The fos gene product undergoes extensive post-translational modification in eukaryotic but not in prokaryotic cells

To investigate the properties of the fos oncogene, we have constructed bacterial and yeast vectors which express the entire fos-coded protein (Fos) and two C-terminal deletion products. In Escherichia coli, Fos proteins were expressed from the phage λ P L promoter under the control of the temperature-sensitive λ repressor. In vitro transcription/translation studies indicate that these vectors produce Fos proteins of the expected sizes. However, in vivo, Fos protein accumulation is observed only in hosts with the Lon − phenotype. In Saccharomyces cerevisiae, the fos gene was expressed from the PHO5 promoter which is induced under low-phosphate conditions. In contrast to the situation in E. coli, in which the heterologous proteins appeared as single major products when subjected to sodium dodecyl sulfate — polyacrylamide gel electrophoresis, the Fos proteins in S. cerevisiae displayed extensive M r heterogeneity. Pulse-chase analyses indicated that this heterogeneity was a consequence of extensive post-translational modification. These modifications occurred to an equivalent extent on the products coded by the fos gene with C-terminal deletions and thus appear not to be controlled by the missing domain.