The Effect of Guanosine Triphosphate, Other Nucleotides, and Aminoacyl Transfer Ribonucleic Acid on the Activity of Transferase I and on Its Binding to Ribosomes

Transferase I loses activity slowly when incubated in Tris buffer with MgCl 2 and NH 4 Cl at 37°; when GTP is added to these incubations, the enzyme loses activity rapidly. In the presence of GTP, aminoacyl-transfer RNA protects transferase I completely against inactivation while, in the absence of the nucleotide, aminoacyl-transfer RNA protects only partly against the temperature-dependent loss of activity. All of the nucleoside diphosphates and 5'-guanylyl-methylenediphosphonate inhibit the aminoacyl transfer reaction. GDP, 5'-guanylyl-methylenediphosphonate, and to some extent UDP, which are competitive inhibitors, also catalyze the rapid nucleotide-stimulated loss of activity of transferase I. Whereas transferase I is protected against inactivation when aminoacyl-transfer RNA is incubated with 5'-guanylylmethylenediphosphonate (as with GTP), aminoacyl-transfer RNA has no effect on the GDP-stimulated loss of activity. Other nucleoside triphosphates have no significant effect on aminoacyl transfer or on the activity of transferase I. When ribosomes, transferase I, GTP, and aminoacyl-transfer RNA are incubated, the ribosomes sedimented from the reaction mixture catalyze aminoacyl transfer in the absence of added transferase I. This observation, which suggests that transferase I is bound to ribosomes, also occurs with 5'-guanylyl-methylenediphosphonate but not with GDP, ATP, or with stripped transfer RNA.