Structure of the protein and DNA in fd filamentous bacterial virus

Abstract

The virion of filamentous bacterial viruses comparises a cylindrical protein shell of o.d. (outer diameter) .apprx. 60 .ANG. and i.d. (inner diameter) 20 .ANG., containing a single-stranded circular DNA molecule which has 2 oppositely directed but not base-paired strands extending the length of the virion. The assembly of the virion involves an intracellular prepackaging of the DNA with a viral DNA-binding protein which is then displaced by the coat protein as the growing virion crosses the bacterial membrane. Studies of the virion by X-ray fiber diffraction show that the protein coat consists largely of a .alpha.-helices oriented roughly parallel to the axis of the virion. As the normal to a planar peptide tends to align normal to a magnetic field, it is possible to significantly improve the orientation of virions in fibers using a strong magnet. The success of this technique with the Pf1 strain of virus led to its use on the better-known fd (f1, M13) strain. New information on the arrangement of protein and DNA in the fd virion obtained from the improved diffraction pattern is reported.