N6-Methyldeoxyadenosine (6mA) has recently been shown to exist and play regulatory roles in eukaryotic genomic DNA (gDNA). However, the biological functions of 6mA in mammals have yet to be adequately explored, largely due to its low abundance in most mammalian genomes. Here, we report that mammalian mitochondrial DNA (mtDNA) is enriched for 6mA. The level of 6mA in HepG2 mtDNA is at least 1,300-fold higher than that in gDNA under normal growth conditions, corresponding to approximately four 6mA modifications on each mtDNA molecule. METTL4, a putative mammalian methyltransferase, can mediate mtDNA 6mA methylation, which contributes to attenuated mtDNA transcription and a reduced mtDNA copy number. Mechanistically, the presence of 6mA could repress DNA binding and bending by mitochondrial transcription factor (TFAM). Under hypoxia, the 6mA level in mtDNA could be further elevated, suggesting regulatory roles for 6mA in mitochondrial stress response. Our study reveals DNA 6mA as a regulatory mark in mammalian mtDNA.

中文翻译：

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哺乳动物线粒体DNA中的N6-脱氧腺苷甲基化。

N6-甲基脱氧腺苷（6mA）最近被证实存在并在真核基因组DNA（gDNA）中起调节作用。然而，6mA在哺乳动物中的生物学功能尚未得到充分的探索，这主要是由于大多数哺乳动物基因组中6mA的丰度较低。在这里，我们报告哺乳动物线粒体DNA（mtDNA）富集了6mA。在正常生长条件下，HepG2 mtDNA中6mA的水平至少比gDNA中高1,300倍，相当于每个mtDNA分子上约有4个6mA修饰。METTL4是一种推定的哺乳动物甲基转移酶，可以介导mtDNA 6mA甲基化，从而导致mtDNA转录减弱和mtDNA拷贝数减少。从机制上讲，6mA的存在可以抑制DNA的结合和线粒体转录因子（TFAM）的弯曲。在缺氧的情况下 mtDNA中的6mA水平可能会进一步升高，表明6mA在线粒体应激反应中的调节作用。我们的研究揭示了DNA 6mA作为哺乳动物mtDNA的调控标记。