Essential for mitochondrial biogenesis. 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. MSU1 and SUV3 are the two components of the mitochondrial degradosome (mtEXO). Present with 1070 molecules/cell in log phase SD medium. Belongs to the RNR ribonuclease family.