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 (PubMed:28887445). In a second step acts as a cyclic phosphodiesterase that hydrolyzes a U6 snRNA 3' end-terminated with a 2',3'-cyclic phosphate to a U6 snRNA 3'-end-terminated with a 3' phosphate (PubMed:28887445, PubMed:22899009). Participates in the U6 snRNA 3' end processing that prevents U6 snRNA degradation (PubMed:28887445, PubMed:23190533, PubMed:22899009). Could be involved in aerobic respiration (PubMed:14690591). a 3'-end uridylyl-uridine-RNA = a 3'-end 2',3'-cyclophospho-uridine-RNA + uridine Optimum pH is 6.5. Monomer. Lethal. Present with 486 molecules/cell in log phase SD medium. Belongs to the 2H phosphoesterase superfamily. USB1 family.