Activation of PKR by varying length duplex RNAs in 75 mM NaCl. Experiments were performed as previously described (10) in AU75 buffer at 32°C. Each reaction contains 200 nM PKR, 0.4 mM ATP, 4 μCi [γ-32P]-ATP and the indicated amount of RNA. The reaction was allowed to proceed for 20 min at 32°C before quenching with SDS loading buffer and resolved on SDS-PAGE. 20bp dsRNA (black), 25bp dsRNA (blue), 30bp dsRNA (red), 35bp dsRNA (green), 40bp dsRNA (brown).