Comparison between the viral transforming gene (src) of recovered avian sarcoma virus and its cellular homolog

Recovered avian sarcoma viruses are recombinants between transformation-defective mutants of Rous sarcoma virus and the chicken cellular gene homologous to the src gene of Rous sarcoma virus. We have constructed and analyzed molecular clones of viral deoxyribonucleic acid from recovered avian sarcoma virus and its transformation-competent progenitor, the Schmidt-Ruppin A strain of Rous sarcoma virus. A 2.0-megadalton EcoRI fragment containing the entire src gene from each of these clones was subcloned and characterized. These fragments were also used as probes to isolate recombinant phage clones containing the cellular counterpart of the viral src gene, termed cellular src, from a lambda library of chicken deoxyribonucleic acid. The structure of cellular src was analyzed by restriction endonuclease mapping and electron microscopy. Restriction endonuclease mapping revealed extensive similarity between the src regions of Rous sarcoma virus and recovered avian sarcoma virus, but striking differences between the viral src's and cellular src. Electron microscopic analysis of heteroduplexes between recovered virus src and cellular src revealed a 1.8-kilobase region of homology. In the cellular gene, the homologous region was interrupted by seven nonhomologous regions which we interpret to be intervening sequences. We estimate the minimum length of cellular src to be about 7.2 kilobases. These findings have implications concerning the mechanism of formation of recovered virus src and possibly other cell-derived retrovirus transforming genes.