Human mitochondrial RNA polymerase primes lagging-strand DNA synthesis in vitro

The mitochondrial transcription machinery synthesizes the RNA primers required for initiation of leading-strand DNA synthesis in mammalian mitochondria. RNA primers are also required for initiation of lagging-strand DNA synthesis, but the responsible enzyme has so far remained elusive. Here, we present a series of observations that suggests that mitochondrial RNA polymerase (POLRMT) can act as lagging-strand primase in mammalian cells. POLRMT is highly processive on double-stranded DNA, but synthesizes RNA primers with a length of 25 to 75 nt on a single-stranded template. The short RNA primers synthesized by POLRMT are used by the mitochondrial DNA polymerase gamma to initiate DNA synthesis in vitro. Addition of mitochondrial single-stranded DNA binding protein (mtSSB) reduces overall levels of primer synthesis, but stimulates primer-dependent DNA synthesis. Furthermore, when combined, POLRMT, DNA polymerase gamma, the DNA helicase TWINKLE, and mtSSB are capable of simultaneous leading- and lagging-strand DNA synthesis in vitro. Based on our observations, we suggest that POLRMT is the lagging-strand primase in mammalian mitochondria.