Structural basis for cooperative ssDNA binding by bacteriophage protein filament P12

Protein-primed DNA replication is a unique mechanism, bioorthogonal to other known DNA replication modes. It relies on specialised single-stranded DNA (ssDNA)-binding proteins (SSBs) to stabilise ssDNA intermediates by unknown mechanisms. Here, we present the structural and biochemical characterisation of P12, an SSB from bacteriophage PRD1. High-resolution cryo-electron microscopy reveals that P12 forms a unique, cooperative filament along ssDNA. Each protomer binds the phosphate backbone of 6 nucleotides in a sequence-independent manner, protecting ssDNA from nuclease degradation. Filament formation is driven by an intrinsically disordered C-terminal tail, facilitating cooperative binding. We identify residues essential for ssDNA interaction and link the ssDNA-binding ability of P12 to toxicity in host cells. Bioinformatic analyses place the P12 fold as a distinct branch within the OB-like fold family. This work offers new insights into protein-primed DNA replication and lays a foundation for biotechnological applications.