3'-5' RNA exonuclease that trims the 3' end of oligo(U) and oligo(A) tracts of the pre-U6 small nuclear RNA (snRNA) molecule, leading to the formation of a mature U6 snRNA 3' end-terminated with a 2',3'-cyclic phosphate. Participates in the U6 snRNA 3' end processing that prevents U6 snRNA degradation. In addition also removes uridines from the 3' end of U6atac snRNA and possibly the vault RNA VTRNA1-1. a 3'-end uridylyl-uridine-RNA = a 3'-end 2',3'-cyclophospho-uridine-RNA + uridine a 3'-end uridylyl-adenosine-RNA = a 3'-end 2',3'-cyclophospho-uridine-RNA + adenosine Belongs to the 2H phosphoesterase superfamily. USB1 family.