Biological and biochemical differences among ecotropic C-type RNA viral isolates chemically induced from C57BL/Ka mouse embryo cells in vitro

Several biological properties of the ecotropic viruses isolated after IUdR induction of C57BL/Ka mouse embryo cells in vitro were found to vary depending on the type of cell from which the viruses were recovered and their passage history in culture. The majority of the induced viral isolates were N-tropic. They could be separated into two classes on the basis of their XC plaque-forming ability and their electrophoretic mobility in SDS-polyacrylamide gels. The exogenous transfer to and propagation in C57BL/Ka fibroblasts of one N-tropic parent virus led to the appearance of a progeny virus population with N- and B-tropic activity and high plaque-forming ability. This preparation, which had B- and N-tropic activity, was either a heterogeneous mixture of N- and B-tropic particles or a heterozygote particle which contained N- and B-tropic genomes. After passage of limiting dilutions of that preparation in C57BL fibroblasts, a novel B-tropic virus was recovered which was distinct from other B-tropic fibrotropic viruses isolated previously from strain C57BL/Ka mice. This new isolate is believed to have originated from recombinational events which had taken place between the B-tropical viral genome endogenous to the C57BL/Ka fibroblasts and the exogenous N-tropic virus used to infect these cells.