RecA

Miller-Messmer M, Kühn K, Bichara M, Le Ret M, Imbault P & Gualberto JM 2012 RecA-dependent DNA repair results in increased heteroplasmy of the Arabidopsis mitochondrial genome. Plant Physiol 159:211-226.

• replication repair of double-strand breaks results in the accumulation of crossovers that increase the heteroplasmic state of the mitochondrial DNA

• only a few factors that influence mtDNA HR and that can affect the SSS process have been identified so far
• the primary effect of the inactivation of these genes is the accumulation of some products arising from recombination between IRs
• SSS involving the amplification of IR recombination products could result from the repair of double-strand breaks (DSBs) by break-induced replication (BIR)
• most examples of SSS involving recombination between IRs are characterized by an asymmetrical amplification of only one of the recombinant forms, which can be explained by a BIR repair pathway
• the involvement of BIR is consistent with models of the plant mtDNA replication by recombination-dependent pathways

• three RECA genes are present in Arabidopsis
• RECA1 and RECA3 are targeted to plastids and mitochondria, respectively
• RECA2 is dual targeted to both organelles
• mutants in either RECA2 or RECA3 displayed the same molecular phenotypes of increased recombination between IRs, suggesting that they act in the same recombination pathways

• we observed an increase mainly in one of the crossover products of repeats I and L (sequences I-2/1 and L-2/1, respectively) in recA3-1 and recA3-2 compared with the wild type
• RECA3 absence triggers asymmetrical recombination at these IRs
• in recA2-1, -3, and -4 plants, HR activity was also increased, as indicated by the high accumulation of these mtDNA recombination products, which in recA2-1, -3, and -4 were about 1 order of magnitude more abundant than in recA3 plants
• the relative increase in crossover products compared with the wild type reached more than 1,000-fold for I-2/1 and X-2/1

• loss of either of the two mitochondrial RECAs results in an increase of the recombination activity that modifies the structure of the mtDNA

• efficient repair of breaks in the mtDNA depends on RECA3, which can explain the reduced fitness of recA3 plants under genotoxic growth conditions

• wild-type plants grown in the presence of MMC or CIP exhibited an important accumulation of crossover products
• recA3 seedlings exposed to genotoxic treatments showed no significant increase in the accumulation of recombination products when compared with untreated plants
• RECA3 is required for HR-dependent mtDNA repair pathways activated upon mtDNA damage and proceeding via IR-mediated recombination
• crossovers, which arise from the activity of these repair pathways, led to a concomitant increase in mtDNA heteroplasmy

• the products of IR-mediated recombination that accumulate in wild-type plants after the repair of DNA breaks induced by MMC or CIP are not the same as those accumulating as a consequence of the inactivation of MSH1, OSB1, or RECA2 and RECA3 in the recombination mutants
• the mechanisms of recombination mobilized for repair may not be entirely the same as the recombination events prevailing in recombination mutants

• the heteroplasmy of plant mtDNA is an important reservoir of genetic variability that is available when required for the rapid evolution of the mtDNA
• in the osb1 and msh1 mutants, it has been shown that the increased heteroplasmy observed in first-generation mutants can lead, in later mutant generations, to new major configurations of the mtDNA by SSS
• RecA-dependent repair has a dual effect on the mtDNA:
• maintaining the integrity of the organellar genome that is important for plant fitness after stress
• favoring the amplification of genome configurations that could be advantageous in the adaptation of plants to environmental changes