F-Type Pyocin Versus Phage λ Tail: Conserved Hub, Divergent Fibers

Bacteriocins are ribosomally synthesized antimicrobial peptides or proteins that offer an alternative to conventional antibiotics against multidrug-resistant pathogens. Phage tail-like bacteriocins (tailocins) are classified into rigid R-type and flexible F-type variants. While R-type pyocins from Pseudomonas aeruginosa are well-characterized, F-type pyocins remain poorly understood, especially with respect to the molecular mechanisms for their Gram-negative bactericidal activity. Here, we report cryo-electron microscopy structures of the F-type pyocin from P. aeruginosa ATCC 15442 at 2.29-3.26 Å resolution, encompassing three modular components: the tail cap, tail tip, and tail fiber. Structural comparisons with bacteriophage λ reveal a conserved tail tip architecture, including the baseplate hub proteins, distal tail protein, tail assembly protein, and tape measure protein. Unexpectedly, we identify three trimeric side fibers that attach not to the distal tail protein, as in canonical systems, but to the α-helical shaft of the central fiber, indicating a previously unrecognized attachment mode. The receptor-binding domain of the side fiber shares structural similarity with LPS-recognizing domains of R-type pyocins. Together, these results define the structural basis of F-type pyocin assembly and host recognition, reveal conserved and unique features relative to phage λ, and provide a framework for engineering tailocins as precision antimicrobials against drug-resistant P. aeruginosa.