Structure, origin, and transforming activity of feline leukemia virus-myc recombinant provirus FTT

A myc-containing recombinant feline leukemia provirus, designated FTT, was molecularly cloned from the cat T-cell lymphoma line F422. Its transforming activity, as well as the nucleotide sequence of the 3' 2.7 kilobases of FTT, including v-myc, was determined. The predicted v-myc protein differs from feline c-myc by three amino acid changes and is truncated by two amino acids at the carboxyl terminus. Comparison with feline leukemia virus (FeLV), feline c-myc, and other FeLV proviruses indicates that recombination junctions involved in the generation of FeLV-onc viruses occur at preferred locations within the virus. They usually follow or occur within the sequence ACCCC at 5' junctions and may result from homologous recombination between sequences of marked purine-pyrimidine strand bias, especially at 3' junctions. Some recombination sites also resemble recombinase recognition sequences utilized in immunoglobulin and T-cell receptor variable-region joining. Transfection of primary rat embryo fibroblasts and subsequent in vivo analysis revealed that morphologic and tumorigenic transformation require cotransfection of FTT with human EJ-ras DNA; neither gene alone is sufficient. FTT v-myc is expressed in these transformed rat cells as a 3.0-kilobase subgenomic RNA; however, in contrast to the depressed level of c-myc expression in v-myc-involved feline tumors, steady-state levels of rat c-myc RNA and protein are apparently unaltered.