Composite structure of the Ff filamentous phage virion. Short segments of the virion near the rounded and pointy tips are depicted at the right (the wild-type virion is 150 times longer than its diameter). The circular single-stranded viral DNA, twisted into a long, thin helix, lies inside the protein coat; the length of the virion is variable, depending on the length of the DNA. The major coat protein pVIII subunits, thousands forming the bulk of the cylindrical outer coat, are bright blue; the minor coat protein pVII and pIX subunits (five each at the rounded tip) are salmon and purple, respectively; and minor coat protein VI subunits (five at the pointy tip) are gold. The domains of the minor coat protein pIII subunits (five at the pointy tip), which are mapped at the lower right and summarized in Table 1, are colored as follows: SS (absent from the virion) is white, N1 is green, flexible linkers L1 and L2 are gray, N2 is cyan, C is dark blue, and TM is gray. TM becomes part of domain C when incorporated into the virion, where it is colored dark blue like the rest of the domain. An end-on view of the pointy tip is shown at the left, with the five pIII subunits artificially arranged symmetrically around the central axis; in reality, these domains have highly variable orientations because of the flexibility of linker L2. Adapted from Overview: Structure, Biology, and Applications of Filamentous Bacteriophages [Rakonjac et al. 2024]) with permission from Cold Spring Harbor Laboratory Press.