Endoribonuclease that catalyzes the hydrolysis of histone-coding pre-mRNA 3'-end. Involved in histone pre-mRNA processing during the S-phase of the cell cycle, which is required for entering/progressing through S-phase. Cleaves histone pre-mRNA at a major and a minor cleavage site after the 5'-ACCCA-3' and the 5'-ACCCACA-3' sequence, respectively, and located downstream of the stem-loop. May require the presence of the HDE element located at the histone pre-RNA 3'-end to avoid non-specific cleavage. a ribonucleotidyl-ribonucleotide-RNA + H2O = a 3'-end ribonucleotide-RNA + a 5'-end (5'-phospho)-ribonucleoside-RNA + H(+) Binds 2 Zn(2+) ions per subunit. Homodimer. Localizes in the nucleus during early S-phase of the cell cycle. Contains four of the five characteristic MBL-fold metal-binding motifs, with two waters completing metal coordination. Belongs to the metallo-beta-lactamase superfamily. Glyoxalase II family.