Fig. (4) Stoichiometry of labeling of IleRS by [¹⁴C]tRNAIleox.IleRS (3 µM) was incubated with 396 μM of unfractionated tRNA sample containing 12 µM of [¹⁴C]tRNA^Ileox (specific radioactivity 9,000 counts.min^-1.nmole^-1). The number of [¹⁴C]tRNA^Ileox molecules (n) incorporated per mol of enzyme is represented as a function of the residual isoleucine-dependent isotopic [³²P]PPi-ATP exchange (●) and tRNAIle aminoacylation (○) activities. The control experiments in the presence of intact [¹⁴C]tRNA^Ile showed no incorporation of radioactivity.