SUMMARYThe efficiency of the transfection of chicken fibroblasts with a single dose (I"5 to I5O #g) of DNA isolated from virogenic RSV (Prague strain)transformed XC cells was increased if chicken fibroblasts were pre-treated with BUdR. Mitomycin or u.v. irradiation in doses used were not efficient. The repeated attempts to transfect duck fibroblasts, not containing activable endogenous chicken virus genome or group-specific antigen, always failed.Both DNA isolated from purified nuclei and whole virogenic cells exerted the same transfecting activity.Transfecting activity was present in the peak fractions obtained after CsC1 gradient sedimentation of DNA obtained from virogenic cells, was absent in RNA preparations and after digestion of DNA with DNase or alkaline denaturation. This indicates that DNA is responsible for transfection. The role of endogenous virus genomes in transfection is discussed.

Two cell lines, RVP3 and RVA4, derived originally from mouse tumors induced by the Prague and Schmidt-Ruppin strain of RSV, respectively, were studied. tall attempts failed to induce infectious virus production in RVP3 cells by fusion with chicken fibroblasts even if the cells were infected with avian leukosis viruses. Also, attempts to rescue the viral genome by transfection were unsuccessful. RVP3 cells harboured 31-45% of the viral genome sequences, as was shown by molecular hybridization, and therefore they were designated cryptovirogenic. The tumour cell line RVA4 did not contain any detectable viral sequences. The significance of the detection of the incomplete Rous virus genome sequences in mammalian cells is discussed.

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