Biochemical comparison of the Neurospora crassa wild type and the temperature‐sensitive and leucine‐auxotroph mutant leu‐5

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Abstract
The cytoplasmic leucyl‐tRNA synthetases of Neurospora crassa wild type (grown at 37°C) and mutant (grown at 28°C) were purified approximately 1770‐fold and 1440‐fold respectively. Additional enzyme preparations were carried out with mutant cells grown for 24 h at 28°C and transferred then to 37°C for 10–70 h of growth. The mitochondrial leucyl‐tRNA synthetase of the wild type was purified approximately 722‐fold. The mitochondrial mutant enzyme was found only in traces. The cytoplasmic leucyl‐tRNA synthetase from the mutant (grown at 37°C) in vivo is subject of a proteolytic degradation. This leads to an increased pyrophosphate exchange, without altering aminoacylation. Proteolysis in vitro by trypsin or subtilisin of isolated cytoplasmic wild‐type and mutant leucyl‐tRNA synthetases, however, did not establish any difference in the degradation products and in their catalytic properties. Comparing the cytoplasmic wild‐type and mutant enzymes (grown at 28°C) via steady‐state kinetics did not show significant differences between these synthetases either. The rate‐determining step appears to be after the transfer of the aminoacyl group to the tRNA, e.g. a conformational change or the release of the product. Besides leucine only isoleucine is activated by the enzymes with a discrimination of ∼ 1:600; however, no Ile‐tRNALeu is released. Similarly these enzymes, when tested with eight ATP analogs, cannot be distinguished. For both enzymes six ATP analogs are neither substrates nor inhibitors. Two analogs are substrates with identical kinetic parameters. The mitochondrial wild‐type leucyl‐tRNA synthetase is different from the cytoplasmic enzyme, as particularly exhibited by aminoacylating Escherichia coli tRNALeu but not N. crassa cytoplasmic tRNALeu. The presence of traces of the analogous mitochondrial mutant enzyme could be demonstrated. Therefore, the difference between wild‐type and mutant leu‐5 does not rest in the catalytic properties of the cytoplasmic leucyl‐tRNA synthetases. Differences in other properties of these enzymes are not excluded. In contrast the activity of the mitochondrial leucyl‐tRNA synthetase of the mutant is approximately 1% of that of the wild‐type enzyme.