POLRMT does not transcribe nuclear genes

Arising from J. E. Kravchenko, I. B. Rogozin, E. V. Koonin & P. M. Chumakov Nature 436, 735–739 (2005); doi:10.1038/nature0384810.1038/nature03848 Mitochondria are involved in a variety of metabolic processes and one of their main functions is to perform oxidative phosphorylation1,2, which requires a crosstalk between the mitochondrial and nuclear genomes to accomplish coordinated gene expression3,4. Splice variants of the mitochondrial RNA polymerase gene (Polrmt) have been reported to encode a nuclear RNA polymerase isoform (spRNAP-IV), which is thought to facilitate this coordination by transcribing a specific subset of nuclear genes5,6,7. Here we report that analysis of Polrmt gene expression, subcellular fractionation and fluorescence microscopy do not support the existence of a nuclear POLRMT isoform in mouse and human cells, and that conditional knockout of Polrmt does not affect expression of the nuclear genes previously reported to be transcribed by spRNAP-IV. We thus conclude that POLRMT has an exclusive mitochondrial role and that it is absolutely required for expression of mitochondrial DNA (mtDNA) in mammalian mitochondria.