Isolation of the avian transforming retrovirus, AS42, carrying the v-maf oncogene and initial characterization of its gene product

A novel avian transforming retrovirus was isolated from a chicken musculoaponeurotic fibrosarcoma. This virus (called AS42) induces tumors histopathologically indistinguishable from the original sarcoma after a long latent period when inoculated into newborn chickens. AS42 also exhibits a weak transforming activity when infected into chicken embryo fibroblasts (CEF). This virus is replication-defective and associated with a helper virus of subgroup A (called ASAV). An AS42-specific protein of about 100 kDa was immunoprecipitated from lysates of AS42-transformed CEF with antiserum directed against avian retrovirus virion proteins. Molecular analysis of the genomic structure of the AS42 virus has revealed that this 100-kDa protein represents a novel oncogene, v-maf of cellular origin, which is fused with a part of the viral gag gene (Nishizawa et al., Proc. Natl. Acad. Sci. USA 86, 7711-7715, 1989). Interestingly, some size variation was observed among the gag-maf fusion proteins found in individual clones of transformed CEF. Consistent with this observation, Southern blot analyses and nucleotide sequence determination of several independent isolates of proviral DNA indicated that this virus segregates multiple forms of deletion mutants, probably through homologous recombinations among the repetitive sequences present within the v-maf coding region.