3'-5'exoribonuclease which is involved in the post-transcriptional processing, editing and degradation of mitochondrial RNAs, including mRNAs, rRNAs and guided RNAs (gRNA) (PubMed:15470249, PubMed:26833087). As part of the mitochondrial 3' processome (MPsome), involved in the maturation of guided RNA (gRNA) precursors by catalyzing the processive 3'-5' degradation of uridylated gRNA precursors (PubMed:26833087). Plays a role in the degradation of 12S rRNA processing intermediates and maturation by-products (PubMed:18032430). Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Component of the mitochondrial 3' processome (MPsome) complex composed at least of terminal uridylyltransferase KRET1/TUT1, 3'-5' exonuclease DSS1, MPSS1, MPSS2 and MPSS3 (PubMed:26833087). Within the complex, interacts with KRET1 and MPSS2 (PubMed:26833087). Component of the mitochondrial degradosome complex composed at least of 3'-5' exonuclease DSS1 and helicase SUV3 (PubMed:19540236). Within the complex, interacts with helicase SUV3 (PubMed:19540236). Expressed at the procyclic stage (at protein level). RNAi-mediated knockdown at the procyclic stage causes moderate growth defect and reduces the levels of several mitochondrial RNAs, including never edited, unedited, and edited mRNAs as well as guided RNAs (gRNA) (PubMed:15470249, PubMed:26833087). Also results in the accumulation of fragments of the flanking region upstream of the mature 5'end of 12S rRNA, the 5' flanking sequence and the truncated 12S rRNA sequence, and the 5' full-length 12S rRNAs (PubMed:18032430). Belongs to the RNR ribonuclease family.