Plant organellar DNA polymerases paralogs exhibit dissimilar nucleotide incorporation fidelity

The coding sequences of plant mitochondrial and chloroplast genomes present a lower mutation rate than the coding sequences of animal mitochondria. However, plant mitochondrial genomes frequently rearrange and present high mutation rates in their noncoding sequences. DNA replication in plant organelles is carried out by two DNA polymerases ( DNAP ) paralogs. In Arabidopsis thaliana at least one DNAP paralog (AtPol IA or AtPol IB ) is necessary for plant viability, suggesting that both genes are partially redundant. To understand how AtPolIs replicate genomes that present low and high mutation rates, we measured their nucleotide incorporation for all 16‐base pair combinations in vitro . AtPol IA presents an error rate of 7.26 × 10 −5 , whereas AtPol IB has an error rate of 5.45 × 10 −4 . Thus, AtPol IA and AtPol IB are 3.5 and 26‐times less accurate than human mitochondrial DNAP γ. The 8‐fold difference in fidelity between both AtPolIs results from a higher catalytic efficiency in AtPol IA . Both AtPolIs extend from mismatches and the fidelity of AtPolIs ranks between high fidelity and lesion bypass DNAP s. The different nucleotide incorporation fidelity between AtPolIs predicts a prevalent role of AtPol IA in DNA replication and AtPol IB in DNA repair. We hypothesize that in plant organelles, DNA mismatches generated during DNA replication are repaired via recombination‐mediated or DNA mismatch repair mechanisms that selectively target the coding region and that the mismatches generated by AtPolIs may result in the frequent expansion and rearrangements present in plant mitochondrial genomes.