The c-fos transcript is targeted for rapid decay by two distinct mRNA degradation pathways.

Rapid degradation of c-fos proto-oncogene mRNA is crucial for transient c-fos gene expression. Experiments were performed to investigate the cellular mechanisms responsible for the extremely short half-life of human c-fos mRNA in growth-factor-stimulated fibroblasts. These experiments demonstrate the existence of two distinct cellular pathways for rapid c-fos mRNA degradation. Each of these pathways recognizes a different, functionally independent instability determinant within the c-fos transcript. One instability determinant, which is located within the c-fos 3'-untranslated region, is a 75-nucleotide AU-rich segment. Insertion of this element into beta-globin mRNA markedly reduces the half-life of that normally long-lived message. Nevertheless, specific deletion of the AU-rich element from c-fos mRNA has little effect on the transcript's cytoplasmic half-life due to the presence of the other c-fos instability determinant, which is located in the protein-coding segment of the c-fos message. Examination of mRNA decay in cells treated with transcription inhibitors indicates that one c-fos mRNA degradation pathway is dependent on RNA synthesis, whereas the other is not.