Electrophoretic and buoyant density variants of southern bean mosaic virus

Abstract

Five electrophoretic variants of southern bean mosaic virus have been found. Each was characterized by its mobility as a function of pH in two buffer systems. In monovalent buffers of 0.10 ionic strength, their isoelectric points ranged between pH 3.95 and 6.03. When sodium chloride was a constituent of the buffer system, the isoelectric points of each of the variants was lowered, suggesting that chloride ions were binding to the viral protein shell. Calculations using these data show that, depending on the variant, chloride ions bind between 7 and 38% of the total available amphoteric charges. A relationship exists among the variants since, for each, approximately the square of the fractional number of chloride ions bound is proportional to the slope of the mobility-pH curve. The “valency” of each variant is similar. Although each variant is homogeneous electrophoretically and in its buoyant density in CsCl, two of the variants display three components in other cesium salts. When the multiple components are recombined and recentrifuged in CsCl density gradient, only a single component is present. This indicates that multiple components do not represent degraded virus. Fourteen “species” have been identified among the five electrophoretic variants by their characteristic buoyant densities.