The induction of interferon (IN) genes by viruses or double-stranded RNA (dsRNA) requires the assembly of a complex set of trnscription factors on responsive DNA elements of ION gene promoters. One of the factors necessary for regulating IFN-P gene transcription is nuclear factor NF-KB, the activation of which is triggered by dsRNA. It has previously been suggested that the dsRNA-activated p68 protein kinase (PKR) may act as an Inducer-receptor, transducing the signal from dsRNA to NF-ucB through phosphorylation of the inhibitor IKB. We present direct evidence that PKR can phosphorylate IKB-a (MAD-3) and activate NF-KB DNA binding activity in vitro. We further show that dsRNA induces an unusual phosphorylated form of IKB-a. The expression of a transdominant mutant PKR is able to perturb the dsRNAmediated signaling pathway in vivo, suggesting a role for this kinase in IFN-P gene induction.

The interferon (IFN)‐induced double‐stranded RNA (dsRNA)‐activated Ser/Thr protein kinase (PKR) plays a role in the antiviral and antiproliferative effects of IFN. PKR phosphorylates initiation factor eIF2α, thereby inhibiting protein synthesis, and also activates the transcription factor, nuclear factor‐κB (NF‐κB), by phosphorylating the inhibitor of NF‐κB, IκB. Mice devoid of functional PKR (Pkr°/°) derived by targeted gene disruption exhibit a diminished response to IFN‐γ and poly(rI:rC) (pIC). In embryo fibroblasts derived from Pkr°/° mice, interferon regulatory factor 1 (IRF‐1) or guanylate binding protein (Gbp) promoter–reporter constructs were unresponsive to IFN‐γ or pIC but response could be restored by co‐transfection with PKR. The lack of responsiveness could be attributed to a diminished activation of IRF‐1 and/or NF‐κB in response to IFN‐γ or pIC. Thus, PKR acts as a signal transducer for IFN‐stimulated genes dependent on the transcription factors IRF‐1 and NF‐κB.