The generality of cation-binding sites in rod-shaped viruses

Hydrogen-ion titration curves have been measured for two filamentous plant viruses (clover yellow mosaic virus and potato virus X) and two filamentous bacterial viruses (fd and Pf1) with and without Ca2+ or Mg2+ ions present, and for the protein of the PM6 mutant of tobacco mosaic virus. The bacterial viruses do not possess the 'strong' cation-binding sites found in all plant viruses, but they have 'weak' sites that can be assigned to juxtaposed carboxylate groups on their external surfaces. The strong sites in plant viruses still cannot be assigned to any particular amino-acid side chains, but they must be located in the region of high electronegativity near the axis.