mtDNA makes a U-turn for the mitochondrial nucleoid

•We introduce how the maintenance and expression of mtDNA control oxidative phosphorylation.•We discuss the role of TFAM in transcription and replication of mtDNA.•TFAM is the main factor for packaging mitochondrial nucleoids.•We review the visualization of mitochondrial nucleoids also in the light of super-resolution microscopy.

Mitochondria contain mtDNA derived from the ancestral endosymbiont genome. Important subunits of the oxidative phosphorylation system, which supplies cells with the energy currency ATP, are encoded by mtDNA. A naked mtDNA molecule is longer than a typical mitochondrion and is therefore compacted in vivo to form a nucleoprotein complex, denoted the mitochondrial nucleoid. Mitochondrial transcription factor A (TFAM) is the main factor packaging mtDNA into nucleoids and is also essential for mtDNA transcription initiation. The crystal structure of TFAM shows that it bends mtDNA in a sharp U-turn, which likely provides the structural basis for its dual functions. Super-resolution imaging studies have revealed that the nucleoid has an average diameter of ∼100 nm and frequently contains a single copy of mtDNA. In this review the structure of the mitochondrial nucleoid and its possible regulatory roles in mtDNA expression will be discussed.