Identification of the internal ribosome entry sites in the 5'‑untranslated region of the c‑fos gene

The Fos proto‑oncogene, activator protein‑1 (AP‑1) transcription factor subunit (c‑fos) gene, a member of the immediate early gene family, encodes c‑Fos, which is a subunit of the AP‑1 transcription factor. The present study aimed to investigate the mechanism by which the translation efficiency of c‑fos mRNA is upregulated when cellular protein synthesis is shut off. The result of western blotting revealed that the protein expression levels of c‑Fos were increased in rhabdomyosarcoma cells infected with enterovirus 71 (EV71) compared with uninfected cells. PCR was used to get the c‑fos 5'‑untranslated region (UTR). The luciferase assay of a bicistronic vector containing the c‑fos 5'UTR revealed that the c‑fos 5'UTR contains an internal ribosome entry site (IRES) sequence and a 175 nucleotide sequence (between 31 and 205 nt) that is essential for IRES activity. Analysis of potential IRES trans‑acting factors revealed that poly(C)‑binding protein 2 (PCBP2) negatively regulated the activity of the c‑fos IRES, whereas the La autoantigen (La) positively regulated its activity. The results of RNA‑protein immunoprecipitation demonstrated that both PCBP2 and La bound to the c‑fos 5'UTR. Furthermore, the IRES activity of in vitro‑transcribed c‑fos mRNA was upregulated during EV71 infection. The present study suggested a mechanism for the effect of viral infection on host genes, and provided a novel target for gene translation regulation.