DIFFERENTIAL AMINOACYLATION OF 3 SPECIES OF ISOLEUCINE TRANSFER RNA FROM ESCHERICHIA COLI

The kinetics of enzymic acylation of an Escherichia coli extract of transfer RNA with isoleucine are biphasic, in that they manifest a rapid first phase during which about one-third of the total isoleucine acceptor capacity is revealed and a slow second phase during which the remainder of the acceptance takes place. Reductions of the concentration of activating enzyme or MgCl2 in the reaction mixture accentuate the biphasic character of this process. These kinetics reflect the presence of three chromatographically distinct species of isoleucine tRNA, of which species I and II, representing about two-thirds of the total isoleucine acceptor capacity, are aminoacylated much more slowly than species III. At low MgCl2 concentrations, aminoacylation of species I and II falls to a negligible rate, while that of species III still proceeds rapidly. Examination of the intracellular condition of isoleucine tRNA demonstrated that species III is not an artifact produced from species I and II during the isolation procedure. Species I and II, which are aminoacylated slowly in vitro, are nearly completely aminoacylated in vivo during both exponential growth and starvation for leucine. Species III, in contrast, is largely non-acylated during exponential growth; during leucine starvation it is largely acylated but is, in addition, in an inactive, probably reduced, state and can accept isoleucine in vitro only after treatment with periodate. © 1969.