A role of Holliday structure in mitochondrial genome maintenance

在复制过程中，人类线粒体DNA（mtDNA）呈现出一种被称为D环的三链结构，它与复制和转录有关。1-甲基-4-苯基吡啶鎓离子（MPP ^+）是一种能诱导帕金森病的毒素，它可能通过分解D环来抑制线粒体DNA的复制。为了启动mtDNA复制，线粒体被认为具有另一种三链结构，即R环。由细菌连接特异性解旋酶RecG分解的R环也可由MPP ^+分解。由于线粒体D环也被RecG解析，因此D环和R环可能具有相似的分支结构。MPP ^+解析超螺旋DNA中的十字形DNA。MPP ^+在合成的霍利迪连接中将堆叠构象转化为延伸构象。1 mM Mg ^<2 +>可逆转这种转化，D环或十字形DNA的分解也是如此。这些观察结果表明，线粒体D环和R环的连接结构受到MPP ^+的影响。线粒体转录因子A（TFAM）是高迁移率族蛋白的一员，对线粒体DNA（mtDNA）的维持至关重要。大多数TFAM和线粒体DNA（两者通常都是可溶的），从人胎盘线粒体的颗粒部分时，用非离子去污剂Nonidet P-40提取回收。用抗TFAM抗体共沉淀mtDNA和TFAM。因此，TFAM和mtDNA彼此紧密相关，并且很可能很少有TFAM或mtDNA分子以未结合的形式存在于线粒体中。基于TFAM的丰度足以完全包裹mtDNA这一事实，这些结果表明人mtDNA被TFAM包裹。

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.