The Zn-finger domain of human PrimPol is required to stabilize the initiating nucleotide during DNA priming.

Human PrimPol is a monomeric enzyme whose DNA primase activity is required to rescue stalled replication forks during nuclear and mitochondrial DNA replication. PrimPol contains an Archeal-Eukaryotic Primases (AEP) core followed by a C-terminal Zn finger-containing domain (ZnFD), that is exclusively required for primer formation and for PrimPol function in vivo. The present study describes the sequential substrate interactions of human PrimPol during primer synthesis, and the relevance of the ZnFD at each individual step. Both the formation of a PrimPol:ssDNA binary complex and the upcoming interaction with the 3'-nucleotide (pre-ternary complex) remained intact when lacking the ZnFD. Conversely, the ZnFD was required for the subsequent binding and selection of the 5'-nucleotide that will become the first nucleotide of the new primer strand. Providing different 5'-site nucleotides, we can conclude that the ZnFD of PrimPol most likely interacts with the γ-phosphate moiety of the 5'-site nucleotide, optimizing formation of the initial dimer. Moreover, the ZnFD also contributes to recognize the cryptic G at the preferred priming sequence 3'GTC5'. Dimer elongation to obtain long DNA primers occurs processively and is facilitated by the 5'-terminal triphosphate, indicating that the ZnFD is also essential in the subsequent translocation/elongation events during DNA primer synthesis.