The interferon-inducible, double-stranded RNA-dependent protein kinase PKR has been implicated in anti-viral, anti-tumor, and apoptotic responses. Others have attempted to examine the requirement of PKR in these roles by targeted disruption at the amino terminal-encoding region of the Pkr gene. By using a strategy that aims at disruption of the catalytic domain of PKR, we have generated mice that are genetically ablated for functional PKR. Similar to the other mouse model of Pkr disruption, we have observed no consequences of loss of PKR on tumor suppression. Anti-viral response to influenza and vaccinia also appeared to be normal in mice and in cells lacking PKR. Cytokine signaling in the type I interferon pathway is normal but may be compromised in the erythropoietin pathway in erythroid bone marrow precursors. Contrary to the aminoterminal targeted Pkr mouse, tumor necrosis factor ␣-induced apoptosis and the anti-viral apoptosis response to influenza is not impaired in catalytic domain-targeted Pkrnull cells. The observation of intact eukaryotic initiation factor-2␣ phosphorylation in these Pkr-null cells provides proof of rescue by another eukaryotic initiation factor-2␣ kinase(s).

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The interferon (IFN)‐inducible double‐stranded (ds) RNA‐dependent protein kinase PKR plays a role in the regulation of gene expression through its capacity to phosphorylate the translation initiation factor eIF‐2 and to inhibit protein synthesis. In addition to translational control, PKR has been implicated in the regulation of gene expression at the transcriptional level. In this regard, we have reported that PKR participates in IFN‐and dsRNA‐mediated signaling pathways by interacting with and modulating the transcriptional activity of the signal transducer and activator of transcription STAT1 [Wong, A.H.‐T., Tam, N.W.N., Yang, Y.‐L., Cuddihy, A.R., Li, S., Kirchhoff, S., Hauser, H., Decker, T. & Koromilas, A.E. (1997) EMBO J. 16, 1291–1304]. Here we report that the STAT1 protein is upregulated in cells lacking PKR (PKR−/–) and in cells expressing dominant negative PKR mutants. This upregulation is specific for STAT1 as increased expression is not observed for other STAT proteins. The inhibitory effect of PKR on STAT1 expression is exerted at the post‐translational level because PKR−/– cells exhibit higher STAT1 protein stability than PKR+/+ cells.

Background:The selenocysteine tRNA (tRNA Sec ) has a uniquely long D-stem containing 6 base pairs. Results: The extended D-stem is not essential for function but is required for stability. Conclusion: Enhanced secondary structure in selenocysteine tRNA compensates for the absence of canonical tertiary interactions. Significance: The flexibility due to the absence of tertiary interactions is required for tRNA Sec function, whereas the enhanced secondary structure compensates for the decreased stability.

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