Required for intron-independent turnover and processing of mitochondrial RNA. Participates in 3'-mtRNA processing where it hydrolyzes single-stranded RNA or partially double-stranded RNA with 3'-single-stranded tails. Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Up-regulated in cells metabolizing nonfermentable carbon sources. Belongs to the RNR ribonuclease family.