The Two Xenopus laevis SRC Genes Are Co-expressed and Each Produces Functional pp60src

The haploid genome of Xenopus laevis contains two src genes, and transcripts from both genes are found in the maternal RNA pool of the oocyte (Steele, R. E. (1985) Nucleic Acids Res. 13, 1747–1761). We have now isolated cDNA clones which contain complete coding sequences from both src mRNAs. In vitro translation of RNAs transcribed in vitro from these clones produces in each case a protein with an apparent molecular mass of 57 kDa. The in vitro-synthesized proteins show identical protease cleavage patterns. Sequence analysis of the coding regions of the two cDNAs revealed that they both produce 532-amino acid polypeptides which differ from each other at only eight sites. Analysis of silent site changes between the two coding sequences suggests that the two genes began diverging about 25 million years ago. Hybridization with probes specific for each of the two src RNAs indicates that the two genes are co-expressed in embryos and in at least some adult tissues as well as during oogenesis. Finally, expression of each of the cDNA clones in yeast causes the appearance of proteins which are recognized by an antibody which binds to phosphotyrosine.