Twinkle is not the mitochondrial DNA replicative helicase in C. elegans, but may have alternate mitochondrial functions

Dysfunction of mitochondrial DNA replication machinery is a common cause of mitochondrial diseases. The minimal mammalian replisome is made up of DNA polymerase gamma, replicative helicase Twinkle, and single-stranded DNA binding protein. The replisome is localized to the inner mitochondrial membrane and serves as the site of mitochondrial DNA replication and mitochondrial fission. Recently, a sequence homolog of Twinkle was uncovered in the nematode Caenorhabditis elegans. Here, we characterized this homolog, twnk-1, and report that twnk-1 does not function as the primary mitochondrial DNA replicative helicase in this species, as loss of twnk-1 does not result in reduce mitochondrial DNA levels, or result in other expected mitochondrial dysfunctions such as reduced oxygen consumption rates, increased sensitivity to metabolic perturbations, or reduced muscle function. Instead, twnk-1 mutants have increased mitochondrial DNA as they age, and exhibit phenotypes associated with mitochondrial stress, including reduced fecundity, an activation of the mitochondrial unfolded protein response, and mitochondrial fragmentation. Our results suggest in Caenorhabditis elegans, twnk-1 does not function as the mitochondrial DNA replicative helicase, but has an alternative function in regulating mitochondrial function.