Competition between 3’ end maturation and degradation drives snRNA quality control

Polymerases and exonucleases act on 3’ ends of nascent RNAs to promote their maturation or degradation but how the balance between these activities is controlled to dictate the fates of cellular RNAs remains poorly understood. Here, we identify a central role for the human DEDD deadenylase TOE1 in distingushing the fates of small nuclear (sn)RNAs of the spliceosome from genome-encoded snRNA variants. We find that TOE1 promotes maturation of all regular RNA polymerase II-transcribed snRNAs of the major and minor spliceosomes by removing post-transcriptional oligo(A)-tails, trimming 3’ ends, and preventing nuclear exosome targeting. By contrast, TOE1 promotes little to no maturation of tested U1 variant snRNAs, which are instead targeted by the nuclear exosome. These observations suggest that TOE1 is positioned at the center of a 3’ end quality control pathway that selectively promotes maturation and stability of regular snRNAs while leaving snRNA variants unprocessed and exposed to degradation in what could be a widespread mechanism of RNA quality control given the large number of non-coding RNAs processed by DEDD deadenylases. We performed experiments to profile 3' ends of U1, U2, U4, U5, U4atac, U11, U12, U3, and U8 RNAs. Samples profiled include immunopurified RNA, purified nascent RNA, and total RNA from HEK293 cells which were subject to various treaments including depletion of TOE1-degron and siRNA depletion of components of the nuclear exosome.