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The Composition of a Population of Vaccinia Virions

The large size and characteristic shape of vaccinia virions make them relatively easy to recognize and count by electron microscopy, even in the midst of substantial quantities of cellular debris. The fact that accurate particle counts of this kind can be made at concentrations as low as 106 virions/ml has permitted their use in locating the position and distribution of virions in rate-zonal sedimentation experiments. The movement of single virions can be studied by this means, quite apart from the effects of aggregation that tend to interfere with the interpretation of experimental data taken at the much greater virion concentrations necessary for optical observations by either absorption, scattering, or refraction of light. These things have been demonstrated in our earlier publication (1) along with the observation that the few slowest-sedimenting virions in the broad single-peaked sedimentation spectrum were more efficient in plaque production than those of the majority that sedimented faster. In view of the significance attached by others to the function of incomplete (2) and deficient (3) interfering virions of other species, we have attempted the difficult task of determining the distribution of DNA, protein, and lipid per virion in the sedimentation spectrum of unpurified vaccinia virus preparations. In this way, we hope to include all minority elements of the virion population that might otherwise be lost through over-zealous purification. The apparent physical heterogeneity of the populations might thus be interpreted in terms of composition and/or structure and possibly later, in terms of phenotypic or genotypic factors. Materials and Methods. Vaccinia virus. WR (mouse neurotropic) strain from the American Type Culture Collection has been passed 162 times in Earle's L cells at a multiplicity of approximately 50 virions/cell. Sedimentation velocity analyses of dilute virion suspensions in the BXIV zonal centrifuge rotor (4) have been made by counting radioactive responses from labeled virions and by counting virions by electron microscopy as previously described (1).