Activation of the transforming potential of the human fos proto-oncogene requires message stabilization and results in increased amounts of partially modified fos protein.

The requirements for activation of the transformation potential of the human c-fos proto-oncogene were investigated. Recombinant plasmids containing the Moloney murine leukemia virus long terminal repeat directing transcription of the c-fos coding region and either the authentic c-fos 3' untranslated region (UTR) or the 3' UTR from human c-myc were inefficient at inducing transformation. In contrast, a recombinant that substituted most of the c-fos 3' UTR with the 3' portion of the simian virus 40 T-antigen gene transformed cells well. This difference in transformation efficiency appeared to be due to significantly higher levels of fos mRNA and protein expressed from the transforming recombinant. This, in turn, was due to the much greater stability of its mRNA compared with those from the poorly transforming recombinants containing the c-fos or c-myc 3' UTR. Thus, the 3' UTR of the human c-fos mRNA is responsible for its rapid degradation and limits the steady-state levels of transcript and protein. Cells transformed by the activated human c-fos plasmids contained increased amounts of partially modified c-fos protein (c-Fos). This form of c-Fos turned over much more rapidly than the highly modified form of c-Fos induced by serum stimulation.