THE 3' REGION OF C-SRC GENE MESSENGER-RNA IS ENTIRELY INCLUDED IN EXON-12 AND DOES NOT ENCODE ANOTHER PROTEIN

We previously reported the isolation of PR2257, a novel replication de fective avian sarcoma virus which transduced the entire c-src coding r egion together with about 900 bp of c-src 3' non coding sequences. Thi s 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 vi ral leader sequences and the first exon of c-src. To understand the me chanism of 3' recombination between viral and cellular sequences, we a nalyzed the genomic organization of the 3' region in chicken and quail src DNA. We report that this sequence is colinear with that of a chic ken src cDNA. Therefore, exon 12, which encodes the last 68 amino acid s 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 kb p) exon of c-src. We also show that the 3' regions of chicken and quai l care genes does not contain the additional open reading frame (orf) which was previously reported (Dorai et al. (1991) Mel. Cell, Biol. 11 , 4165-4176), and that no other significant conserved open reading fra mes could be found in this region for both species. Therefore, this re gion of src does not code for another protein. Taken together, our res ults suggest that PR2257 was generated by recombination at the RNA lev el. 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.