Conserved Sequence Box II Directs Transcription Termination and Primer Formation in Mitochondria

Initiation and beyond: multiple functions of the human mitochondrial transcription machinery

Mitochondria contain their own DNA (mtDNA) that is expressed and replicated by nucleus-encoded factors imported into the organelle. Recently, the core human mitochondrial transcription machinery has been defined, comprising a bacteriophage-related mtRNA polymerase (POLRMT), an HMG-box transcription factor (h-mtTFA), and two transcription factors (h-mtTFB1 and h-mtTFB2) that also serve as rRNA methyltransferases. Here, we describe these transcription components as well as recent insights into the mechanism of human mitochondrial transcription initiation and its regulation. We also discuss novel roles for the mitochondrial transcription machinery beyond transcription initiation, including priming of mtDNA replication, packaging of mtDNA, coordination of ribosome biogenesis, and coupling of transcription to translation.

The C-terminal Tail of Mitochondrial Transcription Factor A Markedly Strengthens its General Binding to DNA

Mitochondrial transcription factor A (TFAM) contains a basic C-terminal tail which is essential for the promoter-specific transcription. TFAM is also a major component of a protein-mitochondrial DNA (mtDNA) complex, called nucleoid, as a non-specific DNA-binding protein. However, little is known about a role of the C-tail in the nucleoid. Overexpression of full-length TFAM decreased the amount of a D-loop form of mtDNA in cells, while overexpression of TFAM lacking its C-tail (TFAM-DeltaC) did not, suggesting that the C-tail is involved in destabilization or formation of the D-loop. An mRNA for mtDNA-derived ND1 was hardly decreased in the former but rather decreased in the latter. Given that the D-loop formation is coupled with the transcription, the decrease in the D-loop is likely due to its destabilization. The recombinant full-length TFAM much strongly unwound DNA than TFAM-DeltaC, which is consistent with the above idea because D-loop is resolved by unwinding of the supercoiling state. Notably, truncation of the C-tail decreased DNA-binding activity of TFAM by three orders of magnitude. Thus, the C-terminal tail of TFAM is important for the strong general binding to mtDNA. This strong DNA-binding conferred by the C-tail may play an important role in the nucleoid structure.