The TFAM-to-mtDNA ratio defines inner-cellular nucleoid populations with distinct activity levels

In human cells, generally a single mitochondrial DNA (mtDNA) is compacted into a nucleoprotein complex denoted the nucleoid. Each cell contains hundreds of nucleoids, which tend to cluster into small groups. It is unknown whether all nucleoids are equally involved in mtDNA replication and transcription or whether distinct nucleoid subpopulations exist. Here, we use multi-color STED super-resolution microscopy to determine the activity of individual nucleoids in primary human cells. We demonstrate that only a minority of all nucleoids are active. Active nucleoids are physically larger and tend to be involved in both replication and transcription. Inactivity correlates with a high ratio of the mitochondrial transcription factor A (TFAM) to the mtDNA of the individual nucleoid, suggesting that TFAM-induced nucleoid compaction regulates nucleoid replication and transcription activity in vivo. We propose that the stable population of highly compacted inactive nucleoids represents a storage pool of mtDNAs with a lower mutational load.

Automated summary

Research has shown that only a minority of nucleoids in human cells are active, with these active nucleoids being larger and involved in both replication and transcription. Inactive nucleoids are characterized by a high TFAM to mtDNA ratio, suggesting a regulation of nucleoid activity. This inactive population may serve as a storage pool for mtDNAs with lower mutational load.