Fagre, C.; Gilbert, W. V.

Pseudouridine ({Psi}) is an important post-transcriptional modification of many noncoding RNAs that is under-characterized in microRNA (miRNA) due to historical limitations in pseudouridine mapping methods. {Psi} modification stabilizes RNA duplex structures and could therefore play an important role in miRNA target binding and repression. To investigate the extent to which mammalian miRNAs are modified with {Psi}, we profiled the modification landscape of short (<30 nt) RNA in human cells and mouse tissues using bisulfite sequencing. Our approach was powered to detect small RNA pseudouridylation based on robust detection of known {Psi} positions in tRNA fragments (tRFs), some of which show tissue-specific patterns of modification. In contrast with tRFs, we find that miRNA pseudouridylation is exceedingly rare, with a single modified miRNA (miR-3068-5p) identified in mouse tissues. Pseudouridylated miR-3068-5p differentially repressed predicted miRNA targets with less stable miRNA:mRNA pairing modes. This study fills a long-standing gap in transcriptome-wide {Psi} profiling and reveals a new potential function for {Psi} as a modulator of activity of small regulatory RNAs.