The DNA-binding domains of the jun oncoprotein and the yeast GCN4 transcriptional activator protein are functionally homologous.

The jun oncoprotein, which causes sarcomas in chickens, and the DNA-binding domain of yeast GCN4, which coordinately regulates the expression of amino acid biosynthetic genes, show significant homology. In yeast cells deleted for the GCN4 gene, GCN4 function can be conferred by a hybrid protein in which the GCN4 DNA-binding domain is replaced by the homologous region of jun. Moreover, in strains containing various mutations of the GCN4 binding site in the HIS3 promoter, HIS3 expression is affected similarly by the hybrid protein and by GCN4. These results indicate that the jun oncoprotein binds the same DNA sequences as GCN4, and strongly suggest that jun is derived from a normal cellular transcription factor (possibly AP-1, which recognizes similar sequences). This provides direct evidence for the idea that alterations in the machinery for proper gene expression can lead to the oncogenic state.