Analysis of X-ray diffraction from fibres of Pf1 Inovirus (Filamentous bacteriophage) shows that the DNA in the virion is not highly ordered

X-ray fibre diffraction patterns of well-aligned Pf1 filamentous bacteriophage show sharp layer-lines attributable to an ordered helical array of protein subunits. Electron density maps calculated from the intensity on these layer-lines show no evidence for DNA following the symmetry of the protein, nor is there evidence on the diffraction patterns for the additional layer-lines expected if ordered DNA follows a symmetry different from that of the protein. We conclude that the interactions between DNA and protein in the Pf1 virion, like those in the If virion, are delocalized rather than specific, and the DNA structure in the virion is less regular than the protein structure. This conclusion has implications for the process of virion assembly, and we suggest a possible model for the change in the viral DNA symmetry as the DNA is passed to the virion from the intracellular complex with the viral gene 5 single-stranded DNA-binding protein. (C) 1998 Academic Press.