ATP-dependent serine protease that mediates the selective degradation of misfolded, unassembled or oxidatively damaged polypeptides as well as certain short-lived regulatory proteins in the mitochondrial matrix. May also have a chaperone function in the assembly of inner membrane protein complexes. Participates in the regulation of mitochondrial gene expression and in the maintenance of the integrity of the mitochondrial genome. Binds to mitochondrial promoters and RNA in a single-stranded, site-specific, and strand-specific manner. May regulate mitochondrial DNA replication and/or gene expression using site-specific, single-stranded DNA binding to target the degradation of regulatory proteins binding to adjacent sites in mitochondrial promoters (By similarity). Endogenous substrates include oxidized aconitase. Hydrolysis of proteins in presence of ATP. Homohexamer. Organized in a ring with a central cavity. The ATP-binding and proteolytic domains (AP-domain) form a hexameric chamber, while the N-terminal domain is arranged as a trimer of dimers. DNA and RNA binding is stimulated by substrate and inhibited by ATP binding. Interacts with TWNK and mitochondrial DNA polymerase subunit POLG. Belongs to the peptidase S16 family.