To test the co-expression effect of those NSs mutants in IFN-b mRNA synthesis, A549 cells were mock-infected or infected with rMP12-NSs-Flag at a moi of 3, and immediately transfected with in vitro synthesized RNA encoding NSs-E253-255A/D257-259A, NSsD257-259A or NSs-E253-255A. Total RNA was extracted at 7 hpi, and Northern blot was performed to detect IFN-b, ISG56 or RVFV S-RNA/N mRNA. None of those mutants attenuated NSsmediated IFN-b mRNA synthesis suppression (Fig. 8B) . These results suggest that attenuation of PKR degradation function might occur due to the accumulation of nonfunctional truncated NSs with some stability (Fig. 3) by the lack of C-terminus 17 amino acids residues.