Expression of FBJ-MSV oncogene (fos) product in bacteria

The protein predicted from the DNA sequence of the FBJ murine osteosarcoma virus (FBJ-MSV) onc gene (v-fos) was expressed in Escherichia coli under the control of the tryptophan operon regulatory region. The 381-amino acid protein was identified by synthesis in minicells isolated from bacteria containing the expression vector plasmid. A 52,000-Da protein was made in these minicells, along with the proteins encoded by the beta-lactamase gene present on the expression vector plasmid. Synthesis of the 52K protein was repressed in trpR+ E. coli minicells in the presence of tryptophan, whereas the protein was synthesized under the same conditions in trpR- (derepressed) minicells. The beta-lactamase proteins, however, were synthesized under both conditions. The synthesis of the viral protein, therefore, was directed by the trp operon promoter. The tryptic peptide map of the 52K bacterial protein labeled with [35S]methionine was compared to that of the 35S-labeled protein immunoprecipitated from FBJ-MSV transformed rat cells using tumor-bearing rat sera. The peptide map of the p55 protein, previously identified as a candidate transforming protein in FBJ-MSV transformed cells, matches exactly that of the bacterial 52K protein. The eukaryotic cell protein, however, differs slightly in electrophoretic mobility in SDS gels, most likely due to post-translational modification which does not occur in bacteria.