Besides its known role as a translational controlling factor, the double stranded RNA-dependent protein kinase (PKR) is a key transcriptional regulator exerting antiviral and antitumoural activities. We have recently described that induction of NF-kappa B by PKR is involved in apoptosis commitment. To define how PKR mediates NF-kappa B activation by dsRNA, we have used two different approaches, one based on expression of PKR by a vaccinia virus (VV) recombinant and the other based on induction of endogenous PKR by poly I:C (pIC) treatment. We found that NF-kappa B complexes induced by PKR are composed primarily of p50-p65 heterodimers and also of c-rel-p50 heterodimers. As described for other stimuli, following pIC treatment, PKR phosphorylates the NF-kappa B inhibitor I kappa B alpha at serine 32 before degradation. Expression by VV recombinants of IKK1 or IKK2 dominant negative mutants together with PKR showed inhibition of PKR-induced NF-kappa B activation, as measured both by gel shift and luciferase reporter assays. Immunoprecipitation analysis revealed that PKR interacts with the IKK complex. Our findings demonstrate that physiological function(s) of PKR involve activation of the I kappa B kinase complex. Oncogene (2000) 19,1369 - 1378.