1. Transferase I from rat liver binds relatively low quantities of GTP when incubated with this nucleotide in the absence of aminoacyl-tRNA. 2. Transferase I reacts with both aminoacyl-tRNA and GTP to form a relatively stable complex that is retained on cellulose nitrate filters. The ternary complex transferase I–aminoacyl-tRNA–GTP is also formed when the transferase I–aminoacyl-tRNA complex is incubated with GTP or during the incubation of the transferase I–GTP complex with aminoacyl-tRNA. Synthesis of this complex does not require the presence of Mg2+. 3. In the presence of Mg2+ the ternary complex becomes readily bound to ribosomes without requirements for any other cofactors. 4. An extensive cleavage of GTP takes place when aminoacyl-tRNA becomes bound to ribosomes. 5. The low interdependence of reactions leading to the formation of transferase I complexes with aminoacyl-tRNA and GTP indicates that the mechanisms of the binding reaction in mammalian systems may be different from those in bacterial cells.

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