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.