Model of Ff phage infection. (A) The pIII N2 domain (cyan oval) binds to the F-pilus tip (gray circles), and the pilus retracts. The question mark below the arrow pointing to B indicates an unknown mechanism by which the phage penetrates the outer membrane. (B) The pIII N1 domain (green oval) binds the TolAIII domain (orange circle). The pIII C domain (Cd, dark blue) goes through conformational changes that begin to expose the pVI and pIII transmembrane helices (see box; a single subunit of pIII is shown in dark blue with its predicted transmembrane helix (THM) pink, and pVI in gold with its predicted transmembrane helices cherry red). The question mark pointing to C indicates an unknown mechanism by which the interaction with TolA results in “opening” of the virion to allow phage DNA entry into the host cell. (C) Hydrophobic helices of pIII and pVI insert into the inner membrane and/or interact with the transmembrane helices of TolQRA (pink ovals). (D) The single-stranded DNA (ssDNA) crosses the inner membrane as pVIII (bright blue) is peeled off and inserts into the membrane. (OM) Outer membrane, (IM) inner membrane. The phage structure is obtained from the cryoEM structures PDB 8B3O and 8B3P; pVII, salmon; pIX, purple; F pilus assembly/retraction system, gray ovals. For simplicity only one N1 and N2 domain of pIIII is shown. The structural transitions shown on phage insertion into the membrane are based on images and the model in Conners et al. (2023).