35S a and b RNA subunits of avian RNA tumor virus strains cloned and passaged in chick and duck cells.

The RNA of transforming and nontransforming avian RNA tumor viruses was heat denatured and examined for its 35S subunit composition by polyacrylamide gel electrophoresis (gel slicing and/or autoradiography). The RNA of virus strains capable of transforming chick embryo fibroblasts in vitro always contained a subunits: the RNA of Pr-RSV of subgroups A, B or C was composed only of a subunits, even after prolonged passage in chick cells, whereas the RNA of SR-RSV-A, SR-RSV-B, SR-RSV-D(H) and B77 also contained b subunits in varying amounts, even when examined as early as possible (8-10 days) after cloning. The b subunit content did not correlate with length of time after cloning. Nontransforming viruses such as RAV-2 and nontransforming derivatives of transforming viruses such as NT-SR-RSV-A, SR-RSV-B AV-3, NC-SRV-D(H), NC-SRV-D(H)(U4), NC-B77-42 and NC-B77-43 contained only b subunits. When B77 virus grown in chick cells was passaged in duck embryo fibroblasts, the ratio of a/b subunits in its RNA did not change markedly. However, when it was cloned in duck embryo fibroblasts, the transformed clones produced virus that contained almost exclusively b subunits. On transfer to chick cells, a subunits once again appeared, and the a/b subunit ratio was soon similar to that of the original B77. B77 virus produced by cloned, transformed duck embryo fibroblasts was found to contain a large excess of nontransforming virus particles.