Cross-strand binding of TFAM to a single mtDNA molecule forms the mitochondrial nucleoid

Abstract

Significance Altered expression of mitochondrial DNA (mtDNA) is heavily implicated in human disease and aging, but the basic organizational unit of mtDNA, the mitochondrial nucleoid, is poorly understood. Here, we have used a combination of biochemistry, superresolution microscopy, and electron microscopy to show that mammalian mitochondrial nucleoids have an irregular ellipsoidal shape and typically contain a single copy of mtDNA. Furthermore, we show that the nucleoid ultrastructure is independent of cellular mtDNA copy number and that the core nucleoid structure is formed by cross-strand binding of mitochondrial transcription factor A (TFAM) to a single copy of mtDNA. The clarification of the ultrastructure of the mammalian mitochondrial nucleoid provides the fundamental basis for the understanding of regulation of mtDNA maintenance and expression in mammals.