Aminoacyl-tRNA synthetases from yeast: generality of chemical proofreading in the prevention of misaminoacylation of tRNA

Abstract

The specificity of valyl-, phenylalanyl- and tyrosyl-tRNA synthetases from yeast [bakers] was examined by a series of stringent tests designed to eliminate the possibility of artefactual interference. Valyl-tRNA synthetase, as well as activating a number of amino acid analoges, will accept alanine, cysteine, isoleucine and serine in addition to threonine as substrates for both ATP-PPi exchange and transfer to some tRNAVal species. The transfer is not observed if attempts are made to isolate the appropriate aminoacyl-tRNAVal-C-C-A but its role in the overall aminoacylation can be suspected from both the formation of a stable aminoacyl-tRNAVal-C-C-A(3''NH2) compound and from the stoichiometry of ATP hydrolysis during the aminoacylation of the native tRNA. Similar tests with phenylalanyl-tRNA synthetase indicate that this enzyme will also activate and transfer other naturally occurring amino acids, namely, leucine, methionine, and tyrosine. The tyrosine enzymes, which lacks the hydrolytic capacity of the other 2 enzymes is probably absolutely specific for tyrosine. Apparently chemical proofreading, in terms of an enzymatic hydrolysis of a misacylated tRNA, plays an important part in maintaining the specificity in the overall reaction and this activity may be more widespread than was suspected.