Autoregulation of the real-time kinetics of the human mitochondrial replicative helicase.

The human mitochondrial helicase Twinkle is essential for mitochondrial DNA (mtDNA) replication and integrity. Using biochemical and single-molecule techniques, we investigated Twinkle's real-time kinetics, including DNA loading, unwinding, and rewinding, and their regulation by its N-terminal Zinc-binding domain (ZBD), C-terminal tail, and mitochondrial SSB protein (mtSSB). Our results indicate that Twinkle rapidly scans dsDNA to locate the fork, where specific interactions halt diffusion. During unwinding, ZBD-DNA interactions and C-terminal tail control of ATPase activity downregulate kinetics, slowing down the helicase. Binding of mtSSB to DNA likely outcompetes ZBD-DNA interactions, alleviating the downregulatory effects of this domain. Furthermore, we show that ZBD-DNA interactions and ATP binding also regulate rewinding kinetics following helicase stalling. Our findings reveal that ZBD and C-terminal tail play a major role in regulation of Twinkle´s real-time kinetics. Their interplay constitutes an auto-regulatory mechanism that may be relevant for coordinating the mtDNA maintenance activities of the helicase.