TUT1-catalyzed U6 snRNA 3'-end maturation is essential for RNA splicing and stem cell survival.

Post-transcriptional maturation of the U6 snRNA 3'-end, important for spliceosome assembly, is catalyzed by sequential actions of TUT1 and USB1. It is believed that the TUT1-catalyzed oligo(U) tail at the U6 snRNA 3'-end serves merely as a substrate for USB1 to generate a final 2',3'-cyclic phosphate group to mature the U6 snRNA. However, biallelic inactivation of TUT1 or USB1 is linked to distinct human developmental disorders, suggesting that they have different physiological functions. Here, using genetically engineered mouse models, we show that Tut1 is required to maintain stem cell pools during embryogenesis, whereas unexpectedly Usb1 is dispensable for this. Loss of Tut1 weakens the interaction of the U6 snRNA with the Lsm2-8 protein complex, causes defective RNA splicing, and triggers massive DNA damage and subsequent cell death. Splicing defects and cell death can be mitigated by recombinant U6 snRNA containing an oligo(U) tail. We propose that the TUT1-catalyzed oligo(U) tail is essential for splicing and cell proliferation. Further modification of this oligo(U) tail by USB1 is ubiquitous but only functionally required in specific cell types.