FIG. 6.

Buoyant density of virus produced by infected CP cells. Supernatant from JSRV₂₁-infected CP cells (passage 19) was concentrated and fractionated by isopycnic centrifugation in a 20 to 55% continuous sucrose gradient. Gradient fractions were assayed for RT activity by the PERT assay. (a) RT activity (solid line) expressed in PhosphorImager units taken from samples visualized in panel b; the broken line shows density (grams per milliliter). The peak of RT activity is in the fraction with a density of 1.15 g/ml. (b) PERT reactions from each gradient fraction visualized by Southern blotting for MS-2 DNA; +, Moloney murine leukemia virus RT (10⁻⁴ U); −, buffer A control.