Preference of human mitochondrial RNA polymerase for superhelical templates with mitochondrial promoters.

The RNA polymerase of HeLa cell mitochondria has been purified free of endonuclease and DNA topoisomerase activities, permitting evaluation of the effect of template topology on transcription in vitro. On single-stranded DNA templates, transcription is nonspecific and does not require mitochondrial DNA sequences. In contrast, duplex DNA templates are efficiently transcribed only when they (1) carry the mitochondrial D-loop region and (2) are negatively supercoiled. These findings suggest a role for template superhelicity in modulating mitochondrial transcription in vivo.