Visualization of the complete primosome reveals the structural mechanisms governing DNA replication restart

Replication restart pathways reinitiate DNA replication processes following their premature termination. In <i>Escherichia coli</i>, this essential process begins with the regulated assembly of the primosome complex, comprising the PriA, PriB, and DnaT proteins, onto an abandoned replication fork. Here, we present two distinct primosome structures. One represents an intermediate stage in primosome assembly with a single DnaT C-terminal domain (DnaT^CTD) bound to PriA/PriB/DNA. The second captures the mature primosome, in which filamentation of multiple DnaT^CTD molecules catalyzes the handoff of the single-stranded DNA lagging strand from PriB to DnaT. The DnaT N-terminal domain forms a separate, independent oligomer in the mature structure. Taken together, our results detail the molecular mechanisms underlying replication restart initiation and regulation and suggest an unexpected mechanistic similarity between DnaT and the canonical initiator protein DnaA.