Topological requirements of the mitochondrial heavy-strand promoters

In vitro studies of mitochondrial transcription often use linear templates that fail to replicate key features of transcription on a circular genome. We developed a plasmid-based system for the analysis of heavy-strand promoters that recapitulates key features of native mtDNA to study topological and protein requirements of promoter activation. The heavy-strand promoters (HSP1 and HSP2) are simultaneously active on a circular template. HSP2 requires supercoiling for maximal activation. Increasing TFAM concentrations suppress HSP2 at levels that result in HSP1 stimulation. This study shows distinct modes of promoter activation, providing opportunities for the regulation of mitochondrial gene expression by promoter selection.