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The N-terminal domain of the human mitochondrial helicase Twinkle has DNA binding activity crucial for supporting processive DNA synthesis by Polymerase γ

Twinkle is the ring-shaped replicative helicase within the human mitochondria with high homology to bacteriophage T7 gp4 helicase-primase. Unlike many orthologs of Twinkle, the N-terminal domain (NTD) of human Twinkle has lost its primase activity through evolutionarily acquired mutations. The NTD has demonstrated no observed activity thus far, hence its role has remained unclear. In this study, we have biochemically characterized the isolated NTD and C-terminal domain with linker (CTD) to decipher their contributions to the activities of the full-length (FL) Twinkle. This novel CTD construct hydrolyzes ATP, has weak DNA unwinding activity, and assists Polγ-catalyzed strand-displacement synthesis on short replication forks. However, CTD fails to promote multi-kilobase length product formation by Polγ in rolling-circle DNA synthesis. Thus, CTD retains all the motor functions but struggles to implement them for processive translocation. We show that NTD has DNA binding activity, and its presence stabilizes Twinkle oligomerization. The CTD oligomerizes on its own, but loss of NTD results in heterogeneously-sized oligomeric species. The CTD also exhibits weaker and salt-sensitive DNA binding compared to FL Twinkle. Based on these results, we propose that NTD directly contributes to DNA binding and holds the DNA in place behind the central channel of the CTD like a ‘doorstop’, preventing helicase slippages and sustaining processive unwinding. Consistent with this model, mtSSB compensate for the NTD loss and partially restore kilobase length DNA synthesis by CTD and Polγ. The implications of our studies are foundational for understanding the mechanisms of disease-causing Twinkle mutants that lie in the NTD.