Structural polymorphism correlated to surface charge in filamentous bacteriophages.

Fiber diffraction studies are used to demonstrate that changes in the helical symmetry of the protein coat of filamentous bacterial viruses fd and M13 are correlated with changes in the surface charge. Comparison of the structure of M13 and fd at pH 2 and 8 indicate that surface charge affects both the helical symmetry and flexibility of the virions. The changes in helical symmetry are similar in magnitude to that observed in the Pseudomanas phage Pf1 and probably reflect an inocuous side effect of the particle flexibility required for protection of the virus particles from damage due to shear. The magnitude of the observed changes in helical symmetry appears to be limited to that which can occur without repacking of the interfaces between the alpha-helices making up the viral protein coat.