Plant mitochondria possess a short-patch base excision DNA repair pathway

Despite constant threat of oxidative damage, sequence drift in mitochondrial and chloroplast DNA usually remains very low in plant species, indicating efficient defense and repair. Whereas the antioxidative defense in the different subcellular compartments is known, the information on DNA repair in plant organelles is still scarce. Focusing on the occurrence of uracil in the DNA, the present work demonstrates that plant mitochondria possess a base excision repair (BER) pathway. In vitro and in organello incision assays of double-stranded oligodeoxyribonucleotides showed that mitochondria isolated from plant cells contain DNA glycosylase activity specific for uracil cleavage. A major proportion of the uracil-DNA glycosylase (UDG) was associated with the membranes, in agreement with the current hypothesis that the DNA is replicated, proofread and repaired in inner membrane-bound nucleoids. Full repair, from uracil excision to thymidine insertion and religation, was obtained in organello following import of a uracil-containing DNA fragment into isolated plant mitochondria. Repair occurred through single nucleotide insertion, which points to short-patch BER. In vivo targeting and in vitro import of GFP fusions showed that the putative UDG encoded by the At3g18 630 locus might be the first enzyme of this mitochondrial pathway in Arabidopsis thaliana.