In vitro translation assays were performed essentially as described by Condò et al. (28) . The samples (25 ml final volume) contained 5 ml of S.solfataricus cell extract, 10 mM KCl, 20 mM Tris-HCl, pH 7.0, 20 mM Mg acetate, 3 mM ATP, 1 mM GTP, 5 mg of bulk S.solfataricus tRNA, 2 ml of [ 35 S]methionine (1200 Ci/mmol at 10 mCi/ml) and $10 pmol of each mRNA. The mixtures were incubated at 70 C for 45 min. After this time, the synthesized proteins were resolved by electrophoresis 12.5% acrylamide-SDS gels and revealed by autoradiography of the dried gels on an Instant Imager apparatus.