The mitochondrial RNA polymerase contributes critically to promoter specificity in mammalian cells

Initiation of transcription in mammalian mitochondria depends on three proteins: mitochondrial RNA polymerase (POLRMT), mitochondrial transcription factor A (TFAM) and mitochondrial transcription factor B2 (TFB2M). We show here that the recombinant mouse and human transcription machineries are unable to initiate transcription in vitro from the heterologous light-strand promoter (LSP) of mitochondrial DNA. This species specificity is dependent on the interaction of TFAM and POLRMT with specific distal and proximal promoter elements. A sequence element localized from position -1 to -2 relative to the transcription start site in LSP functionally interacts with POLRMT. The POLRMT/TFB2M heterodimer is unable to interact with promoter elements and initiate even abortive transcription in the absence of TFAM. TFAM is thus an integral part of the mammalian transcription machinery, and we propose that TFAM induces a structural change of the promoter that is required for POLRMT-dependent promoter recognition.