Accessory subunit of DNA polymerase gamma solely responsible for replication of mitochondrial DNA (mtDNA). Acts as an allosteric regulator of the holoenzyme activities. Enhances the polymerase activity and the processivity of POLG by increasing its interactions with the DNA template. Suppresses POLG exonucleolytic proofreading especially toward homopolymeric templates bearing mismatched termini. Binds to single-stranded DNA. {ECO:0000269|PubMed:11477093, ECO:0000269|PubMed:11477094, ECO:0000269|PubMed:11504725, ECO:0000269|PubMed:15167897, ECO:0000269|PubMed:19837034, ECO:0000269|PubMed:26056153, ECO:0000269|PubMed:30157269, ECO:0000269|PubMed:31778857, ECO:0000269|PubMed:37202477}. This is the function of POLG2 (DNA polymerase gamma 2, accessory subunit, Ensembl gene identifier ENSG00000256525).