The Ff virion structure. (A) Helper phage R408 and a phagemid imaged by atomic force microscopy (J Rakonjac, M Russel, and P Model, unpubl.). (B) Major coat protein pVIII subunit structure (minus the four residues at the amino terminus) in the virion extracted from the cryo-electron microscopy (cryoEM) structure (PDB 8B3Q; Conners et al. 2023). (C) Composite structure of an f1 phage virion obtained from the cryoEM structures PDB 8B3O, 8B3P, and 8B3Q, and a prediction of the N1–N2 domains and linkers, based on the data in Conners et al. (2023), pVII is shown in salmon, pIX in purple, pVIII in bright blue, pVI in gold, the C domain of pIII in dark blue, the pIII linkers in gray, the pIII N1 domain in green, and the pIII N2 domain in cyan. (D) The structure in C, viewed from the pIII–pVI end. (E) A schematic of the nascent pIII protein. (SS) Signal sequence, (N1, N2, C) domains of pIII; (L) glycine-rich linkers; (TM) transmembrane helix. (F) f1-derived nanorods (50 nm in length) (Adapted from Sattar et al. 2015, under a Creative Commons License Attribution (CC BY); http://creativecommons.org/licenses/by/4.0/.)