The absence of structural relationship between mitochondrial and mitochondrial and cytoplasmic leucyl-tRNA synthetases from Tetrahymena pyriformis.

Two leucyl-tRNA synthetases (EC 6.1.1.4) have been purified to near homogeneity, the one from mitochondria and the other from cytoplasm of Tetrahymena pyriformis. Both enzymes were found to be structurally unrelated, single polypeptides with molecular weights of approximately 100,000 as determined by gel permeation, sucrose gradient centrifugation, and sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. These enzymes behaved differently in elution profiles through hydroxyapatite- and diethylaminoethyl cellulose-column chromatography and isoelectric focusing. The two enzymes also showed some differences in responses to various salts for charging and in pH optima and temperature sensitivity, but no significant difference was found in their affinities (K_m) for ATP and leucine. These enzymes recognized different leucyl-tRNA isoaccepting species as revealed by reversed-phase column chromatography. The mitochondrial enzyme can charge six isoaccepting leucyl-tRNA species, while the cytoplasmic enzyme can recognize only four species.