Abstract
Methionyl transferRNA (tRNA) synthetase fromEscherichia colistrain K‐12 was purified 675 fold from extracts of kilogram quantities of cell paste as a product that appears to be homogenous on chromatography, electrophoresis and ultracentrifugation. The activities measured by the ATP‐PPi exchange assay and by methionyl tRNA formation did not vary significantly during the purification. The final product, which catalyzed the exchange of 1,000 μmoles of pyrophosphate into ATP/mg protein in 15 min at 37°, was obtained with a yield of 50%.Some general properties of purified methionyl tRNA synthetase are described. The enzyme has a sedimentation coefficient (s20,w) of 6.5 S, and a molecular weight of 173,000. In the presence of dissociating reagents (8M urea or 6M guanidine) it undergoes dissociation into subunits which have a molecular weight of about 43,000. Within the limits of the accuracy of the ultracentrifugation analyses, there was no indication of heterogeneity in mass among these subunits. Furthermore, acrylamide gel electrophoresis in the presence of 8 M urea failed to reveal any significant heterogeneity of the dissociated enzyme. The results indicate that methionyl tRNA synthetase fromE. coliK‐12 is composed of four, possibly identical, subunits.
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