We next tested the effect of co-expression of NSs mutants in the inhibition of IFN-b mRNA synthesis by MP-12 NSs. A549 cells were mock-infected or infected with rMP12-NSs-Flag using moi of 3 and then either mock-transfected or immediately transfected with in vitro synthesized RNA encoding CAT or NSs mutants. Total RNA was extracted at 7 hpi, and Northern blot was performed as described above. None of cells transfected with RNA encoding NSs mutants increased the synthesis of IFN-b mRNA (Fig. 5B) . On the other hand, cells transfected with RNA encoding NSD249-265 slightly increased ISG56 mRNA abundance (Fig. 5B) . These results suggest that nonfunctional NSs encoding the C-terminus self-association domain do not have dominant-negative function, while those lacking the C-terminus domain slightly inhibit PKR degradation as well as ISG56 mRNA synthesis.