Evidence for a regulatory element controlling amino acid transport system L in Chinese hamster ovary cells.

We have used the technique of somatic cell hybridization to study the regulation of the neutral amino acid transport system L in Chinese hamster ovary (CHO) cells. The cell line CHO-ts025C1 has a temperature-sensitive mutation in leucyl-tRNA synthetase. At the nonpermissive temperature of 39 degrees C, CHO-ts025C1 cells are unable to charge leucyl-tRNA and behave as though starved for leucine by increasing their system L transport activity two- to fourfold. From the temperature-sensitive cell line, we have isolated a regulatory mutant cell line, CHO-C11B6, that has constitutively elevated system L transport activity. The CHO-C11B6 cell line retains the temperature-sensitive leucyl-tRNA synthetase mutation, but growth of this cell line is temperature resistant because its increased system L transport activity leads to increased intracellular leucine levels, which compensate for the defective synthetase. Hybrid cells formed by fusion of the temperature-sensitive CHO-ts025C1 cells and the temperature-resistant CHO-C11B6 cells show temperature-sensitive growth and temperature-dependent regulation of leucine transport activity. These data suggest that the system L activity of CHO cells is regulated by a dominant-acting element that is defective or absent in the regulatory mutant CHO-C11B6 cell line.