Purification of an Escherichia coli Leucine Suppressor Transfer Ribonucleic Acid and Its Aminoacylation by the Homologous Leucyl-Transfer Ribonucleic Acid Svnthetase*

The suppression of an amber mutation in a permissive strain of Escherichia coli can be achieved by a new tRNA species, a suppressor tRNA (for a review, see Reference 1). The tyrosine amber suppressor tRNA arises from a redundant tRNA species in the Sustrain by a single base change in the anticodon (2). However, this may not be the only mechanism to generate suppressor tRNA species, as suggested recently from studies involving a UGA suppressor, tryptophan tRNA (3). The aminoacylation of a suppressor tRNA is believed to be carried out by the cognate (for the amino acid) aminoacyl-tRNA synthetase. This, of course, implies that the enzyme recognition site on the tRNA cannot involve all bases of the anticodon. This view is well supported by many independent lines of evidence (for a review, see Reference 4). However, the anticodon may play a role in aminoacyl-tRNA formation. The Michaelis constant for this reaction with a glycine missense suppressor tRNA was 1500 times higher than that seen with normal glycine tRNA (5). A detailed study of this reaction with pure suppressor tRNA and pure aminoacyl-tRNA synthetases has not yet been reported. In this paper we report the purification of the E. coli leucine amber suppressor tRNA (6) and its interaction with pure E. coli