Isolation and characterization of two methionine: tRNA ligases from wheat germ

Two methionine: tRNA ligases (here called ligase A and ligase B) with distinctly different enzymatic and molecular properties were isolated in homogenous form from extracts of raw wheat germ. Both the A and B enzyme are composed of single polypeptide chains of Mr 105000 and 70000 respectively. The smaller molecule (B) has been shown not to be a proteolytic fragment of the larger one (A). The catalytic properties of both the A and B enzymes have been established and the Mg2-dependent capacity to charge six purified methionine-accepting tRNAs have been compared to those of the methionine: tRNA ligases from Escherichia coli and bakers' yeast. The possible reasons for the presence of two methionine: tRNA ligases and their unusual monomeric nature are discussed.