Structural atlas of Pakpunavirus P7-1 reveals determinants of virion stability and genome ejection

Bacteriophages of the Pakpunavirus genus exhibit a broad host range and potent bacteriolytic activity, making them promising candidates for clinical use. Here, we present a structural atlas of the therapeutic phage Pakpunavirus P7-1, a component of a phage cocktail targeting Pseudomonas aeruginosa that has undergone Phase 1/2 clinical trials. We determine the near-atomic structure of the extended virion and obtain a medium-resolution reconstruction of the contracted tail. Atomic models are presented for 20 structural proteins comprising the icosahedral capsid, neck, contractile tail, and baseplate. We identify six upward-pointing Short Tail Fibers that stabilize the extended sheath and six highly flexible Long Tail Fibers likely involved in host recognition. Ordered fragments of the Tape Measure Protein reveal six copies inside the tail tube, forming a 3-helix cork at the tail tip. Sheath contraction repositions the baseplate, projecting all twelve tail fibers outward, yet contraction alone is insufficient to trigger genome ejection.