Ribosomal L10⅐L7/L12 protein complex and L11 bind to a highly conserved RNA region around position 1070 in domain II of 23 S rRNA and constitute a part of the GTPase-associated center in Escherichia coli ribosomes. We replaced these ribosomal proteins in vitro with the rat counterparts P0⅐P1/P2 complex and RL12, and tested them for ribosomal activities. The core 50 S subunit lacking the proteins on the 1070 RNA domain was prepared under gentle conditions from a mutant deficient in ribosomal protein L11. The rat proteins bound to the core 50 S subunit through their interactions with the 1070 RNA domain. The resultant hybrid ribosome was insensitive to thiostrepton and showed poly(U)-programmed polyphenylalanine synthesis dependent on the actions of both eukaryotic elongation factors 1␣ (eEF-1␣) and 2 (eEF-2) but not of the prokaryotic equivalent factors EF-Tu and EF-G. The results from replacement of either the L10⅐L7/L12 complex or L11 with rat protein showed that the P0⅐P1/P2 complex, and not RL12, was responsible for the specificity of the eukaryotic ribosomes to eukaryotic elongation factors and for the accompanying GTPase activity. The presence of either E. coli L11 or rat RL12 considerably stimulated the polyphenylalanine synthesis by the hybrid ribosome, suggesting that L11/RL12 proteins play an important role in post-GTPase events of translation elongation.The "GTPase center" of the ribosome is a region involved in interaction with GTP-bound translation factors, GTP hydrolysis (1), and post-GTPase events including tRNA movements on the ribosome (2, 3). Translation elongation is markedly stimulated by the interaction of this region with two elongation factors in a GTP-dependent manner. The GTPase center includes two essential RNA regions around positions 1070 and 2660 (Escherichia coli numbering) of the 23 S/28 S rRNA (4), which appear to bind to the elongation factors (5, 6). Despite the highly conserved structure of the 1070 and 2660 RNA regions, ribosomes show a kingdom-dependent accessibility for translation factors, i.e., prokaryotic ribosomes do not engage in translation elongation with the eukaryotic factors instead of the prokaryotic factors (7-9). Furthermore, there are differences in the rate of GTPase turnover between the two systems; in vitro eukaryotic eEF-2 1 /80 S ribosomedependent GTP hydrolysis is 10-fold slower than the prokaryotic EF-G/70 S ribosome system (10). This may reflect, in part, the elaborate regulation of eukaryotic translation.The other important component of the GTPase center is the acidic stalk protein, termed L7/L12 in prokaryotes (11-15). Four copies of this proteins bind to protein L10 and form a stable complex (16), designated here as L10⅐L7/L12. This protein complex and another protein, L11, are assembled on the 1070 RNA domain (17). Flexible property of L7/L12 protein in the ribosome (16, 18 -20) seems to be correlated with the fast turnover of EF-G-dependent GTPase. The eukaryotic counterparts of the prokaryotic L7/L12 and L10 are P1/P2 and P0, respectively (21,22)....

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