MicroRNA maturation: stepwise processing and subcellular localization
Yoontae Lee1, Kipyoung Jeon2, Jun-Tae Lee1, Sunyoung Kim1,2 and V.Narry Kim2,3
1 Institute of Molecular Biology and Genetics and 2 School of Biological Science, Seoul National University, Seoul 151-742, Korea 3 Corresponding author
Received April 12, 2002; revised July 16, 2002; accepted July 17, 2002
MicroRNAs (miRNAs) constitute a novel, phylogenetically extensive family of small RNAs (22 nucleotides) with potential roles in gene regulation. Apart from the finding that miRNAs are produced by Dicer from the precursors of 70 nucleotides (pre-miRNAs), little is known about miRNA biogenesis. Some miRNA genes have been found in close conjunction, suggesting that they are expressed as single transcriptional units. Here, we present in vivo and in vitro evidence that these clustered miRNAs are expressed polycistronically and are processed through at least two sequential steps: (i) generation of the 70 nucleotide pre-miRNAs from the longer transcripts (termed pri-miRNAs); and (ii) processing of pre-miRNAs into mature miRNAs. Subcellular localization studies showed that the first and second steps are compartmentalized into the nucleus and cytoplasm, respectively, and that the pre-miRNA serves as the substrate for nuclear export. Our study suggests that the regulation of miRNA expression may occur at multiple levels, including the two processing steps and the nuclear export step. These data will provide a framework for further studies on miRNA biogenesis.
Keywords: microRNA/nuclear export/processing/subcellular localization/transcription