Characterization of Two Species of Methionine Transfer Ribonucleic Acid from Bakers' Yeast

Abstract

Abstract The biochemical properties of two species of methionine transfer ribonucleic acid from bakers' yeast, separated by diethylaminoethyl Sephadex column chromatography, have been studied. Each species of the two methionine tRNAs, designated methionine tRNA I and methionine tRNA II, respectively, has been purified by benzoylated DEAE-cellulose column chromatography. Methionine tRNA I can be esterified with methionine by Escherichia coli aminoacyl-tRNA synthetase as efficiently as by yeast aminoacyl-tRNA synthetase, whereas methionine tRNA II can be charged only partially by the E. coli enzyme under the same conditions used in the charging of methionine tRNA I. Methionine tRNA I has been shown to be converted to N-formylmethionyl-tRNA I by E. coli methionyl-tRNA transformylase in the presence of N10-formyltetrahydrofolate, while methionine tRNA II is not. In ribosomal binding studies, 14C-methionyl-tRNA I is recognized by the condons AUG, GUG, and UUG, while 14C-methionyl-tRNA II responds principally to AUG. In addition, methionine has been found to be incorporated from both methionyl-tRNAs I and II into polypeptide as directed by messenger RNA obtained from bacteriophage f2 in a cell-free system from E. coli. Our present report also provides evidence that yeast N-formylmethionyl-tRNA I serves as an initiator of protein synthesis programmed with the natural messenger in an E. coli system in vitro.