It is now recognized that mammalian mitochondrial DNA forms a higher structure called the nucleoid, corresponding to the nucleosome of nuclear DNA. Mitochondrial transcription factor A (TFAM), which was cloned as a transcription factor for mitochondrial DNA, is essential for the maintenance of mitochondrial DNA. In fact, TFAM markedly enhances the promoter-specific transcription of mitochondrial DNA. In addition, TFAM has an ability to bind to DNA in a sequence-independent manner and is abundant enough to cover an entire region of mitochondrial DNA. Over-expression of human TFAM in cells increases the amount of mitochondrial DNA almost in parallel with the TFAM. TFAM may stabilize mitochondrial DNA by packaging and regulate (or titrate) the amount of mitochondrial DNA. Thus, TFAM may play a crucial role in maintaining mitochondrial DNA as a main component of the nucleoid (or more appropriately mitochromosome).
