FIG. 4.

Northern blot of RNAs extracted from peak fractions of CsCl gradients containing viral particles derived from the two-plasmid packaging assay. RNAs were extracted from equal amounts of fractions and run on a 1.4% nondenaturing agarose gel, and a Northern blot was performed as previously described (8). The minus-strand RNA probe was specific for vector portions of the test transcript. Relative packaging efficiencies were calculated from the PhosphorImager scan of the gel normalized to the amount of cells harvested for particles in each case.