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.