3'-5' RNA exonuclease that trims the 3' end of oligo(U) tracts of the pre-U6 small nuclear RNA (snRNA) molecule, leading to the formation of a U6 snRNA 3' end-terminated with a 2',3'-cyclic phosphate.d (PubMed:26213367). Participates in the U6 snRNA 3' end processing that prevents U6 snRNA degradation (PubMed:26213367, PubMed:23022480). a 3'-end uridylyl-uridine-RNA = a 3'-end 2',3'-cyclophospho-uridine-RNA + uridine Deletion of mpn1 decreases cell population growth at low temperature and leads to a generalized pre-mRNA splicing defect, decrease of mature U6 snRNA levels, abnormal U6 snRNA 3'-end processing, decrease levels of U4/U6 di-snRNPs. Belongs to the 2H phosphoesterase superfamily. USB1 family.