Endoribonuclease; cleaves preferentially 3' to purine-pyrimidine dinucleotide motifs in single-stranded RNA. The cleavage product contains a free 3' -OH group. Has no activity with double-stranded RNA or DNA. Required for normal mitochondrial function and cell viability. Binds 2 Zn(2+) ions per subunit. Belongs to the metallo-beta-lactamase superfamily. Glyoxalase II family.