POLRMT overexpression increases mtDNA transcription without affecting steady-state mRNA levels

POLRMT is the sole RNA polymerase in human mitochondria, where it generates primers for mitochondrial DNA (mtDNA) replication and transcribes the mtDNA to express genes encoding essential components of the oxidative phosphorylation (OXPHOS) system. Elevated POLRMT levels are found in several cancers and in mouse models with severe mitochondrial dysfunction. Here, we generated and characterized mice overexpressing <i>Polrmt</i> to investigate the physiological and molecular consequences of elevated POLRMT levels. Increasing POLRMT levels did not result in any pathological phenotype but led to increased exercise performance in male mice under stress conditions. <i>Polrmt</i> overexpression increased mtDNA transcription initiation, resulting in higher steady-state levels of the promoter-proximal L-strand transcript <i>7S</i> RNA. Surprisingly, the abundance of mature mitochondrial RNAs was not affected by the elevated POLRMT levels. Furthermore, ubiquitous simultaneous overexpression of <i>Polrmt</i> and <i>Lrpprc</i>, which stabilizes mitochondrial messenger RNAs, did not increase steady-state levels of mitochondrial transcripts in the mouse. Our data show that POLRMT levels regulate transcription initiation, but additional regulatory steps downstream of transcription initiation and transcript stability limit OXPHOS biogenesis.