A substantial reduction in the abundance of PKR occurred in both the cytoplasmic and nuclear fractions of MP-12-infected cells, but not in the mock-infected cells and in those infected with MP12 lacking NSs (Figure 7) . Expression of NSs alone resulted in a reduction in the amount of PKR ( Figure 7B ). These data established that NSs protein induced PKR downregulation. The NSs-induced PKR downregulation occurred as early as 4 h.p.i. of MP-12 ( Figure 7G ). We also demonstrated the presence of PKR mRNA in NSs-expressing cells ( Figure 7C ) and that ActD treatment had little effect on the NSs-induced PKR downregulation ( Figure 7B ). We failed to detect PKR at 16 h.p.i. of MP-12infected 293 cells, and yet we easily detected the PKR after a 16 hlong incubation of 293 cells with puromycin ( Figure 7D ). Accordingly, the putative NSs-induced translational inhibition was not a main reason for the reduction in the abundance of PKR. Furthermore, pulse-chase experiments showed that NSs expression reduced the abundance of radiolabelled myc-tagged PKR ( Figure 7E ). All of these data strongly suggested that the NSs induced PKR downregulation at post-transcriptional levels.