Identification of a unique insertion in plant organellar DNA polymerases responsible for 5'-dRP lyase and strand-displacement activities: Implications for Base Excision Repair.

Plant mitochondrial and chloroplast genomes encode essential proteins for oxidative phosphorylation and photosynthesis. For proper cellular function, plant organelles must ensure genome integrity. Although plant organelles repair damaged DNA using the multi-enzyme Base Excision Repair (BER) pathway, the details of this pathway in plant organelles are largely unknown. The initial enzymatic steps in BER produce a 5'-deoxyribose phosphate (5'-dRP) moiety that must be removed to allow DNA ligation and in plant organelles, the enzymes responsible for the removal of a 5'-dRP group are unknown. In metazoans, DNA polymerases (DNAPs) remove the 5'-dRP moiety using their intrinsic lyase and/or strand-displacement activities during short or long-patch BER sub-pathways, respectively. The plant model Arabidopsis thaliana encodes two family-A DNAPs paralogs, AtPolIA and AtPolIB, which are the sole DNAPs in plant organelles identified to date. Herein we demonstrate that both AtPolIs present 5'-dRP lyase activities. AtPolIB performs efficient strand-displacement on a BER-associated 1-nt gap DNA substrate, whereas AtPolIA exhibits only moderate strand-displacement activity. Both lyase and strand-displacement activities are dependent on an amino acid insertion that is exclusively present in plant organellar DNAPs. Within this insertion, we identified that residue AtPollB-Lys593 acts as nucleophile for lyase activity. Our results demonstrate that AtPolIs are functionally equipped to play a role in short-patch BER and suggest a major role of AtPolIB in a predicted long-patch BER sub-pathway. We propose that the acquisition of insertion 1 in the polymerization domain of AtPolIs was a key component in their evolution as BER associated and replicative DNAPs.