Single-nucleoid architecture reveals heterogeneous packaging of mitochondrial DNA [RNA-seq]
Cellular metabolism relies on the regulation and maintenance of mitochondrial DNA (mtDNA). Hundreds to thousands of copies of mtDNA exist in each cell, yet how mtDNA is packaged into individual mitochondrial nucleoids remains poorly resolved, as mitochondria lack histones or other chromatin machinery important for nuclear genome compaction. Here, we used long-read single-molecule accessibility mapping to resolve the compaction of individual full-length mitochondrial nucleoids at single-nucleotide resolution. We find that, unlike the nuclear genome, human mtDNA largely undergoes an all-or-none global compaction, with the majority of nucleoids existing in an inaccessible, inactive state. Highly accessible mitochondrial nucleoids are occupied by transcription and replication machinery, and selectively form a D-loop structure. In addition, we demonstrate that the primary nucleoid-associated protein TFAM directly modulates the fraction of inaccessible nucleoids both in vivo and in vitro, and acts via a 'nucleation and spreading' mechanism to coat and compact mitochondrial nucleoids. Overall, these findings expose the primary architecture of mtDNA packaging and regulation in humans. Overall design: RNA-seq of human skeletal muscle myoblasts at three time points: 0d, 3d, and 6d in differentiation media.