A role of Holliday structure in mitochondrial genome maintenance

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During replication, human mitochondrial DNA (mtDNA) takes on a triple-stranded structure known as a D-loop, which is implicated in replication and transcription. 1-Methyl-4-phenylpyridinium ion (MPP^+), a toxin inducing Parkinsonism, inhibits mtDNA replication possibly by resolving the D-loops. For initiation of mtDNA replication, mitochondria are thought to have another triple-stranded structure, an R-loop. The R-loop, which is resolved by a bacterial junction-specific helicase RecG, is also resolved by MPP^+. Because mitochondrial D-loops are resolved by RecG as well, the D- and R-loops may share a similar branched structure. MPP^+ resolves cruciform DNA in supercoiled DNA. MPP^+ converts a stacked conformation to an extended conformation in a synthetic Holliday junction. This conversion is reversed by 1 mM Mg^<2+>, as is the resolution of the D-loops or cruciform DNA. These observations suggest that the junction structure of mitochondrial D- and R-loops is affected by MPP^+. Mitochondrial transcription factor A (TFAM), a member of the high mobility group proteins, is essential for maintenance of mitochondrial DNA (mtDNA). Most TFAM and mtDNA (both of which are normally soluble) was recovered from the particulate fraction of human placental mitochondria when extracted with the non-ionic detergent Nonidet P-40. mtDNA and TFAM were co-immunoprecipitated by anti-TFAM antibodies. Thus, TFAM and mtDNA are tightly associated with each other, and it is likely that few TFAM or mtDNA molecules exist in an unbound form in mitochondria. Based on a fact that TFAM is abundant enough to wrap mtDNA entirely, these results suggest that human mtDNA is packaged with TFAM.