Methylation protects miRNAs and siRNAs from a 3'-end uridylation activity in Arabidopsis.
Small RNAs of 21-25 nucleotides (nt), including small interfering RNAs (siRNAs) and microRNAs (miRNAs), act as guide RNAs to silence target-gene expression in a sequence-specific manner. In addition to a Dicer homolog, DCL1, the biogenesis of miRNAs in Arabidopsis requires another protein, HEN1. miRNAs are reduced in abundance and increased in size in hen1 mutants. We found that HEN1 is a miRNA methyltransferase that adds a methyl group to the 3'-most nucleotide of miRNAs, but the role of miRNA methylation was unknown. Here, we show that siRNAs from sense transgenes, hairpin transgenes, and transposons or repeat sequences, as well as a new class of siRNAs known as trans-acting siRNAs, are also methylated in vivo by HEN1. In addition, we show that the size increase of small RNAs in the hen1-1 mutant is due to the addition of one to five U residues to the 3' ends of the small RNAs. Therefore, a novel uridylation activity targets the 3' ends of unmethylated miRNAs and siRNAs in hen1 mutants. We conclude that 3'-end methylation is a common step in miRNA and siRNA metabolism and likely protects the 3' ends of the small RNAs from the uridylation activity.
Li
Junjie
J
Department of Botany and Plant Sciences, Institute of Integrative Genome Research, University of California, Riverside, Riverside, California 92508, USA.
Yang
Zhiyong
Z
Yu
Bin
B
Liu
Jun
J
Chen
Xuemei
X
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism; D003001:Cloning, Molecular; D008745:Methylation; D035683:MicroRNAs / Q000378:metabolism*; D008969:Molecular Sequence Data; D026721:RNA 3' End Processing / Q000502:physiology*; D034741:RNA, Small Interfering / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D043244:Ribonuclease III / Q000378:metabolism; D014529:Uridine / Q000378:metabolism
D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; C450825:HEN1 protein, Arabidopsis; D035683:MicroRNAs; D034741:RNA, Small Interfering; C402383:DCL1 protein, Arabidopsis; D043244:Ribonuclease III; D014529:Uridine