Avian sarcoma virus PR2257 transduced the c- gene and about 900 bp of 3′ non-coding sequences belonging to the locus. This virus contains only one mutation in the c- coding sequence causing a reading frame shift after Pro-525. The molecular clone studied was derived from a cell line of transformed quail fibroblasts, C7. It contains endogenous virus () derived sequences in the U5 and 3′ non-coding regions, indicating that multiple recombination occurred with endogenous virus. Here we investigated the possible evolution of PR2257 when the original tumour was repeatedly passaged . After 16 passages a new virus, designated PR2257/16, appeared with a tenfold higher titre. The sequence of PR2257/16 was determined and showed that PR2257/16 resulted from recombination of PR2257 with the gene of the helper virus (PR-C). This recombination expanded the gene content in PR2257/16 and, in addition, five point mutations occurred in its genome. Because we thought that an endogenous virus might be involved in the mechanism of c- transduction, we also reinvestigated the presence of 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 such sequences. These results do not support the hypothesis that endogenous sequences were involved in the transduction process.


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