Nuclear DNA is tightly packed into nucleosomal structure. In contrast, human 
mitochondrial DNA (mtDNA) had long been believed to be rather naked because 
mitochondria lack histone. Mitochondrial transcription factor A (TFAM), a member 
of a high mobility group (HMG) protein family and a first-identified 
mitochondrial transcription factor, is essential for maintenance of 
mitochondrial DNA. Abf2, a yeast counterpart of human TFAM, is abundant enough 
to cover the whole region of mtDNA and to play a histone-like role in 
mitochondria. Human TFAM is indeed as abundant as Abf2, suggesting that TFAM 
also has a histone-like architectural role for maintenance of mtDNA. When human 
mitochondria are solubilized with non-ionic detergent Nonidet-P40 and then 
separated into soluble and particulate fractions, most TFAM is recovered from 
the particulate fraction together with mtDNA, suggesting that human mtDNA forms 
a nucleoid structure. TFAM is tightly associated with mtDNA as a main component 
of the nucleoid.