Structural snapshots of transcription initiation process by human and yeast mitochondrial RNA polymerases.

Transcription of genes are highly regulated process. The eukaryotic and bacterial gene transcription are accomplished by multi-subunit RNA polymerases (RNAPs). The mitochondrial genes are transcribed by single-subunit RNAPs. Regardless of differences in details, all RNAPs catalyze transcription initiation by a common sequence of events consisting of (i) promoter-specific recognition, (ii) formation of a transcription bubble, (iii) DNA scrunching to make a specific length of small RNA products, and finally (iv) promoter release and transition from initiation to elongation phase. Understanding of mitochondrial DNA transcription initiation lags that of bacterial and nuclear DNA transcription. Mitochondrial DNA is transcribed by single-subunit mitochondrial RNAPs (mtRNAPs), which unlike their phage counterparts, such as T7 RNA polymerase, depend on one or more transcription factors for promoter-specific transcription initiation. The human mtRNAP transcription initiation requires assembly of the catalytic subunit h-mtRNAP (also referred as POLRMT) and two transcription factors, TFAM and TFB2M. Whereas, the yeast mtRNAP transcription initiation requires y-mtRNAP (also known as RPO41) and the transcription factor MTF1.