Identification of nucleotide sequences which may encode the oncogenic capacity of avian retrovirus MC29

The retrovirus strain MC29 induces a variety of tumors in chickens, including myelocytomatosis and carcinomas of the kidney and liver. In addition, the virus can transform cultures of embryonic avian macrophages and fibroblasts. We have characterized the genome of MC29 virus and have identified nucleotide sequences that may encode the oncogenic potential ofthe virus. MC29 virus can replicate only with the assistance of a related helper virus. The defect in replication is apparently a consequence of a deletion in one or more viral genes: the haploid genome of the MC29 virus has a molecular weight of ca. 1.7 X 10(6), whereas the genome of the helper virus MCAV has a molecular weight of ca. 3.1 X 10(6). Although MC29 virus transforms fibroblasts in culture, its genome has no detectable homology with the gene src that is responsible for transformation of fibroblasts by avian sarcoma viruses. We prepared radioactive single-stranded DNA complementary to nucleotide sequences present in the genome of MC29 virus but not in the genome of MCAV (cDNA(MC29)). If they are contiguous, these sequences (ca. 1,500 nucleotides) are sufficiently complex to encode at least one protein. Homologous sequences were not detectable in several strains of avian sarcoma viruses or in an endogenous virus of chickens. Our findings confirm and extend recent reports from other laboratories and lead to the conclusion that MC29 virus may contain a previously unidentified gene(s) that is capable of transforming several distinct target cells. The evolutionary origins of this putative gene and its location on the viral genome can be explored with cDNA(MC29).