Unraveling 3'-end RNA uridylation at nucleotide resolution.

Post-transcriptional modification of RNA, the so-called 'Epitranscriptome', can regulate RNA structure, stability, localization, and function. Numerous modifications have been identified in virtually all classes of RNAs, including messenger RNAs (mRNAs), transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), microRNAs (miRNAs), and other noncoding RNAs (ncRNAs). These modifications may occur internally (by base or sugar modifications) and include RNA methylation at different nucleotide positions, or by the addition of various nucleotides at the 3'-end of certain transcripts by a family of terminal nucleotidylyl transferases. Developing methods to specifically and accurately detect and map these modifications is essential for understanding the molecular function(s) of individual RNA modifications and also for identifying and characterizing the proteins that may read, write, or erase them. Here, we focus on the characterization of RNA species targeted by 3' terminal uridylyl transferases (TUTases) (TUT4/7, also known as Zcchc11/6) and a 3'-5' exoribonuclease, Dis3l2, in the recently identified Dis3l2-mediated decay (DMD) pathway - a dedicated quality control pathway for a subset of ncRNAs. We describe the detailed methods used to precisely identify 3'-end modifications at nucleotide level resolution with a particular focus on the U1 and U2 small nuclear RNA (snRNA) components of the Spliceosome. These tools can be applied to investigate any RNA of interest and should facilitate studies aimed at elucidating the functional relevance of 3'-end modifications.