Methylation protects microRNAs from an AGO1-associated activity that uridylates 5' RNA fragments generated by AGO1 cleavage.

In plants, methylation catalyzed by HEN1 (small RNA methyl transferase) prevents microRNAs (miRNAs) from degradation triggered by uridylation. How methylation antagonizes uridylation of miRNAs in vivo is not well understood. In addition, 5' RNA fragments (5' fragments) produced by miRNA-mediated RNA cleavage can be uridylated in plants and animals. However, the biological significance of this modification is unknown, and enzymes uridylating 5' fragments remain to be identified. Here, we report that in Arabidopsis, HEN1 suppressor 1 (HESO1, a miRNA nucleotidyl transferase) uridylates 5' fragments to trigger their degradation. We also show that Argonaute 1 (AGO1), the effector protein of miRNAs, interacts with HESO1 through its Piwi/Argonaute/Zwille and PIWI domains, which bind the 3' end of miRNA and cleave the target mRNAs, respectively. Furthermore, HESO1 is able to uridylate AGO1-bound miRNAs in vitro. miRNA uridylation in vivo requires a functional AGO1 in hen1, in which miRNA methylation is impaired, demonstrating that HESO1 can recognize its substrates in the AGO1 complex. On the basis of these results, we propose that methylation is required to protect miRNAs from AGO1-associated HESO1 activity that normally uridylates 5' fragments.