A relevant (rl) of the temperature-sensitive leucyl-tRNA synthetase mutant (tsHl) of Chinese hamster ovary cells has been analyzed. The enzyme in the revertant is also temperature sensitive, but its activity at every temperature tested is slightly higher than that of the mutant. Both tsHl and the revertant are sensitive to a decreased concentration of leucine in the growth medium, but the revertant is less sensitive than the mutant. The tsHl synthetase has a Km for leucine which is 80-fold higher at 39°C than that of wildtype CHO cells, whereas the Km of the rl enzyme is 20-fold higher than the wild type at this temperature.
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