Analysis of a deleted MC29 provirus: gag sequences are not required for fibroblast transformation.

Recovered avian myelocytomatosis virus HBI is an MC29-related virus that induces lymphoid tumors in chickens rather than the predominant neoplastic disease induced by wild-type MC29 (namely, endotheliomas). An analysis of the structure of the HBI provirus(es) in the tumors demonstrated that the provirus(es) could be either full size or deleted. One tumor was found to be clonal in that it contained a single provirus which had been partially deleted; this raised a question concerning the role of this provirus in the maintenance of tumor growth. To characterize the detailed structure of this provirus and determine its biological activity, it was molecularly cloned from tumor DNA. Sequencing confirmed that the provirus contained a deletion which effectively removed the whole gag gene. However, the provirus was shown to encode a myc-specific protein, presumably initiating from within the myc gene, and to be biologically active when it was transfected onto quail embryo fibroblasts. Our results suggest that myc alone is sufficient to transform quail embryo fibroblasts and to maintain tumor growth in vivo.