The effect of amino acids on the temperature sensitive phenotype of the mammalian leucyl‐tRNA synthetase mutant ts Hl and its revertants

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Abstract
The temperature sensitive leucyl‐tRNA synthetase mutant tsHl and two revertants have been compared to the parental Chinese hamster ovary cells with respect to the effects of amino acid concentrations in the medium on growth. Elevating the leucine concentration 30‐ or 100‐fold allowed tsHl to grow exponentially at 38.5°C, normally the nonpermissive temperature. Partial revertants that had recovered some enzyme activity required smaller supplements for growth. Measurements of the leucine pools indicated that they respond directly to the extracellular leucine concentration and may mediate the effect. Use of combinations of amino acids confirmed that isoleucine has a similar though weaker effect on tsHl and identified an even weaker protection by valine. The triple combination of leucine, isoleucine and valine was a much more efficient medium supplement and three times normal concentrations of these amino acids supported growth of tsHl at 38.5°C. It is postulated that they are acting at their respective aminoacyl‐tRNA synthetases to help stabilize a complex which also contains the mutant leucyl‐tRNA synthetase. The pool size measurements also showed that the leucine pools of tsHl and a revertant increased 2‐fold more in a response to increased temperature than those of WT. It is suggested that this is a regulatory response to low leucyl‐tRNA synthetase activity and is important in determining growth phenotypes.
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