The 3' region of c-src gene mRNA is entirely included in exon 12 and does not encode another protein.

We previously reported the isolation of PR2257, a novel replication defective avian sarcoma virus which transduced the entire c-src coding region together with about 900 bp of c-src 3' non coding sequences. This virus originated from a chicken sarcoma induced by inoculation of a transformation-defective Rous sarcoma virus carrying only replicative genes. The 5' end of PR2257 was formed by a splice junction between viral leader sequences and the first exon of c-src. To understand the mechanism of 3' recombination between viral and cellular sequences, we analyzed the genomic organization of the 3' region in chicken and quail src DNA. We report that this sequence is colinear with that of a chicken src cDNA. Therefore, exon 12, which encodes the last 68 amino acids of c-src, also contains all 3' non coding sequences present in c-src mRNA and consequently, appears to be the last and largest (about 2 kbp) exon of c-src. We also show that the 3' regions of chicken and quail c-src genes does not contain the additional open reading frame (orf) which was previously reported (Dorai et al. (1991) Mol. Cell. Biol. 11, 4165-4176), and that no other significant conserved open reading frames could be found in this region for both species. Therefore, this region of src does not code for another protein. Taken together, our results suggest that PR2257 was generated by recombination at the RNA level. However, because of the absence of introns in this region of c-src, we cannot formally rule out the possibility that this recombination took place at the DNA level.