Coordination of primer initiation within the catalytic domain of human PrimPol

To facilitate the eukaryotic repriming pathway of DNA damage tolerance, PrimPol synthesises de novo oligonucleotide primers downstream of polymerase-stalling obstacles. These primers enable replicative polymerases to resume synthesis to ensure timely completion of DNA replication. Initiating synthesis de novo requires the coordination of single-stranded DNA, initiating nucleotides, and metal ions within PrimPol's active site to catalyze formation of the first phosphodiester bond. Here we examine the interactions between human PrimPol's catalytic domain, nucleotides, and DNA template during each of the various catalytic steps to determine the 'choreography' of primer synthesis, where substrates bind in an ordered manner. Our findings show that the ability of PrimPol to conduct de novo primer synthesis is underpinned by a network of stabilising interactions between the enzyme, template, and nucleotides, as we previously observed for related primase CRISPR-Associated Prim-Pol (CAPP). Together, these findings establish a detailed model for the initiation of DNA synthesis by human PrimPol, which appears highly conserved.