. Intriguingly, it is the sense strand of the miRNA:miRNA* duplex that preferentially has a U tail added, hinting that the addition might occur after passage through the RISC complex, when the antisense miRNA* strand has already been jettisoned. The authors propose that methylation may stabilize small RNAs, a speculation supported by the previous observation that cleaved mRNA targets have also been observed to be U-tailed. — GR

Curr. Biol. 15, 1501 (2005).

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