Mutations in the structural genes of CHO cell histidyl-, valyl-, and leucyl-tRNA synthetases.

Forty-three temperature-sensitive mutants were isolated in the CHO cell line by selecting for noncycling cells using [3H]TdR and cytosine arabinoside. Cell division was extremely temperature sensitive in eight of the mutants, and these were studied in more detail. In seven of these eight mutants, the in vitro specific activity of a single aminoacyl-tRNA synthetase was greatly reduced; four had reduced levels of histidyl-tRNA synthetase, two of valyl-tRNA synthetase, and one of leucyl-tRNA synthetase. Cell hybridization studies showed that the mutants formed three complementation groups. In six of the seven mutants the aminoacyl-tRNA synthetase which had reduced activity was also more thermolabile than the wild-type enzyme. The spontaneous reversion frequency was low for these mutants, and in most cases could be increased by treatment with a chemical mutagen. The isolation of the valyl-tRNA synthetase mutant reported here brings to eight the number of different aminoacyl-tRNA synthetase mutants isolated in the CHO cell line.