Biochemical studies with DNA polymerase β and DNA polymerase β-PAK of Trypanosoma cruzi suggest the involvement of these proteins in mitochondrial DNA maintenance

Mammalian DNA polymerase β is a nuclear enzyme involved in the base excision and single-stranded DNA break repair pathways. In trypanosomatids, this protein does not have a defined cellular localization, and its function is poorly understood. We characterized two Trypanosoma cruzi proteins homologous to mammalian DNA polymeraseβ, TcPolβ and TcPolβPAK, and showed that both enzymes localize to the parasite kinetoplast. In vitro assays with purified proteins showed that they have DNA polymerization and deoxyribose phosphate lyase activities. Optimal conditions for polymerization were different for each protein with respect to dNTP concentration and temperature, and TcPolβPAK, in comparison to TcPolβ, conducted DNA synthesis over a much broader pH range. TcPolβ was unable to carry out mismatch extension or DNA synthesis across 8-oxodG lesions, and was able to discriminate between dNTP and ddNTP. These specific abilities of TcPolβ were not observed for TcPolβPAK or other X family members, and are not due to a phenylalanine residue at position 395 in the C-terminal region of TcPolβ, as assessed by a site-directed mutagenesis experiment reversing this residue to a well conserved tyrosine. Our data suggest that both polymerases from T. cruzi could cooperate to maintain mitochondrial DNA integrity through their multiple roles in base excision repair, gap filling and translesion synthesis.