Development and Modification of Pre-miRNAs with a FRET Dye Pair for the Intracellular Visualization of Processing Intermediates That Are Generated in Cells.

microRNAs (miRNAs) are small non-coding ribonucleic acids (RNAs), which regulate gene expression via the RNA interference (RNAi) system. miRNAs have attracted enormous interest because of their biological significance and disease relationship. In cell systems, the generation of miRNA is regulated by multiple steps: the transfer of primary miRNA from the nucleus to the cytosol, the generation of the precursor-miRNA (pre-miRNA), the production of double-stranded RNA from pre-miRNA by the Dicer, the interaction with protein argonaute-2 (AGO2), and the subsequent release of one strand to form miRISC with AGO2. In this study, we attempt to visualize the intermediates that were generated in the miRNA-maturation step in the cells to acquire a detailed understanding of the maturation process of miRNA. To achieve this, we developed pre-miRNAs labeling with a Dicer- or AGO2-responsible fluorescence resonance energy transfer (FRET) dye pair. We observed that modifications with the dye at suitable positions did not interfere with the biological activities of pre-miRNAs. Further, imaging analyses employing these pre-miRNAs demonstrated that the processing of pre-miRNA promoted the accumulation of miRNA at the specific foci in the cytosol. The FRET-labeled pre-miRNA would further elucidate the mechanisms of the RNAi process and provide the basis for development of nucleic acid drugs working in the RNAi system.