Transformation of primary chick-embryo fibroblasts by virally-expressed C-jun requires no mutation in the protein when a sufficient level of C-jun protein is reached.

Proto-oncogene c-jun transforms primary chick embryo fibroblasts as a single gene when expressed from the replication competent retrovirus R (or RCAS). We have investigated whether transformation is caused by accumulating mutations within the proto-oncogene sequence during the retrotranscription process. c-jun coding sequences carried by three independently isolated R-cJUN viruses have been cloned and entirely sequenced. Two of these-were shown to carry no mutation demonstrating that deregulated expression of a normal c-Jun product is sufficient for transformation. On the contrary when expressed from non replicative Rneo vector lacking gag and pol sequences, the avian c-jun no longer transformed. This difference is correlated with weaker c-Jun accumulation in the second case. When expressed from the same Rneo vector, low level of v-Jun or of the heterologous mouse c-Jun still transform. Taken together, these data strongly suggest that a threshold level of c-Jun product is required for transformation and that this critical level can be lowered by mutations in c-jun or by the synthesis of an heterologous protein.