Poly(U)-specific endoribonuclease involved in the processing of intron-encoded box C/D snoRNAs, such as U16 and U86. Releases products that have 2',3'-cyclic phosphate termini at the 3'-end. uridylyl-uridylyl-ribonucleotide-RNA = a 3'-end uridylyl-2',3'-cyclophospho-uridine-RNA + a 5'-end dephospho-ribonucleoside-RNA In vitro, optimal manganese concentration is in the range of 5-10 mM. Optimum pH is 7.5. Monomer. Belongs to the ENDOU family.