Evidence for One Leucyl Transfer Ribonucleic Acid Synthetase with Suecificitv for Leucine Transfer Ribonucleic Acids with Different Coding Characteristics*

Leucyl transfer ribonucleic acid synthetase (EC 6.1.1.4, L-leucine: tRNA ligase (AMP)) has been purified from Escherichia coli and is free of other aminoacyl-tRNA synthetases, tRNA-methylating enzymes, and CpCpA-adding enzymes. Several criteria suggest that there is one synthetase with specikity for two forms of leucine-specik tRNA which can be separated by countercurrent distribution: purification studies, the mixed substrate method, and, most conclusively, an enzyme-dependent leucine exchange from countercurrent distribution fractions tRNAFeU to tRNAp. Leucine exchange from tRNAF to tRNAp occurs in the presence of AMP, magnesium chloride, 12C-leucine, and synthetase. This exchange can best be explained by an AMP-dependent reaction in which leucine is transferred from one kind of tRNA molecule to the other through a leucyladenylate-synthetase complex. This exchange implies the existence of a single leucyl-tRNA synthetase which acylates the multiple forms of leucine-tRNA in E. coli.