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. Monomer. RNAi-mediated down-regulation results in rapid cell death. The reason for this is not clear; there are only minor changes in mitochondrial mRNA levels. Belongs to the metallo-beta-lactamase superfamily. Glyoxalase II family.