A common pattern of DNase I footprinting throughout the human mtDNA unveils clues for a chromatin-like organization

Human mitochondrial DNA (mtDNA) is believed to lack chromatin and histones. Instead, it is coated solely by the transcription factor TFAM, which binds the mtDNA without sequence specificity and packs it into a bacterial-like nucleoid in a dose dependent fashion. We asked whether mtDNA packaging is more regulated than once thought. As a first step to address this question, we analyzed mtDNA DNase-I-seq experiments in 324 different human cell types and found, for the first time, a pattern of 29 Genomic footprinting (DGF) sites throughout the mtDNA shared by ~90% of the tested samples. Low SNP density at the DGF sites, and their conservation in mouse DNase-seq experiments, reflect strong selective constraints. Co-localization of the DGFs with known mtDNA regulatory elements and with recently-discovered transcription pausing sites, suggest a role for such DGFs in mtDNA transcription. Altered mtDNA DGF pattern in IL-3 treated CD+34 cells offer first clue to their physiological importance. Taken together, human mtDNA has a conserved and regulated protein-DNA organization, which is likely involved in regulation of mtDNA gene expression.