ORIGIN AND EVOLUTION OF THE C-SRC-TRANSDUCING AVIAN-SARCOMA VIRUS PR2257

Avian sarcoma virus PR2257 transduced de novo the c-src gene and about 900 bp of 3' non-coding sequences belonging to the src locus. This vi rus contains only one mutation in the c-src coding sequence causing a reading frame shift after Pro-525. The molecular clone studied was der ived from a cell line of transformed quail fibroblasts, C7. It contain s endogenous virus (ev) derived sequences in the U5 and 3' non-coding regions, indicating that multiple recombination occurred with endogeno us virus. Here we investigated the possible evolution of PR2257 when t he original tumour was repeatedly passaged in vivo. After 16 passages a new virus, designated PR2257/16, appeared with a tenfold higher titr e. The sequence of PR2257/16 was determined and showed that PR2257/16 resulted from recombination of PR2257 with the env gene of the helper virus (td daPR-C). This recombination expanded the env gene content in PR2257/16 and, in addition, five point mutations occurred in its geno me. Because we thought that an endogenous virus might be involved in t he mechanism of c-src transduction, we also reinvestigated the presenc e of ev sequences in PR2257 proviruses from several early passages of the original tumour. We found that in contrast with the first isolate from the C7 cell line, the provirus in these tumours did not contain s uch sequences. These results do not support the hypothesis that endoge nous sequences were involved in the transduction process.