A second promoter on the human mitochondrial light strand

Human mitochondria contain multiple copies of a circular, double-stranded DNA genome which encode for unique protein components required for the synthesis of ATP. On each strand of mitochondrial DNA (mtDNA), polycistronic transcription is initiated from a dedicated promoter. Transcription from the light strand promoter can produce both near-genome length transcripts and short RNA primers for mtDNA replication. In this way, light strand gene expression and mitochondrial DNA replication are seen as coupled events, although there is no consensus as to how this is regulated. Here we report the discovery of a second light strand promoter (LSP2) in human mtDNA, located immediately upstream of the first gene on the light strand. LSP2 possesses nearly identical characteristics to canonical mitochondrial promoters. Our results challenge the prevailing model in mammals wherein regulation of transcription from the light strand promoter is a prerequisite for the synthesis of light strand genes, and suggests that light strand transcription can be decoupled from replication primer formation. Overall design: Cappable-seq and tagRNA-seq library preparation and sequencing on RNA isolated from HeLa (human) mitochondria were performed by Vertis Biotechnologie (Freising, Germany) with duplicate.