Processing of tRNA is accomplished by a high-molecular-weight enzyme complex

An enzyme complex is a multifunctional catalytic unit that efficiently associates substrates with functionally related enzymes. The enzyme complex provides for the cellular regulation of enzymatic activities by physical interaction of the proteins with each other and by prior alteration of one enzyme's substrate by a related enzyme. Such regulatory abilities may go awry in neoplasia. Components of the protein biosynthetic machinery, such as aminoacyl-tRNA synthetases, have been thought to exist freely in the cytoplasm. However, high-molecular-weight enzyme complexes with aminoacyl-tRNA synthetase activities have been found in mammalian cells. We have been the first to report that the mammalian cell enzymes responsible for modification of tRNA occur in enzyme complexes (molecular weight 900000 daltons) associated with aminoacyl-tRNA synthetases and that the activities of these enzymes differ in normal and leukemic cells. Thus the enzymes responsible for the methylation of tRNA occur in enzyme complexes that provide efficient maturation of tRNA and possible regulation of protein synthesis. In FLC cells a unique enzyme complex composed of tRNA-methyltransferase and aminoacyl-tRNA synthetase activities has also been shown to contain a specific ribonuclease activity and a cysteine-tRNA sulfurtransferase activity. Sulfurtransferase activity has been characterized and optimized for its tRNA and cysteine substrates and mercaptoethanol and cation cofactors. Abnormal activity of this enzyme during neoplasia could result in improper acylation of tRNA and/or infidelity of coding by tRNA. Specific RNase is important in the sizing of percursor tRNA into mature tRNA. Results showed that this sizing was dependent upon the presence of the enzyme complex and the length of the incubation time. Many of the 20 aminoacyl-tRNA synthetases are also found in the complex. Electron microscopy has verified the subunit nature of the complex, seen previously by density gradient centrifugation and gel filtration. Three subunits, each of 300 000 daltons, comprise a complex approximately 200 A in diameter.