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Common Location of Determinants in Initiator Transfer RNAs for Initiator-Elongator Discrimination in Bacteria and in Eukaryotes*

Initiator tRNAs are used exclusively for initiation of protein synthesis and not for elongation. We show that both Escherichia coli and eukaryotic initiator tRNAs have negative determinants, at the same positions, that block their activity in elongation. The primary negative determinant in E. coliinitiator tRNA is the C1xA72 mismatch at the end of the acceptor stem. The primary negative determinant in eukaryotic initiator tRNAs is located in the TΨC stem, whereas a secondary negative determinant is the A1:U72 base pair at the end of the acceptor stem. Here we show thatE. coli initiator tRNA also has a secondary negative determinant for elongation and that it is the U50·G64 wobble base pair, located at the same position in the TΨC stem as the primary negative determinant in eukaryotic initiator tRNAs. Mutation of the U50·G64 wobble base pair to C50:G64 or U50:A64 base pairs increases the in vivo amber suppressor activity of initiator tRNA mutants that have changes in the acceptor stem and in the anticodon sequence necessary for amber suppressor activity. Binding assays of the mutant aminoacyl-tRNAs carrying the C50 and A64 changes to the elongation factor EF-Tu·GTP show marginally higher affinity of the C50 and A64 mutant tRNAs and increased stability of the EF-Tu·GTP· aminoacyl-tRNA ternary complexes. Other results show a large effect of the amino acid attached to a tRNA, glutamineversus methionine, on the binding affinity toward EF-Tu·GTP and on the stability of the EF-Tu·GTP·aminoacyl-tRNA ternary complex.