A complex of Mgm101 (green) and Mmm1 (red) controls mtDNA (blue) replication.

On page 503, Meeusen and Nunnari show that yeast mitochondria harbor self-sustaining DNA replication factories.

The mitochondrial genome (mtDNA) is packaged into nucleoids, some of which attach to the mitochondrial membrane at sites that contain the outer membrane protein Mmm1. Mmm1 binds to the actin cytoskeleton, and mitochondrial movement depends on actin, so one obvious hypothesis is that this DNA–protein structure segregates mtDNA into buds. But Meeusen and Nunnari suggest that Mmm1 and associated proteins replicate, rather than actively segregate, the genome.

The authors find that Mmm1-linked nucleoids are associated with both replicating mtDNA and proteins necessary for its duplication, including Mgm101, an essential DNA repair protein, and Mip1, the mtDNA polymerase. These proteins make up an independent structure, spanning two membranes, that is replicated and inherited even in the absence of mtDNA, indicating that the association is not simply a byproduct of DNA-binding abilities. It is not yet clear which components are needed for its replication.

The structures do not seem to segregate DNA actively, as they had limited movement except in concert with the organelle as a whole. However, they might still ensure faithful mtDNA inheritance by anchoring nucleoids throughout the organelle. This model is consistent with the uniform spacing of the complexes seen in vivo. ▪