The sizes of non-covalently linked RNA subunits isolated from highly leukaemogenic Friend virus derived from the plasma (PV, plasma virus) of leukaemic mice were compared to the RNA subunits isolated from low leukaemogenic Friend virus grown in tissue culture (TCV, tissue culture virus). Histograms derived from electron microscope measurements showed that about one-half of the plasma virus RNA was 1.4 to 2.5 µm in length, corresponding to a mol. wt. range from 1.8 × 10 to 3.2 × 10 and the other half less than 1.4 µm. In contrast, approx. 50% of the TCV RNA was only 0.7 to 1.6 µm in length (mol. wt. 0.9 × 10 to 2.0 × 10) and the remainder less than 0.7 µm in length regardless of whether the virus RNA was isolated from 3, 9, 36 or 72 h cultures. The histograms suggest size classes for both TCV and PV derived RNA subunits. Data obtained from sucrose gradient sedimentation of heat-denatured FLV RNA agreed with the electron microscope length measurements. The smaller sizes of the TCV RNA subunits are hypothetically related to the limited biological activity of Friend leukaemia virus produced from leukaemic cells in culture. Comparable results were obtained using two different RNA extraction procedures. Contamination of TCV nucleic acid preparations by cellular DNA was observed even when the virions were harvested from short term cultures and purified by isopycnic sucrose gradient centrifugation. Consequently, preparations of intact virus were treated with DNase prior to sucrose gradient purification of the virions.


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