FIG. 3.

Viral particle density analysis. (A) Viral particles obtained from stably transfected QT6 cells with wt RSV proviral DNAs were fractionated through 10 to 40% iodixanol gradients. Eleven fractions were collected, and the density of each fraction was measured on a refractometer. RT assays of each fraction were performed, as described in Materials and Methods. (B) QT6 cells transfected with mutant plasmid DNAs were labeled with [³⁵S]methionine, and viral pellets were centrifuged in 10 to 40% iodixanol gradients. Ten fractions were collected, and the density of each fraction was measured on a refractometer. The labeled Gag proteins in each fraction were immunoprecipitated and electrophoresed on SDS-polyacrylamide gels, as described in Materials and Methods. The relative amount of Gag proteins in each fraction were quantitated by using ImageQuant software.