R-loop in the replication origin of human mitochondrial DNA is resolved byRecG, a Holliday junction-specific helicase

Stable RNA-DNA hybrids (R-loops) prime the initiation of replication in Esc herichia coli cells. The R-loops are resolved by Escherichia coli RecG prot ein, a Holliday junction specific helicase. A stable RNA-DNA hybrid formati on in the mitochondrial D-loop region is also implicated in priming the rep lication of mitochondrial DNA. Consistent with this hypothesis, the 3' ends of the mitochondrial R-loop formed by in vitro transcription are located c lose to the initiation sites of the mitochondrial DNA replication. This mit ochondrial R-loop is resolved by RecG in a dose-dependent manner. Since the resolution by RecG: requires ATP, the resolution is dependent on the helic ase activity of RecG. A linear RNA-DNA heteroduplex is not resolved by RecG , suggesting that RecG specifically recognizes the higher structure of the mitochondrial R-loop. This is the first example that R-loops of an eukaryot ic origin is sensitive to a junction-specific helicase. The resolution of t he mitochondrial R-loop by RecG; suggests that the replication-priming R-lo ops have a common structural feature recognized by RecG. (C) 1999 Academic Press.