Prevalent N6-methyldeoxyadenosine methylation in mammalian mitochondrial DNA

N6-methyldeoxyadenosine (6mA or m6dA), a well-known prokaryotic DNA modification has recently been shown to exist and play regulatory roles in eukaryotic genomic DNA. However, biological functions of 6mA in mammals remain to be explored largely due to its low abundance in most mammalian genomes. Here, we report a significant enrichment of 6mA in mammalian mitochondrial DNA (mtDNA), and identify METTL4 as a methyltransferase that installs 6mA. The level of 6mA in mtDNA is elevated up to over 0.02% of the total deoxyadenosines under hypoxia, representing ~7,00-fold enrichment over 6mA in the genomic DNA (gDNA) under normal growth conditions. The presence of 6mA negatively regulates mitochondrial transcription and balances mitochondrial function for cellular adaption to hypoxic stress. While DNA 5-methylcytosine (5mC) is a well-established, prevalent epigenetic mark in mammalian genomic DNA, our study reveals for the first time DNA 6mA methylation as a regulatory mark in mammalian mtDNA. Overall design: We performed CLIP-exo sequencing for 6mA under normoxia and hypoxia conditions, respectively. Furthermore, the transcriptome-wide changes between wild-type cells and cells with METTL4 knockdown were characterized using RNA-Seq.