Influenza C virus NS1 protein up-regulates viral mRNA splicing

Pre-mRNAs of the influenza A virus M and NS genes are poorly spliced in virus-infected cells. By contrast, in influenza C virus-infected cells, the predominant transcript from the M gene is spliced mRNA. The present study was performed to investigate the mechanism by which influenza C virus M gene-specific mRNA (M mRNA) is readily spliced. Ribonuclease protection assays showed that the splicing of M mRNA in infected cells was much higher than that in M gene-transfected cells, suggesting that viral protein(s) other than M gene-translational products facilitates the splicing of viral mRNAs. The unspliced and spliced mRNAs of the influenza C virus NS gene encode two nonstructural (NS) proteins, NS1 (C/NS1) and NS2 (C/NS2), respectively. The introduction of translational premature termination into the NS gene, which blocked the synthesis of C/NS1 and C/NS2 proteins, drastically reduced the splicing of NS mRNA, raising the possibility that C/NS1 or C/NS2 enhances the splicing of viral mRNAs. The splicing of influenza C virus M mRNA was increased by co-expression of C/NS1, whereas it was reduced by co-expression of influenza A virus NS1 protein (A/NS1). The splicing of influenza A virus M mRNA was also increased by co-expression of C/NS1, whereas it was inhibited by that of A/NS1. These results suggest that influenza C virus NS1, but not A/NS1, can up-regulate the splicing of viral mRNAs.